trimaran ama design

Professional BoatBuilder Magazine

Folding multihulls.

By John Marples , Jan 28, 2023

Folding multihulls and their beam-reduction strategies.

A Farrier 33R trimaran with amas folded and secured on a road-legal trailer exem- plifies the advantages of adjustable- beam multihulls, which are easy to trans- port and store out of the water.

“I’ll be surprised if you can find space in the harbor for that thing,” I heard him say as my new 37 ‘ (11.3m) trimaran was about to be launched. I hadn’t given it much thought, but now this legitimate question was raised, and where to moor was the next issue at hand. Space, particularly width of a slip, becomes the limiting requirement. But then, space also is one of the attractive features of multihulls—deck space to move around on, free from the confines of the cockpit.

Step aboard any multihull and it is obvious how much real estate they offer. Even small models seem expansive. The beam of the typical catamaran is half its length, and trimarans are even wider, sometimes as wide as they are long. Aside from increasing space, beam also boosts stability without adding ballast. The early Pacific Islanders created these form-stable craft for fishing and interisland commerce and voyaging where natural island harbors were few, so the boats had to be light enough for crew to carry them up the beach. Today’s modern multihulls are still lighter than contemporary monohulls, but the larger ones preclude the option of dry sailing them from the beach. They require more marina space than monohulls, and the limited number of slips to accommodate them can be a problem. As someone once put to me, “Multihulls have a poor ‘stacking factor.’”

With catamarans and trimarans becoming more popular, they demand mooring solutions. Some marinas offer shallow-water slips to multihulls, typically at the inboard ends of docks, next to the seawall, because multihulls either have shallow keels or retractable boards. Some marinas also designate the end ties as multihull slips in areas not used for transients. Even though these boats protrude farther into the channel than monohulls, the extra space their beam occupies is relatively small. With multihulls crowding waiting lists for marina slips, builders were prompted to consider folding systems to “improve their stacking factor.”

Without ballast, smaller multihulls up to about 30 ‘ (9.1m) can be dry-stored on a trailer, and most launch ramps easily accommodate over-width boats. If the boat’s beam can be reduced to the legal highway width of 8.5 ‘ (2.6m), the owner can store the boat at home. Today, folding trimarans and catamarans are common sights on trailers in storage yards and backyards. Various folding systems have evolved to support this need, especially for the backyard builder. Some beam-reduction systems allow the boats to be folded and stored in the water in conventional-size marina slips, while other systems facilitate efficient storage or provide street-legal trailering.

Folding Systems

Basic folding systems are separated into several categories:

take-apart akas, the simplest beam-reduction method
telescoping akas (sliding beams)
simple horizontal hinge
complex horizontal hinge systems
vertical hinges (swing wing)
complex swing wing

The following overview of folding systems illustrates how these mechanisms work. It is not intended to be an exhaustive listing of available folding designs. I’ll address two-hull boats (catamarans and proas) first, followed by trimarans. Each type has its challenges and advantages. All are separated into two more categories: transportable boats and trailerable boats. The characteristic distinguishing between them is the time and effort required to launch, starting from an on-the-trailer folded condition. Trailerable implies the possibility of daily launching, requiring less than an hour from trailering to sailing. Transportable denotes a road-legal trailer package but with a longer assembly time to sail away. It might even take more than one trailer load, and considerable assembly time. Legal width in this category could extend to 10 ‘ (3m) wide if OVERSIZE LOAD signs are used (consult local laws). Transportable boats usually require seasonal transport with storage in the water during sailing season and dry storage in the winter. Both categories benefit from the ability to “go to weather at 65 mph” to reach any suitable launching site, even hundreds of miles from home. This opens the possible sailing venue to any water body with a launch ramp and road access, and some trailerable and transportable boats can be delivered anywhere in the world in standard shipping containers.

Catamarans and Proas

The WindRider 17′ trima- ran’s telescoping tubular akas are secured with pins

Hobie Cats and other beach cats are familiar sights around lakes, beaches, and harbors. They are usually built to 8 ‘ (2.4m) beam and do not need folding systems. The 19 ‘ (5.8m) Tornado class catamaran at 10 ‘ wide uses a side tilt-up trailer to reduce beam. Larger catamarans needing folding systems have greater challenges than trimarans of the same length, for a few reasons: The hulls are normally bigger (and heavier) than amas for the same length trimaran; the mast is stepped on the center of an aka, midway between the hulls, which means the aka must be extra strong; and there is no easy means of supporting the hulls while the beam is being expanded to the sailing position, requiring that the trailer have an expanding-beam function. As a result, folding systems are less common on cats and are usually of three types:

folding akas along the centerline or to a center pod
telescoping akas
take-apart akas

On this Wharram cat, the akas are securely lashed into “deck alleys.”

Designers have used telescoping akas, but production boats generally avoid the associated complexity and cost. The mechanically straightforward take-apart feature has successfully been used by many boats, like the 27 ‘ (8.2m) Stiletto Cat and others. Generally, the assembly of these boats takes some time and muscle, which relegates them to the transportable category. Stiletto Cat advertising suggests a four-hour setup time, but in reality, it is much longer. All the James Wharram–designed catamarans up to 63 ‘ (19.2m) are held together with rope lashings and can be dismantled for transport. The required time and effort are generally proportional to the length of the boat.

Note that the Gougeon 32 ‘ (9.7m) sailing catamaran is unique, at 8 ‘ wide, without folding capability but with a water ballast system to make up for the lack of form stability.

The large main hull of a typical smaller trimaran offers a larger interior space than a comparably sized catama ran, a deep footwell in the cockpit for comfortable seating, and a folding system for trailering with the amas connected to a well-supported main hull. In addition, the mast is stepped on the main hull, with the headstay attached to the bow, not to the akas.

Trailerable trimarans come in all sizes to about 32 ‘ long, with transportable designs somewhat longer. The latter types tend to have larger interior spaces and less complex connectives. To a certain extent, manufacturers were willing to add cost to the folding system to reduce setup time. Folding capability on or off the water also adds to the design challenges.

Take-Apart Aka Systems

This is the least expensive method and easiest to achieve for the home builder or the manufacturer. The akas may be built-up wood box beams or tubular metal. Each beam is secured to the hulls by through-bolts, bolted straps, plug-in sockets, or lashings. Tubular aluminum beams are the lightest but most expensive. Regardless of attachment method, the hulls must be supported in their respective positions for the akas to be installed. In small vessels, this can be an abbreviated procedure, but larger vessels will require a special trailer to hold the disconnected amas while on the road.

The Chesapeake Light Craft 15′ single outrigger canoe akas lash into saddles on both hulls.

Telescoping Aka Systems

The telescoping option is limited to boats where the total stack-up width and length dimensions of the hulls and fully retracted akas do not exceed the legal road limits. The WindRider 17 is a good example. The boat is supported on “high bars” on the trailer, leaving the amas free to be moved in or out. The simplicity of the akas and trailer-support system reduces cost and launching time.

In larger vessels, this system has been applied to reduce width for storage in marina slips. For these boats, the sliding system is large and complex, usually requiring some sort of power to make the telescope slide. Because the sliding mechanism requires a small clearance between the sliding members, the akas will move slightly during sailing, which is difficult to avoid.

The 1970s-vintage Telstar 26 features a simple hinge-down system with a bolted con- nection on deck and a bolted strut below.

Simple Horizontal Hinge Systems

Early trailerable trimaran designs often incorporated a simple hinged beam-reduction system to fold both sides down. Boats to about 25 ‘ (7.6m) with a 16 ‘ (4.9m) beam could be made to fold to 8 ‘ . At the ama end, lifting the hull, sometimes with attached wing deck, could require substantial muscle or a mechanical lift. Even for smaller boats this task may be beyond one person’s capability. Normally, bolts and plates between the members secure the hull for sailing. On the Searunner 25 and Constant Camber 26 (7.9m), double-hinged tubes are bolted to tangs on the main hull.

The Searunner 25 trimaran has a hinge mechanism on its metal-tube A-frame akas that secures with bolts at both ends.

Commonly, simple hinge systems require that the main hull be positioned rather high on the trailer so the amas clear the trailer wheels beneath. A disadvantage is that the trailer must be submerged more deeply than usual for the boat to float off. Compared to the Telstar system, the Searunner 25 offered some improvement by positioning the hinge point at the top of the cabinside, raising the folded ama slightly.

Complex Aka Hinge Systems

A complex system for folding multihulls, much like a garage door lift linkage, was developed and patented by Ian Farrier for his trailerable trimaran designs. It allows one person to fold or unfold the boat while it’s afloat. Before launching, the mast is stepped and secured with lower stays. Note that folded storage in the water for long periods is not practical because the immersed ama’s topsides will gather marine fouling. In addition, the arrangement of the support linkage arms has a very shallow angle with the aka, causing them to be highly stressed, which adds significant weight and cost.

Unfolding it requires help from friends.

A complex folding system I developed has only four attachment bolts and a wide-angle strut brace. It is very light but requires folding prior to launching. It relies on a simple roller dolly on a beam attached to the trailer to support the ama during folding and unfolding.

Swing-Wing Systems

In-water storage of folding trimarans is generally limited to swing-wing designs, where the hulls all float on their respective waterlines, either folded or unfolded. Many variations have been used in production boats, and among the most successful is the Quorning-designed Dragonfly. It has hinged arms supported by a “waterstay”— a diagonal cable under the arm to counteract cantilever aka loads. The outer end of the arm, on the ama deck, pivots on a single pin. The waterstay becomes slack when the boat is folded, leaving only the hinge to support the ama in the folded configuration. I’ve seen one folded boat that was damaged while moored at the dock in strong harbor waves when the ama climbed onto the dock. Swing-wing designs stored in the water must provide strong vertical support for the ama in the folded condition

A swing-wing aka system on the Borg Quorning–designed Dragonfly 32 is further supported by a waterstay when rigged for sailing. The akas can be adjusted in and out while in the water and for storage at the dock.

The main challenge of the swing-wing system is to get all the pivot axes parallel because they must rotate about 90° without binding. If there is any depth to the structure, this accuracy is critical, as the pins or pivot axles could be quite long, so even a small inaccuracy will make the system difficult to assemble, let alone pivot smoothly.

Folding Multihulls with Flat Swing-Wing Akas

The most basic swing-wing system is the flat aka configuration developed by Jim Brown. He avoided the need for perfect parallel alignment of all hinge axes because the beams are not very thick, and the pivot-pin holes can have additional clearance. For the swing system to operate without binding, spacing of the pivot points must be identical on all the swing arms. The system’s downside is strength, because the aka must support all the heeling loads in a relatively narrow beam. For some boats, a waterstay may need to be added to increase cantilever strength and reduce deflections when sailing.

The Seaclipper 16 flat swing- wing akas are made from common dimensional lumber and pivot using steel bolts.

A logical improvement in strength for swing arms is to add a truss, with triangulated strength that will easily bear all the heeling loads from the ama. Here again, it is essential that pivot axes be in perfect alignment to avoid binding. To my eye, open trusses in sleek yachts are never beautiful, but they offer higher strength for lower weight.

Complex Swing-Wing Systems

If the akas are not flat along their full length, it is more difficult to achieve a smoothly pivoting system. My latest boat, Syzygy (pronounced, sis-a-gee), is a case in point. Flat akas offer little variation in styling—flat is flat. To add underwing clearance and more attractive aesthetics, many designers favor the arched aka. This configuration allows the aka to approach the ama hull from above and connect through the deck for more usable immersion of the ama buoyancy, and to keep the aka above the wavetops.

This system has arched akas with an upward angle (dihedral) as they extend from the main hull and descend with a smooth curve onto the ama deck. The pivot axis must also be inclined, normal to the surface, to allow it to pivot. To make life simple, the vertical centerline of the ama is inclined inboard at the top by the same amount, which aligns all the pivot axes with the ama vertical centerline. If the beam is level fore-and-aft, when the ama is folded inboard, it is positioned rather low, due to the arch. To compensate, the akas must be given a negative angle of attack to make the folded ama arrive in the same position as a simple flat aka system. It’s a good challenge for any boatbuilder to get it right and a good use of a digital level. The angles in Syzygy were 8° dihedral, and a nega tive 5° angle of attack. The aka pivot surfaces must be perfectly parallel on both ends—at the inboard aka pivots and the ama deck pivot tables.

Jan Gougeon designed and built strings, a 40′ swing-wing catamaran with carbon- tubular-truss swing akas built over foam mandrels.

A late iteration of the Telstar 26 became the Telstar 28 with a vertical-axis swing-wing system. This production boat is no longer manufactured but was unique for its faired wing and attempt to hide the folding system from view. It also featured an electric linear drive to fold/unfold the heavy akas.

For transporting folding multihulls on the highway, road trailers must have some specific attributes to properly support the hulls. Most models use transverse cradle supports under the hull at major interior bulkhead positions. It is important to install bow guides on the trailer to get the hull to settle in exactly the right place when retrieved from the water. Rollers beneath the hull are not recommended, as they tend to distort it and potentially cause damage. The amas require enough support so the folding mechanism is not carrying the load when being towed.

For swing-wing boats, there is a significant change in the center of gravity between folded to unfolded configurations. Normally, the amas swing back when folded and swing forward for the sailing position. If the trailer has the proper tongue weight for towing on the hitch with the boat folded, the weight will increase when unfolded. For trailers with telescoping tongues, tongue design must accomodate that weight; otherwise, the extended tongue may bend severely during launching or retrieval.

Homebuilt wooden trailers are popular for these specialized boats, and some designers provide plans for them. Without much metal in them, they will probably float, which sometimes leads to difficulty at launching. Adding some steel channel to the bunks can solve that. However, floating is not an undesirable feature if a trailer floats level but is submerged enough to maneuver the hull into the bunks, and the hull settles into the right place automatically. Floating trailers also never run off the end of the ramp.

Conclusions

There’s truth in the humorous claim that “the new family yacht has to look good behind your SUV.” But while many of the latest small boats are daysailers, folding multihulls have expanded the trailerable and transportable boat size to include those with weekend cruising capability, up to about 32 ‘ . As we’ve seen, those essential folding or retraction mechanisms are not simple and must be carefully designed and engineered, even by the home builder. But for owners of these boats, seasonal storage and slip availability are no longer problems. And the overall reduction in total cost can bring owning a boat within reach for many more people. What’s not to like about that?

About the Author: John Marples has designed, built, and rigged many sail- ing vessels. His portfolio includes doz- ens of wood-epoxy composite sailing and power multihulls to 110′ (33.5m). He operates Marples Marine , a multihull design and engineering firm in Penobscot, Maine

Dieter Loibner | Professional BoatBuilder Magazine

Nomenclature

Multihull designers have developed some useful, specific names for components, mostly derived from the Pacific Islander language.

Aka (ah-kah) refers to the crossbeam structure of any multihull. Designers used to call them “cross-beams,” but writing that on hand-drawn plans took up too much space and time, so this shorter Polynesian name became the standard.

Ama (ah-mah) is the Polynesian name for the outer hull of a trimaran or proa. They were formerly named “floats” or “outer hulls” (never pontoons), but again, ama is shorter.

Vaka (vah-kah) is the Polynesian name for the main (largest) hull of a trimaran or proa. Since it can be confused with the other names and is not very descriptive, most designers have opted for the term main hull.

Waterstay is a diagonal stay, metal or synthetic rope, below the aka, between the main hull near the waterline and aka near its outboard end. This stay counteracts the upward load from ama buoyancy when the ama is immersed.

—John Marples

The Crossbeam (Aka) Structure

T he essential function of any crossbeam (aka) system on a multihull is to structurally connect the hulls in a way that resists all the forces generated when sailing. Heeling forces from lift on the sails must be transferred to the leeward hull by the aka structure. The forces on the akas are complex, composed of cantilever bending due to heeling loads, twisting of the structural platform, and horizontal bending caused by drag from the ama’s forward motion through the water. The heeling force, resisted by the buoyancy of the ama, pushes up, causing cantilever bending loads in the akas similar to the forces on an airplane wing. Torsion is created when the sails’ lift pushes the leeward ama bow down, while the shrouds supporting the mast pull the weather-side ama stern up. Drag from the leeward ama tries to bend the akas toward the stern, and forces from the windward shroud tend to pull the aka forward as well as up. These forces all act together at the attachment points on the hulls. In most cases, torsion is resisted by the tubular hull and cabin structure itself. Heeling is countered by the cantilever strength of the aka beams and is sometimes strengthened by diagonal waterstay cables beneath. Drag forces can be resolved by the fore-and-aft strength of the akas or by adding diagonal cables between the akas. Each folding system must accommodate these loads through all the pivoting components in the structure.

Of key interest in aka design are the loads imposed on the ama hulls by the seaway when sailing to windward. These hulls are subject to significant loads on the outboard sides. The windward ama is pummeled by wavetops, and the leeward ama is pushed sideways due to leeway. Since the aka system is characteristically attached through the ama deck, these forces are trying to rotate the ama keel inboard, toward the main hull, in either case. The same is true for catamarans, concerning the aka loads where they emerge at the hull inboard sides. These loads can be calculated to estimate the strength required for any configuration and should be part of the design’s stress analysis. If centerboards or daggerboards are located in the amas, those rotating forces are significantly increased.

Of further interest in swing-wing designs is the clearance between pins and brackets in vertical pivot mechanisms. When sailing, the forces at the hinge pins can change from positive to negative repetitively, creating noise and wear. The wear will eventually elongate the holes, reduce pin diameter, and become a maintenance problem. Designs like the flat wing can be tightened to eliminate movement, which will eliminate wear. Amas with waterstays tend to put the akas in compression and stop the vertical deflection that would be normally carried through the hinge pins. In that case, the pins would be loaded in only one direction and not be subject to cyclic ± loads. —J.M.

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Sailboat Reviews

Multihull Capsize Risk Check

Waves, squalls, and inattention to trim and helm contribute to instability..

In recent years we’ve seen a surge in interest in multihulls. Thirty years ago, when my experience with cruising multihulls began, nearly all of the skippers served an apprenticeship with beach cats, learning their quirks by the seat of their pants. They hiked out on trapezes and flew head-over-heels past their pitch-pole prone Hobie 16s, until they learned the importance of keeping weight way aft on a reach and bearing off when the lee bow began to porpoise.

By contrast, the new generation of big cat buyers skipped this learning process, learning on monohulls or even choosing a big stable cat as their first boat. Heck, nobody even builds real beach cats anymore, only pumped up racing machines and rotomolded resort toys. So we’re guessing there are a few things these first-time cruising multihull sailors don’t know, even if they have sailed cruising cats before.

It is extremely hard to capsize a modern cruising cat. Either a basic disregard for seamanship or extreme weather is required. But no matter what the salesman tells you (“none of our boats have ever …”), it can happen. A strong gust with sail up or a breaking wave in a survival storm can do it. And when a multihull goes over, they don’t come back.

Trimarans tend to be more performance oriented than catamarans. In part, this is because it’s easier to design a folding trimaran, and as a result Farrier, Corsair, and Dragonfly trimarans had a disproportionate share of the market.

In spite of this and in spite of the fact that many are raced aggressively in windy conditions, capsizes are few, certainly fewer than in equivalent performance catamaran classes. But when they do go over, they do so in different ways.

Trimarans have greater beam than catamarans, making them considerably more resistant to capsize by wind alone, whether gusts or sustained wind. They heel sooner and more than catamaran, giving more warning that they are over powered.

Waves are a different matter. The amas are generally much finer, designed for low resistance when sailing deeply immersed to windward. As a result, trimarans are more susceptible to broach and capsize when broad reaching at high speed or when caught on the beam by a large breaking wave.

In the first case, the boat is sailing fast and overtaking waves. You surf down a nice steep one, into the backside of the next one, the ama buries up to the beam and the boat slows down. The apparent wind increases, the following wave lifts the transom, and the boat slews into a broach. If all sail is instantly eased, the boat will generally come back down, even from scary levels of heel, but not always.

In the second case a large wave breaks under the boat, pulling the leeward ama down and rolling the boat. Catamarans, on the other hand, are more likely to slide sideways when hit by a breaking wave, particularly if the keels are shallow (or raised in the case of daggerboards), because the hulls are too big to be forced under. They simply get dragged to leeward, alerting the crew that it is time to start bearing off the wind.

Another place the numbers leave us short is ama design. In the 70s and 80s, most catamarans were designed with considerable flare in the bow, like other boats of the period. This will keep the bow from burying, right? Nope. When a hull is skinny it can always be driven through a wave, and wide flare causes a rapid increase in drag once submerged, causing the boat to slow and possibly pitchpole.

Hobie Cat sailors know this well. More modern designs either eliminate or minimize this flare, making for more predictable behavior in rough conditions. A classic case is the evolution of Ian Farrier’s designs from bows that flare above the waterline to a wave-piercing shape with little flare, no deck flange, increased forward volume, and reduced rocker (see photos page 18). After more than two decades of designing multihulls, Farrier saw clear advantages of the new bow form. The F-22 is a little faster, but more importantly, it is less prone to broach or pitchpole, allowing it to be driven harder.

Beam and Stability

The stability index goes up with beam. Why isn’t more beam always better? Because as beam increases, a pitchpole off the wind becomes more likely, both under sail and under bare poles. (The optimum length-to-beam ratios is 1.7:1 – 2.2:1 for cats and 1.2:1-1.8:1 for trimarans.) Again, hull shape and buoyancy also play critical roles in averting a pitchpole, so beam alone shouldn’t be regarded as a determining factor.

Drogues and Chutes

While monohull sailors circle the globe without ever needing their drogues and sea anchors, multihulls are more likely to use them. In part, this is because strategies such as heaving to and lying a hull don’t work for multihulls. Moderate beam seas cause an uncomfortable snap-roll, and sailing or laying ahull in a multihull is poor seamanship in beam seas.

Fortunately, drogues work better with multihulls. The boats are lighter, reducing loads. They rise over the waves, like a raft. Dangerous surfing, and the risk of pitchpole and broach that comes with it, is eliminated. There’s no deep keel to trip over to the side and the broad beam increases the lever arm, reducing yawing to a bare minimum.

Speed-limiting drogues are often used by delivery skippers simply to ease the motion and take some work off the autopilot. By keeping her head down, a wind-only capsize becomes extremely unlikely, and rolling stops, making for an easy ride. A properly sized drogue will keep her moving at 4-6 knots, but will not allow surfing, and by extension, pitch poling.

For more information on speed limiting drogues, see “ How Much Drag is a Drogue? ” PS , September 2016.

Capsize Case Studies

Knock wood, we’ve never capsized a cruising multihull (beach cat—plenty of times), but we have pushed them to the edge of the envelope, watched bows bury, and flown multi-ton hulls to see just how the boat liked it and how fast she would go. We’re going to tell you about these experiences and what can be learned from them, so you don’t have to try it.

First, it helps to examine a few examples of some big multihull capsizes.

Techtronics 35 catamaran, John Shuttleworth design

This dramatic pitchpole occurred in a strong breeze some 30 years ago. In order to combine both great speed and reasonable accommodation, the designer incorporated considerable flare just above the waterline, resulting in hulls that were skinny and efficient in most conditions, but wide when driven under water in steep chop.

The boat was sailing fast near Nova Scotia, regularly overtaking waves. The bows plowed into a backside of a particularly steep wave, the submerged drag was huge, and the boat stopped on a dime. At the same time, the apparent wind went from about 15 knots into the high 20s, tripling the force on the sails and rapidly lifting the stern over the bow. Some crew were injured, but they all survived.

PDQ 32 Catamaran

On July 4, 2010, the boat’s new owners had scheduled time to deliver their new-to-them boat up the northern California coast. A strong gale was predicted, but against all advice, they left anyway. The boat turned sideways to the confused seas and a breaking wave on the beam capsized the boat. There were no injuries, and the boat was recovered with only moderate damage a few weeks later. Repaired, she is still sailing.

Another PDQ 32 was capsized in the Virgin Islands when a solo sailor went below to tend to something and sailed out of the lee of the island and into a reinforced trade wind.

Sustaining speed with wider tacking angles will help overcome leeway.

Cruising cats can’t go to windward. That’s the rumor, and there’s a kernel of truth to it. Most lack deep keels or dagger boards and ex-charter cats are tragically under canvassed for lighter wind areas, a nod to near universal lack of multihull experience among charter skippers. Gotta keep them safe. But there are a few tricks that make the worst pig passable and the better cats downright weatherly. Those of you that learned your craft racing Hobies and Prindles know most of this stuff, but for the rest of you cruising cat sailors, there’s some stuff the owner’s manual leaves out.

“Tune” the Mast

Having no backstay means that the forestay cannot be kept tight unless you want to turn your boat into a banana and over stress the cap shrouds. Although the spreaders are swept back, they are designed primarily for side force with just a bit of pull on the forestay. The real forestay tension comes from mainsheet tension.

Why is it so important to keep the forestay stay tight? Leeward sag forces cloth into the luff of the genoa, making it fuller and blunting the entry into the wind. The draft moves aft, the slot is pinched, and aerodynamic drag increases. Even worse, leeway (sideslip) increases, further increasing drag and sliding you away from your destination. Sailing a cruising cat to windward is about fine tuning the lift to drag ratio, not just finding more power.

How do you avoid easing the mainsheet in strong winds? First, ease the traveler instead. To avoid pinching the slot, keep the main outhaul tight to flatten the lower portion of the main. Use a smaller jib or roll up some genoa; overlap closes the slot. Reef if need be; it is better to keep a smaller mainsail tight than to drag a loose mainsail upwind, with the resultant loose forestay and clogged slot. You will see monos with the main twisted off in a blow. Ignore them, they are not cruising cats. It is also physically much easier to play the traveler than the main sheet. Be glad you have a wide one.

Check Sheeting Angles

Very likely you do not have enough keel area to support large headsails. As a result, you don’t want the tight genoa lead angles of a deep keeled monohull. All you’ll do is sail sideways. Too loose, on the other hand, and you can’t point. In general, 7-10 degrees is discussed for monos that want to pinch up to 40 degrees true, but 14-16 degrees makes more sense for cruising cats that will sail at no less than 50 degrees true. Rig up some temporary barber haulers and experiment. Then install a permanent Barber-hauler; see “ Try a Barber Hauler for Better Sail Trim ,” Practical Sailor , September 2019.

The result will be slightly wider tacking angles, perhaps 105 degrees including leeway, but this will be faster for you. You don’t have the same hull speed limit, so let that work for you. Just don’t get tempted off onto a reach; you need to steer with the jib not far from luffing.

Watch the fore/aft lead position as well. You want the jib to twist off to match the main. Typically it should be right on the spreaders, but that depends on the spreaders. If you have aft swept shrouds, you may need to roll up a little genoa, to 110% max.

Use your Tell-Tales

On the jib there can be tell-tale ribbons all over, but on the main the only ones that count are on the leech. Keep all but the top one streaming aft. Telltales on the body of sail are confused by either mast turbulence (windward side) or pasted down by jib flow (leeward side) and won’t tell you much. But if the leach telltales suck around to leeward you are over sheeted.

Keep Your Bottom Clean

It’s not just about speed, it’s also pointing angle. Anything that robs speed also makes you go sideways, since with less flow over the foil there will be less lift. Flow over the foils themselves will be turbulent. Nothing slows you down like a dirty bottom.

Reef Wisdom

Push hard, but reef when you need to. You will have the greatest lift vs. windage ratio when you are driving hard. That said, it’s smart to reef most cruising cats well before they lift a hull to avoid overloading the keels. If you are feathering in the lulls or allowing sails to twist off, it’s time to reef.

Don’t Pinch

Pinching (pointing to high) doesn’t work for cats. Get them moving, let the helm get a little lighter (the result of good flow over the rudder and keel), and then head up until the feeling begins to falter. How do you know when it’s right? Experiment with tacking angles (GPS not compass, because you want to include leeway in your figuring) and speed until the pair feel optimized. With a genoa and full main trimmed in well, inside tracks and modified keels, and relatively smooth water, our test PDQ can tack through 100 degrees with the boat on autopilot. Hand steering can do a little better, though it’s not actually faster to windward. If we reef or use the self-tacking jib, that might open up to 110-115 degrees, depending on wave conditions. Reefing the main works better than rolling up jib.

Boats with daggerboards or centerboards. The comments about keeping a tight forestay and importance of a clean bottom are universal. But the reduction in leeway will allow you to point up a little higher, as high as monohulls if you want to. But if you point as high as you can, you won’t go any faster than similar monohulls, and quite probably slower. As a general rule, tacking through less than 90 degrees, even though possible, is not the best strategy. A slightly wider angle, such as 100 degrees, will give a big jump in boat speed with very little leeway.

Chris White Custom 57

In November 2016, winds had been blowing 25-30 knots in stormy conditions about 400 miles north of the Dominican Republic. The main had two reefs in, and the boat was reaching under control at moderate speed when a microburst hit, causing the boat to capsize on its beam. There were no serious injuries.

Another Chris White 57 capsized on July 31, 2010. It had been blowing 18-20 knots and the main had a single reef. The autopilot steered. The wind jumped to 62 knots in a squall and changed direction so quickly that no autopilot could be expected to correct in time.

Gemini 105mC

In 2018, the 34-foot catamaran was sailing in the Gulf of Mexico under full sail at about 6 knots in a 10-15 knot breeze. Squalls had been reported on the VHF. The crew could see a squall line, and decided to run for cover. Before they could get the sails down, the gust front hit, the wind shifted 180 degrees, and the boat quickly went over.

38-foot Roger Simpson Design

The catamaran Ramtha was hit head-on by the infamous Queen’s Birthday storm in 1994. The mainsail was blown out, and steering was lost. Lacking any control the crew was taken off the boat, and the boat was recovered basically unharmed 2 weeks later. A Catalac catamaran caught in the same storm trailed a drogue and came through unharmed. Of the eight vessels that called for help, two were multihulls. Twenty-one sailors were rescued, three aboard the monohull Quartermaster were lost at sea.

15 meter Marsaudon Ts

Hallucine capsized off Portugal on November 11 of this year. This is a high performance cat, in the same general category as the familiar Gunboat series. It was well reefed and the winds were only 16-20 knots. According to crew, it struck a submerged object, and the sudden deceleration caused the boat to capsize.

Multihulls We’ve Sailed

Clearly seamanship is a factor in all of our the previous examples. The watch needs to be vigilant and active. Keeping up any sail during squally weather can be risky. Even in the generally benign tropics, nature quickly can whip up a fury. But it is also true that design choices can impact risk of capsize. Let’s see what the numbers can tell us, and what requires a deeper look.

Stiletto Catamaran

We’ve experienced a number of capsizes both racing and while driving hard in these popular 23-foot catamarans. The combination of light displacement and full bow sections make pitchpoling unlikely, and the result is very high speed potential when broad reaching. Unfortunately, a narrow beam, light weight, and powerful rig result in a low stability factor. The potential for capsize is real when too much sail is up and apparent wind is directly on the beam. The boat can lift a hull in 12 knots true. This makes for exciting sailing when you bring your A-game, but limits the boat to coastal sailing.

Corsair F-24 MK I trimaran

Small and well canvased, these boats can capsize if driven hard (which they often are), but they are broad beamed, short-masted, and designed for windy sailing areas. F-24s are slower off wind than the Stiletto, in part because of greater weight and reduced sail area, but also because the main hull has more rocker and does not plane as well. They are faster to weather and point considerably higher than a Stiletto (90-degree tacking angle vs. 110 degrees). This is the result of greater beam, a more efficient centerboard design, and slender amas that are easily driven in displacement mode. The boat is quite forgiving if reefed.

Going purely by the numbers, this boat seems nearly identical to the F-24. In practice, they sail quite differently. The Dash uses a dagger board instead of centerboard, which is both more hydrodynamic and faster, but more vulnerable to damage if grounded at high speed.

The rotating mast adds power that is not reflected in the numbers. The bridgedeck clearance is higher above the waterline, reducing water drag from wave strikes. The wave-piercing amas create greater stability up wind and off the wind. The result is a boat that is slightly faster than the original F-24 and can be driven much harder off the wind without fear of pitchpole or broach.

Without proper testing, calculating stability yields only a rough picture.

Evaluating multihull performance based on design numbers is a bit more complicated than it is with ballasted, displacement monohulls, whose speed is generally limited by hull form. [Editor’s note: The formula for Performance Index, PI has been updated from the one that originally appeared in the February 2021 issue of Practical Sailor.

The following definitions of units apply to the adjacent table:

SA = sail area in square feet

D (displacement) = weight in pounds

LWL = length of waterline in feet

HCOE = height of sail center of effort above the waterline in feet

B = beam in feet

BCL = beam at the centerline of the hulls in feet.

Since a multihull pivots around the centerline beam, the overall beam is off the point and is not used in formulas. Calculate by subtracting the individual hull beam from the overall beam.

SD ratio = SA/(D/64)^0.66

This ratio gives a measure of relative speed potential on flat water for monohulls, but it doesn’t really work for multihulls.

Bruce number = (SA)^0.5/(D)^0.333

Basically this is the SD ratio for multihulls, it gives a better fit.

Performance index = (SA/HCOE)^0.5 x (D/1000)^0.166

By including the height of the COE and displacement, this ratio reflects the ability of the boat to use that power to sail fast, but it understates the importance of stability to the cruiser.

Stability factor = 9.8*((0.5*BCL*D)/(SA*HCOE))^0.5

This approximates the wind strength in knots required to lift a hull and includes a 40% gust factor. In the adjacent data sheet, we compare the formula’s predicted stability to observed behavior. Based on our experience on the boats represented, the results are roughly accurate.

Ama buoyancy = expressed as a % of total displacement.

Look for ama buoyancy greater than 150% of displacement, and 200 is better. Some early trimaran designs had less than 100 percent buoyancy and would capsize well before flying the center hull. They exhibited high submerged drag when pressed hard and were prone to capsize in breaking waves.

Modern tris have ama buoyancy between 150 and 200 percent of displacement and can fly the center hull, though even racing boats try to keep the center hull still touching. In addition, as a trimaran heels, the downward pressure of wind on the sail increases, increasing the risk of capsize. The initial heel on a trimaran is more than it is on catamarans, and all of that downward force pushes the ama even deeper in the water. Thus, like monohulls, it usually makes sense to keep heel moderate.

These numbers can only be used to predict the rough characteristics of a boat and must be supplemented by experience.

This is the first real cruising multihull in our lineup. A few have capsized. One was the result of the skipper pushing too hard in very gusty conditions with no one on watch. The other occurred when a crew unfamiliar with the boat ignored local wisdom and set sail into near gale conditions.

Although the speed potential of the PDQ 32 and the F-24 are very similar, and the stability index is not very different, the feel in rough conditions is more stable, the result of much greater weight and fuller hull sections.

Like most cruising cats, the PDQs hulls are relatively full in order to provide accommodation space, and as a result, driving them under is difficult. The increased weight slows the motion and damps the impact of gusts. Yes, you can fly a hull in about 25 knots apparent wind (we proved this during testing on flat water with steady winds), and she’ll go 8-9 knots to weather doing it, but this is not something you should ever do with a cruising cat.

Stability by the Numbers

The “stability factor” in the table above (row 14) is based on flatwater conditions, and ignores two additional factors. Unlike monohulls, the wind will press on the underside of the bridgedeck of a multihull once it passes about 25 degrees of heel, pushing it up and over. This can happen quite suddenly when the boat flies off a wave and the underside is suddenly exposed to wind blowing up the slope of the wave. A breaking wave also adds rotational momentum, pitching the windward hull upwards.

Multihulls by the Numbers

Autopilot is a common thread in many capsizes. The gust “came out of no place…” No it didn’t. A beach cat sailor never trusts gusty winds. The autopilot should be disengaged windspeeds and a constant sheet watch is mandatory when gusts reach 30-40 percent of those required to fly a hull, and even sooner if there are tall clouds in the neighborhood. Reef early if a helm watch is too much trouble.

“But surely the sails will blow first, before the boat can capsize?” That would be an expensive lesson, but more to the point, history tells us that well-built sails won’t blow.

“Surely the rig will fail before I can lift a hull?” Again, that could only be the result of appallingly poor design, since a rig that weak will not last offshore and could not be depended on in a storm. Furthermore, good seamanship requires that you be able to put the full power of the rig to work if beating off a lee shore becomes necessary.

Keeping both hulls in the water is up to you. Fortunately, under bare poles and on relatively flat water even smaller cruising cats can take 70 knots on the beam without lifting … but we don’t set out to test that theory, because once it blows for a while over even 40 knots, the real risk is waves.

Everything critical to safety in a blow we learned on beach cats. Like riding a bike, or—better yet—riding a bike off-road, there are lessons learned the hard way, and those lessons stay learned. If you’ve been launched into a pitchpole a few times, the feeling you get just before things go wrong becomes ingrained.

Perhaps you are of a mature age and believe you monohull skills are more than enough to see you through. If you never sail aggressively or get caught in serious weather, you’re probably right.

However, if there’s a cruising cat in your future, a season spent dialing in a beach cat will be time well spent. Certainly, such experience should be a prerequisite for anyone buying a performance multihull. The statement might be a little pointed, but it just makes sense.

Capsize by Wind Alone

Capsizing by wind alone is uncommon on cruising multihulls. Occasionally a performance boat will go over in squally weather. The crew could easily have reefed down or gone to bare poles, but they clung to the idea that they are a sail boat, and a big cat feels so stable under sail—right up until a hull lifts.

Because a multihull cannot risk a knockdown (since that is a capsize), if a squall line is tall and dark, the smart multihull sailors drops all sail. Yes, you could feather up wind, but if the wind shifts suddenly, as gusts often do, the boat may not turn fast enough. Off the wind, few multihulls that can take a violent microburst and not risk a pitchpole. When a squall threatens, why risk a torn sail for a few moments of fast sailing?

You can’t go by angle of heel alone because of wave action. Cat instability begins with the position of the windward hull. Is it flying off waves?

A trimaran’s telltale is submersion of leeward ama. Is the leeward ama more than 30-40 percent under water? The maximum righting angles is a 12-15 degrees for cats and 25-30 degrees for trimarans, but that is on flat water. Once the weather is up, observation of motion becomes far more important. Is the boat falling into a deep trough, or is at about to launch off a steep wave and fly?

Just as monohulls can surprise a new sailor by rounding up and broaching in a breeze, multihulls have a few odd habits that only present themselves just before things go wrong. Excuse the repetition, but the best way to learn to instinctively recognize these signs is by sailing small multihulls.

Sailing Windward

Because of the great beam, instead of developing weather helm as they begin to fly a hull, multihulls can suddenly develop lee helm, causing the boat to bear away and power up at the worst possible moment. This is because the center of drag moves to the lee hull, while the center of drive remains in the center, causing the boat to bear away.

If the boat is a trimaran, with only a center rudder, this rounding up occurs just as steering goes away. This video of a MOD 70 capsize shows how subtle the early warning signs can be ( www.youtube.com/watch?v=CI2iIY61Lc8 ).

Sailing Downwind

Off the wind, the effect can be the reverse. The lee hull begins to bury, and you decide it is time to bear off, but the submerged lee bow acts like a forward rudder. It moves the center of effort far forward and prevents any turn to leeward. Nearly all trimarans will do this, because the amas are so fine. The solution is to bear away early, before the ama buries—or better yet, to reef.

Conclusions

We’re not trying to scare you off multi-hulls. Far from it. As you can probably tell, I am truly addicted. Modern designs have well-established reputation seaworthiness.

But multihull seaworthiness and seamanship are different from monohulls, and some of those differences are only apparent when you press the boat very hard, harder than will ever experience in normal weather and outside of hard racing. These subtle differences have caught experienced sailors by surprise, especially if their prior experience involved only monohulls or cruising multihulls that were never pressed to the limit.

Although the numbers only tell part of the story, pay attention to a boat’s stability index. You really don’t want an offshore cruising boat that needs to be reefed below 22-25 knots apparent. Faster boats can be enjoyable, but they require earlier reefing and a more active sailing style.

When squalls threaten or the waves get big, take the appropriate actions and take them early, understanding that things happen faster. And don’t forget: knockdowns are not recoverable. It is satisfying to have a boat that has a liferaft-like stability, as long as you understand how to use it.

Technical Editor Drew Frye is the author of “Rigging Modern Anchors.” He blogs at www.blogspot/sail-delmarva.com

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Small Trimaran; Ama design?

Discussion in ' Sailboats ' started by science abuse , May 27, 2010 .

science abuse Junior Member

The central hull does not play any role hydrodynamically when the boat gets going..... Obviously if an ama is the only hull in the water then it is best to fit the rudders and dagger boards to the amas rather than the main hull. Click to expand...

messabout Senior Member

Planeing amas mgiht work on a high speed boat, but the canoe is definitely not in the planeing realm. So if you design the amas with a planeing planform you will be shooting yourself in the foot.

Fanie Fanie

I mean to find out. Click to expand...

That's the nature behind my screen name; I'm not using science practically or professionally, I'm using it for fun... Since it should be used for the greater good, I consider this abuse. Click to expand...

Doug Lord Flight Ready

A little science... As I said in my first post I don't think planing amas would work too well on a "normal" canoe. However, a little science shows that "hull speed"(1.34 X sq.rt. LWL) for a 17' wl canoe is 5.52knts. The planing threshold(2 X sq. rt. LWL) of a 4' "ama"(designed as a planing hull with appropriate shape) is 4 knots...... Food for thought.......

Doug Lord said: ↑ As I said in my first post I don't think planing amas would work too well on a "normal" canoe. However, a little science shows that "hull speed"(1.34 X sq.rt. LWL) for a 17' wl canoe is 5.52knts. The planing threshold(2 X sq. rt. LWL) of a 4' "ama"(designed as a planing hull with appropriate shape) is 4 knots...... Food for thought....... Click to expand...

science abuse said: ↑ thanks for this, these are the sorts of things I'm hoping to learn here. Hull speed calculations aught to be fairly accurate in the case of the "scanoe", since the hull is so basic in its shape. To get more accuracy, I aught to weigh the additional "sailing stuff" and load the boat up, with me in it, to see where the waterline will be. The Amas will will be shorter than the boat itself, would it be best to have them biased toward the front of the boat, toward the stern, or neutral about the center of gravity? I'll be positioning the sail so that it's windload is centered amidships. Click to expand...

He he... The ama's will always keep up with the main hull

DrCraze Junior Member

If you wanna go real fast on that canoe fit it out with some leeboards and a hiking board. Just one thing...you'll get real wet too.

Getting wet may prove an issue, depending on how well my boy can operate a bucket. The canoe isn't a self-bailer, if she leans a bit too far she'll probably swamp entirely... thus the trimaran. Along those lines, though, this came to me this morning: http://img708.imageshack.us/img708/4420/projection.png Leeboards fixed at an angle, so they penetrate a fair amount of water when heeled over, yet contribute very little to the draft. I really lucked out this week, I found a fella on Craigslist with a heap of aluminum extrusions. For $150 I got: 2x 24ft long, 2" od, 1/8" wall (main mast, Aka's) 1x 11ft long, 2" od, .062 wall (boom) 1x 11ft long, 2" od, .062 wall (tiller, misc) The tubing supplier I'd been talking to couldn't get anything over 12ft in length.

DennisRB Senior Member

Nice project. I have thought of doing something similar myself with my kayak. One thing is you are putting your amas on the wrong way! The flat side goes outward. Look at a hobiecat. It is done this way to reduce the need for centerboards. The flat side acts as the board.

Thanks for the tips and the visual. This isnt the first timei've heard it, so I'll deffinately be trying it that way. I've been puzzling over the best way to mount a centerboard, or a small fin on each ama. If I can get around that, it would save some labor and make beaching easier.

I would certainly just try the amas with flat side out first. It works fine on a hobie which has no boards. At the very least it would reduce the size of the board needed.

peterAustralia Senior Member

Solway Dory makes a double outrigger sailing canoe There are 2 models, one is called the Osprey, the other.... something else. Looking at teh photos, the osprey looks as though it has amas about 10ft long. The other model has shorter amas. There is a video of 2 of these small trimarans sailing along. Unladen weight is approx 80kg for the 16ft Osprey double outrigger. I have looked at it, and it seems a pretty good setup. My suggestion would be, just copy 100 percent what Solway Dory did with the Osprey, their outriggers look to have a very nice shape to me. (I have built 3 outriggers for various single outrigger canoes, well 5 if u include some very small outriggers) If u live in the UK u could buy one, an Osprey that is. link is here http://www.solwaydory.co.uk/ from memory I don't think they sell plans unfortunately, but i may be mistaken. n peter evans www.tacking-outrigger.com

ancient kayaker aka Terry Haines

Here's my own experience for what it is worth: Years ago when I knew squat about sailing I put a single flat-bottomed 7' x 8" x 8" ama on a conventional 14' fiberglass canoe with about 35 sq ft of sail, plywood leeboard and rudder. Sailing was not very exciting, as you would expect with such a small sail, and I had added a seat on the gunnel so the ama was well down in the water. One day I found myself gliding along much faster than usual, looked around and realized that the ama was clear of the water. That, to me, is the key to the whole thing. With a pair of amas it is easy to balance the boat so the amas are clear of the water in which case their shape is irrelevant. They are only there to add static stability for fishing and while you are stomping around raising the sail and such, and to provide backup stability when you get caught by a gust. Just make sure they do not have so much drag that the boat slews violently when one touches the water at speed, and they'll be fine just hanging out in the breeze. Long skinny amas are probably best mounted bow up so they enter the water gradually without digging in. Just make sure they are high enough that you can get both of them clear of the waves. From a practical point of view, deep amas would require high-mounted beams or curved beams to get the amas above the water, shallow hulls on a straight beam would be simpler; if I did it again I would try mounting the amas on a single beam, with a short wide vee-bottomed ama hull to minimize torque on the beam. IMHO most people make the amas too large: I know I did; it never got even close to being pushed under the water and could have had its buoyancy reduced by 2 or 3 times. Crew weight does most of the work of balancing the sail's heeling moment. A last-minute thought: there is no reason why the amas have to be identical: why not try out 2 different designs at a time to see which works best?

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Home » Outriggers on canoes and sailboat- Proas, Trimarans, even Catamarans.

Outriggers on canoes and sailboat- Proas, Trimarans, even Catamarans.

Shas Cho on my forum wrote a number of questions to do with our outrigger plan that can be added to a range of different canoes and kayaks –

Volumes, submersible, non submersible.
How do different types handle and so on.

Just as a first example … here are some of our Drop in Outriggers fitted to a paddling canoe in a medium breeze with some strong gusts.

There are certainly ways to go faster. But what general conclusions can we draw from looking at different types?

Very general questions – which are often the most interesting type.

Please excuse me if this has already been discussed (I’m reading the archived articles as fast as I can!) ; I’ve been looking at commercial and home-built outriggers for canoes and am puzzled at the apparent lack of either rhyme or reason regarding the optimum size of the equipment.

There are very many more poor designs of ama than there are excellent ones, but there are a bunch of reasonable compromise ones too.

The amas (dismissing the crab pot traps and the swimming pool noodles) appear to be anything from one quarter the length of the boat to the full boat length.

Great observation. For excellent speed the best solution is for something like the length of the middle hull. But this can be too complicated and expensive and hard to move around. So often a smaller size is chosen. Amas, single or double, add two things to the boat and have to be analysed from both of those directions. 1/ a slender low wetted surface hull compared to the main hull 2/ lots of stability This doesn’t really differ from normal multihull design – all the standard texts will fill in details.

“Submersible Amas”

So if you push the ama down in the water to deck level – it will have a maximum displacement. My drop in outriggers have a displacement around (I can’t really remember) – but maybe 140 t0 200 lbs. This is my version Lets do the Maths. Main hull – 45lbs amas 2 x 10 lbs = 20lbs crossbeams 2 x 10lbs = 20lbs Rudder, spars and stuff – 30lbs maybe Person – use a 170lb average. That all comes to 300lbs. So what is going to happen when the boat catches the wind and starts heeling?

The weight is gradually transferred to the leeward ama. It will disappear under the water when the main hull is half lifted out of the water. This will slow the boat down but also it provides LOTS of warning that the boat is loading up and the rudder cannot come out of the water as the main hull cannot lift.

This is termed a submersible ama design. It doesn’t use all the available power from the sails and it tells you to ease the sheet or start climbing out to windward yourself

“Non-submersible Amas”

You can make the ama longer and more buoyant than the overall weight. This is another boat that was at the SAIL OKLAHOMA EVENT last weekend. So nice to have fresh photos!

The event is on every year and had (appropriately) multihull designer Richard Woods, Small boat adventurer Howard Rice, Skin on frame designer Dave Gentry, polymath designer/instructor David Nicholls and Michael Storer (er, me)

Ok … cutting to the chase. These hulls are double the length also wider and deeper so you would have to guess that the volume was around 400 to 500lbs per ama. If the wind loads up the sail there might be enough volume in the hull to allow the main hull to lift out of the water

But at that point the sails will have to be developing twice the power of my outrigger. As the main hull lifts there will be an increase of speed as wetted surface is reduced, then a progressive loss of control as the rudder is pulled up out of the water.

The same smart helmsperson that sailed my hypothetical submersible hull boat would be waiting for any slushy feeling in the rudder and ease the sheet whenever he or she felt it. Some ocean going trimarans have rudders in all three hulls so they can keep pushing hard with the main rudder out of the water. Yet another way is sometimes seen on Proas and some indigenous boats. Dave Gentry’s super elegant outrigger at the same weekend has this setup.

This gives resting and low speed stability to a very narrow hull. It can be used for sailing if the crew transfer their weight to windward (a windward ama, platform, leaning plank or this can be to windward as a Pacific type Proa)

This has little ability to support much of the boat weight at all but is there to slow the roll of the boat as the load comes on. A sharp crew notices this and adjusts sails, their position or other factors to either provide the stability needed or reduce the heeling force.

A nice case study – amas for foiling multis

An interesting variant of this idea is the record sailing boat Hydroptere.

The foils out on the amas provide all the righting moment in the photo, but to get some lift from them the boat needs to be moving.

But it can’t get moving without any stability to it has small buoyant amas – not enough to get the boat to much speed but enough to start the hydrofoils developing the full sailing lift.

The forces are pretty much similar to a non submersible ama in practice as it is difficult to lose lift – if you are moving fast the foils can support. But if the boat goes slow in lots of wind it will overpower those little amas easily. Like the very tiny ama of the skin on frame boat.

I have not seen any reference to the length of the floats produced by your plans (though your pdf file “outriggers drop in Eureka version b” shows the ama as about one half the boat length) … nor have I seen the function of ama length discussed anywhere. Is there no formula for the relative lengths of boat to amas? And aside from the effect on bouyancy, does the width and depth of the amas affect sailing performance? What about the effective beam of the outriggers relative to the beam or length or displacement of the ‘host’ boat? And we haven’t even touched on the fore-and-aft placement of the amas and, separate but related, the akas.

Ama volume, volume distribution, aesthetics – no rules but common sense bounds

Relationship between main hull volume and centres of ama volume is a big area. if the ama volume is too far aft the bow of the main hull will be forced deeper into the water. Not good for steering – also the trim pushes the bow of the ama under.

Crazy far forward and the stern of the main hull will drag as the boat heels. So something similar in volume position of the main hull. But remember that heeling forces are forward and to leeward too so maybe it makes sense to have the volume of the amas slightly forward of the volume of the main hull. Ok – back to my drop in outriggers .. my starting criteria were that…

The hulls had to be shorter than an 8ft piece of ply. Then, after that…
Second criterion was to try and get two decent volume amas (as we have seen volume and beam are what create power) out of a sheet of ply. It wasn’t, but some other designer’s amas have this feature.
Then I had to choose how much volume. Slenderness ratio of beam to length is much less important than it was once mooted to be. You can have very skinny hulls like a Crowther Twiggy or a very wide main hull like a Farrier – there is much more choice here than people realise – within reason of course.
So reviewing the simplified cross section there was an aesthetic choice too – that it had to match a reasonable number of potential main hulls.

Decision Points – working up through the grades

I did see a brief mention of fine vs blunt ama entry when someone considered scaling down his Quick Canoe plans to one-third for use as amas, but I don’t recall any conclusions being drawn, other than a mention of lateral resistance when the ama is submerged (No doubt one must consider the intended purpose of the outriggers, whether they are part of the sailing equipment or merely ‘training wheels’).

Yes … from the training wheels of minimal volume amas To the moderate power of the submersible ama – remember that the volume of the ama is about 150lbs – the weight of a person and that person is 5ft from the middle of the boat pushing upwards hard! Much more power than a single person trapezing on a performance dinghy To the full power of non submersible amas.

All of it is a game to balance performance in terms of increased stability pushing towards maximum volume and maximum boat width. as one factor against

required stability for intended performance.
acceptable amount of bulk
building materials cost
encumberance for the main hull
trailer/roofrack weight
boatshed storage or building space.

Beam and volume add to performance, all the others tend to reduce it. More you can check out design books – this is a link on Amazon .

I remember the Chris White book is pretty reasonable. The Norwood book which I have in storage is rather nice too.

Any back issues of Multihull magazine have articles from reviews on boats but also a good ongoing discussion about design criteria and practicality that goes back four decades. You can also see a whole bunch of articles on the net by looking for specific things. Maybe readers can put some notes to articles that are more specific in the comments below.

2 thoughts on “Outriggers on canoes and sailboat- Proas, Trimarans, even Catamarans.”

Do slanted Amas give decreased leeway instead of drop keels.They may reduce tacking performance?ie stability in a straight line is very good with veed Amas. On larger cats or Tris, slanted amas are considered a safety factor as they allow the vessel to slide sideways when beam on to large waves, they reduce RI . resistance to Inertia. May be not as applicable in smaller vessels.

Hi Anthony,

One good comparison point for leeway resistance comparison for canted hulls vs centreboards (drop keels) is the Hobie 16. The boat has deep vee hulls that are well immersed.

But through the years the Hobie sailors have raked (leaned) the rig back to transfer as much load as possible to the deep rudder foils.

This indicates the experience of multihull sailors (actually all sailors, no matter whether mono or multi in general that a deep foil centreboard (drop keel) will give much better resistance to leeway than an immersed hull or shoal keel.

In general foil depth trumps hull area in just about every case.

As far as resistance to flipping in a beam sea a round hull with daggerboard retracted will sideslip more than a deeper vee hull with no centreboard. It is purely about the immersed area which a vee hull has more of. Hull slanting might make some difference but much less than the effect of the pure lateral area presented by the immersed leeward hull.

Best Regards Michael Storer

A Comprehensive Guide & Introduction to Trimaran Yachts

When it comes to sailing boats, there are several options to choose from, including monohull boats, cruising catamarans, and multihull boats such as catamarans and trimarans. While catamarans are a popular choice due to their spaciousness and stability, trimarans offer some unique advantages. With three hulls instead of two, trimarans like the Corsair 880 and Rapido Trimaran have a wider beam, which can help increase speed and stability in high winds.

Additionally, trimaran sailing techniques can differ from those used on catamarans due to the placement of the centerboard and the differences in the hull shape. In fact, trimarans can be faster than catamarans in certain conditions, as the wind speed doubles between the sails, creating more lift. Ultimately, the choice between a catamaran and a trimaran will depend on the sailor’s preferences and intended use.

When it comes to choosing a sailboat, there are many factors to consider, including boat speed, comfort, and cost. Trimarans have become increasingly popular due to their speed and stability, and the Dragonfly 32 Trimaran and Strike 18 Trimaran are two great examples. The Neel Trimaran line, including the Neel 43 and Neel 51, offer even more comfort with their aft cabins and spacious designs.

However, the cost of these larger trimarans can be significant, and buyers may want to consider a basic boat incl from Quorning Boats or even a used Neel Trimaran for sale. Ultimately, the right choice depends on the buyer’s priorities, but all trimarans offer the advantage of being able to sail faster than the apparent wind due to their multiple hulls. Whether testing a new model or enjoying an established design, a trimaran is a fantastic choice for sailors looking to maximize their speed and stability on the water.

Give it up for the multihull sector of the boating industry.

Any industry insider will easily see how fast growing that sector currently is. In fact, it is the fastest growing sector if the number of new cruising boat manufacture and sales is anything to go by.

Far from the prejudice of the earliest days, multihulls now seem to be the in-thing and you don’t have to look far away before you’ll see the next catamaran, trimaran yacht for sale.

Sales of catamarans are booming with each passing day. Trimarans, on the other hand, are like a niche within another niche.

Most trimarans are high performing vessels. These sailing yachts are usually designed for purposes ranging from racing to recreational use. This article will provide readers with a brief but comprehensive introduction into the world of trimarans.

So, what then is a trimaran boat in the first place?

For anyone looking for an exciting and versatile sailboat, the aluminum trimaran is a great option. One of the most popular types of trimaran boats is the Piver trimaran, which is known for its stability and maneuverability on the water. The Piver trimaran features three hulls, with the middle hull being the largest and most stable.

This design allows for smooth sailing and comfortable overnight cruising, making it a great choice for those looking to spend a weekend or longer on the water. The aluminum construction also ensures durability and low maintenance, making it a cost-effective choice for sailboat enthusiasts. Whether you’re a seasoned sailor or a beginner, the Piver trimaran offers a unique and exciting sailing experience that is sure to impress.

What is a Trimaran Yacht?

According to Wikipedia, a trimaran is also known a double outrigger. Trimaran yachts are multihull yachts comprised of the main hull and two other smaller outrigger hulls (which may also be called floats).

These three parts are the basic components of every trimaran sailboat. Each of the outrigger hulls is connected to the main hull with lateral beams.

While most trimaran yachts are usually sailing yachts designed for recreational and racing purposes, some others are still designed to be used as ferries and warships.

(NOTE: Have you been looking for a Trimaran Yacht? Check out this 2006 Racing Trimaran Tantrum Too , a fun & fast competitive seaworthy racer & offshore cruiser. Winner of the ‘2018 Bintam Trophy’ and holder of ‘Around Bintam Record’. Learn more here! )

The name trimaran was coined from ‘tri’ and ‘maran’

The ‘trimaran’ name has its origin in two words. It was formed from ‘tri’ and ‘maran’. While ‘tri’ means three, the ‘maran’ part means wood or tree.

Trimarans have a kind of interesting design and architecture that makes them exceptionally fast and powerful. It is thus the preferred vessel for most lovers of speed and water adventure.

Read more: What is the best hull material for a houseboat ?

Trimarans have been around since 2000 BC

Although today’s popular sailing trimarans did not get mainstream acceptance until the 60’s and 70’s, these vessels have been in use and existence long before global acceptance, dating back to about 2000 BC.

In fact, the earliest trimarans are still very much in use today.

The development started with the Austronesia people.

Today, traditional fishermen of the maritime region of Southeast Asia, Micronesia, Madagascar, as well as, the Polynesia regions still use trimaran sailboat similar to the first trimarans.

Trimarans are composed of three main parts

We’ve mentioned this in our description of what a trimaran yacht is.

Every trimaran would have a main or center hull and then two other small outrigger hulls attached to the center hull with lateral beams.

These three components form the basic design for all trimarans and modern trimaran sailboat designs always take this trimaran base.

The center hull is the main hull and is also known as vaka. The two stabilizers on the sides are known as the outrigger hulls, or ama(s).

The main hull offers the most buoyancy

The amas are mainly there to offer more stability to the vessel. Most of a trimaran yacht’s buoyancy is provided by the center or main hull.

This arrangement no doubt offers more flexibility to the design.

Because there is usually a wide separation between the hulls, the trimaran yacht requires only a little buoyancy to keep the yacht stable. This reason is why you’d rarely find trimarans having their outrigger hulls or amas in the water.

Trimarans offer better stability and a super stellar performance

Trimarans, when compared to catamarans which are composed of only two hulls, are more stable.

They offer a better stability and a stellar performance which have even been improved with new technologies infused into the design of these boats.

Trimarans are lightweight and are the preferred choice for boat aficionados and connoisseurs.

The stellar performance advantage which is made possible by the design makes trimarans some of the fastest boats when it comes to yacht racing.

They are perfect for sports sailing and are often winners in water races and competitions.

Different types of trimarans

There are basically just two types of trimarans. These are the regular trimaran and the open trimaran.

Instead of plating, the open trimaran rather has a trampoline in between the hulls.

Trimaran yacht advantages over monohull yachts

When compared to single-hulled yachts, trimarans no doubt have some advantages. Some feature differences also exist.

A trimaran, when compared with a monohull of the same length would have a shallower draft. It would also have a wider beam and a less wetted area.

The trimaran though will be able to fly more sail area compared to the single-hulled yacht.

Because of the wider beam, the trimaran has a better straight-line performance when compared with a monohull.

The trimaran can sail in shallower waters and can still perform well while maintaining its stability in the face of stronger winds.

Trimarans are almost unsinkable

Trimarans have a kind of design that is considered almost unsinkable.

This is because even when it is filled with water, the buoyancy of a single outrigger hull or ama can easily keep the entire yacht afloat.

The ama offers greater resistance to the boats and unlike monohulls, trimarans rarely sink.

Because of the high level of safety and stability, sailors with restricted mobility often prefer some special types of trimarans.

Trimarans are like the bridge between catamarans and monohulls

Catamarans, as well as monohulls, have their areas of excellence. Nevertheless, trimaran yachts have certain capabilities which have made them seem like the bridge between monohulls and catamarans at which there is little or no compromise.

Trimaran sailboats are excellent for high speed.

Trimarans generally have moderate weight carrying capability, moderate space below the main deck, and a larger deck area.

The cross deck design between the amas and the main deck is another advantage.

Catamarans usually have large empty gaps in their cross deck bridges.This complexity then necessitates the need for stronger structures, implying more weight, and higher costs. A trimaran yacht would usually have a much smaller cross deck.

There will be a shorter gap between the hulls and the empty space would not extend through the entire yacht’s length.Because of these, less structural weight is required and heavier loads can also be carried on the cross deck.

While catamaran and trimaran yachts are usually built for different purposes, the most obvious difference out of racing is usually the center hull.The center hull of a trimaran yacht will be much larger than the catamaran hulls.

Trimarans are easy to trail, they have a kind of superior handling and are easier to tack, compared to catamarans.

They are easy to manhandle ashore due to their lightweight design.

The lightweight design also makes it easier to use small and inexpensive engines when motoring. With a trimaran yacht, you’d be getting a stellar performance that will get you higher speeds.

If you want more excitement and thrill, perhaps a trimaran yacht such as the racing trimaran Tantrum Too will be very much suitable for you. Tantrum Too is a 12.7m trimaran yacht designed as a concept yacht for a fast ocean cruiser and cruising trimaran.

If you’re in the market for a good buy, the trimaran yacht for sale is currently available on auction. You can register here so you can tender your bid for this cruising trimaran.

The purpose of this article is not to pick the better yacht between a trimaran yacht and a catamaran, or a trimaran and a single-hulled boat.

Each of these different watercraft has different areas where they excel. Your choice will most likely have to do with the purpose for which you’re buying.

If you’ve discussed with your sailing crew and have decided to choose a trimaran as your next yacht, all well and good, you’re in for a lot of thrills and fun.

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Your Ultimate Guide to Yacht Financing in Singapore

More Trimaran Folding Systems — Part 3

Hinge and latch system (gull-wing).

This system works well on small trimarans and is employed on the Cross 18, Discovery 20/21 and the W17 .

The Discovery 20 has large vertical reinforced plywood palms, the full height of its very deep beams, held in place with large bolts. Very strong, because the beams are large and deep. But as initially designed, the amas rose high in the air when folded, and as this created considerable windage on the highway, most Discovery owner/builders have since dispensed with the original hinging and they now remove the amas completely at the bolted joint. This permits them to be stowed much lower on a custom trailer and while this solves the windage issue, the craft should now more correctly be designated as a ‘ demountable trimaran’ , rather than folding (see photo).

Added 2022 . Due to greater ama weight on larger boats and the resulting difficulty to then lift them ‘up and over’, this system was initially limited to trimarans of 18ft or less, However, by building amas lighter (foam cored) and using a portable lifting derrick designed in 2022. the system can now be used for tris up to 20ft.

So, as long as 1) the ama lifting height is within practical range; 2) one has a wide enough place to open the amas before launching and 3), you either have a helper with you, or the above noted lifting derrick to lift the ama when alone, then this Gull Wing system will be tough to beat for boats under 16ft beam. It’s lightweight, inexpensive, strong, rigid, noiseless, fast to deploy, compact, good looking and creates no spray . What is not to like about that ?

New 2022: For boats under 20ft, the basic W17 system can also be used as a demountable if the hinges and latches are reversed * . This would allow the amas to drop-down about 30 deg. to a low support (or down 45 deg to the ground), on each side. Once both the upper and lower pins are pulled, the ama (with its fixed outer akas) can be brought in close to the main hull to sit on supports built out (or telescoping out) from the trailer, yet still not exceeding the original trailing width (7'-3 or 2.21m for the W17). With this arrangement, the mast could be left up and even used for lifting the amas up again to allow the latch bolts (now on top) to be slipped in place. As these hinge and latch bolts only work in shear. they do not need tools to hard-tighten them as just hand-tightened nyloc nuts serve very well. This adds yet another option to the use of this design that does not change the strength or sailing appearance of the boat in any way.

* 2023 NOTE to existing W17 owner s. As the fore and aft beams have a slightly different depth, this reversal would theoretically require that the hinge alignment now be made on the UNDERSIDE of the beams instead of the upper side, so a Gull-Wing W17 cannot be fully converted to this 'demountable system only' without that change first being made. However , if prior to lowering, the lower pins were temporarily replaced with smaller ones of just 1/4" dia, this would create enough slack to offset the underbeam misalignment, allowing the ama to drop down to at least the height of the trailer bed, when all pins would be removed. This may prove to be a useful way to remove an ama for maintenance, with the main hull still on the trailer. (See more in Part 4 of these articles on Folding Systems)

Telescoping Beams

A few designs use telescoping beams, such as the French Astus, the L7 by Mike Leneman (no longer being offered). and some boats designed by Kurt Hughes and a few small kayak-based craft like the Windriders etc.

While theoretically quick and neat to use, common characteristics and issues are, .............

1) that unless the beams are very short, the two sides cannot be symmetric and identical. If one telescoping set must be behind the other, the beams are not in line Port and Starboard. This adds complexity to the building of both the main hull and amas, and arguably, creates a less attractive boat.

3) that many (and to different degrees) historically have a tendency to jamb in operation, as keeping them perfectly in line and free of sand, is a constant issue. Giving the tubes (or boxes) added clearance is sometimes the solution used, but then there can be constant movement while sailing, which, while generally acceptable technically, can be very annoying and disconcerting. Teflon sleeves can help.

4) that the overall expanded beam is geometrically limited by the arrangement, as each aka tube length cannot exceed the permissible road width without a complete dismantling.

For these reasons, this system is generally limited to relatively inexpensive small trimarans under 7m.

Finally, a WRAP-UP re cost, complexity and value (2022)

At the lower cost end, it’s hard to beat having no folding system at all, but with any trimaran over 8ft sailing beam, it has to be at least demountable. But there are at least two systems that can cost no more than a demountable one. These are the simple flat plate swing arm solution devised by Jim Brown and the up-and-over Gull Wing that I developed for the W17. Both can be set up to eliminate all annoying noise, with the raised height and greater physical depth of the Gull Wing making it more suitable for sea conditions requiring both strength and wave clearance. The swing arm is physically easier to fold by one person, whereas the Gull Wing takes less beam on a trailer (or in a garage) and if there are not 2 persons handy (when it’s an easy 2 minute task), a derrick can be made to fold each ama with only one person. See here.

Other systems like the telescopic one could be 10-15% of the cost and have potential friction issues and offer less attractive appearance, while a more developed swing arm with folding nets etc, could be in the 10-20% range. The 'king of cost' has to be the original Farrier system that could be 20-25%. This is also so complex to set up to work correctly, that Ian finally sold it as modules, pre-assembled, that could be built into the hulls, but even 20 years ago, this was a $6000 kit. The other option is to work from plans and build it piece by piece. Ray Kendrick offers this for the Scarab, but it’s also expensive and time consuming to build and set up, and there is no guarantee it will work as smoothly as a factory Farrier. I once saw a photo of the nearly 100 parts for a Scarab folding system that is similar, and despite being assembled by a fine engineer who works on helicopters, it still did not work 100%.

The Farrier system is quite a masterpiece and when properly built and installed works great … but the downside is its cost, the amas fouling in the water and the spray these braced systems create as they can go fairly low to the waterlevel. Neither of the lowest cost systems have these issues, but not all have the strength to match the Farrier system for say ocean sailing plus the easy of in-water folding. Clearly, you need to match your boats operational needs with the most suitable system. I'd personally recommend adding fairings around the Farrier brackets to drive the wave tops down.

POSTSCRIPT (2019) for the Farrier System.

Having a good friend with a folding Farrier, I was encouraged to look at whether anything could be done to solve the main negative issue of the otherwise brilliant design … the issue of the side-turning ama when folded. This not only removes the relatively flat deck of the ama that is so useful and safer for docking and boarding on the Swing-Arm boats, but there's also the issue of side fouling when left afloat for long periods while folded – as in a marina (see Header photo above).

A review of the geometry shows that IF the ama could be left free to pivot while the arm is folded, it would take up a fairly level position and could readily be held there with a simple retaining line. (A possible solution is now entry #126 in an Ideas & Inventions Record Book I started back in 1952 ... with this one being 'just an idea'. Even a documented 'idea' needs to have a prototype built and proven to become an 'invention' ;)

To achieve this, several modifications would be required. While much easier to build-in during initial construction, it could be possible to retrofit an existing Farrier with some effort and ama surgery.

Here is what I am suggesting, though test experience could expect to bring about a few tweaks.

The installation of two heavily reinforced bulkheads or webs under the deck, both forward and after of each hole and added longitudinal reinforcements to compensate for the hole
The attachment of a solid, strong foot-shaped part at the outer end of each aka .. solidly made of aluminum, fiberglass or carbon fiber and well integrated into the aka beam end. This foot would fit snuggly between the two new bulkheads and be drilled to take strong bolts through the heel and toe of the end piece. (see photo)
The heel would have a semi-permanently installed pivot bolt, while the toe would take a locking bolt that would only be installed after the aka was lowered to the horizontal sailing position
Naturally, hinged covers would cover the deck holes once securely bolted down, but if the area was created as a watertight well (like an anchor locker) with a drainage hole to the inboard side, that deck cover would not need to be 100% watertight.
Detailed design would need to take care of bolt access and also of all loads typically expected on the ama.

It could be expected that initially, it would take about 20 mins. to attach each ama for sailing, but that this time could be cut in half with practice and planning, making the whole operation one of about 10 mins. for 2 persons. Probably worth it for the convenience of a flat deck in most cases and if fouling is not an issue, one may decide to turn only one side, purely for easier boarding. Just some thoughts to chew on ;) m/

Click here for Part 1 of this article - comparing basic types

Click here for Part 2 of this article - comparing with demountables

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Payware Airports and Scenery Reviews

Scenery Review : Moscow City XP by Drzewiecki Design

drzewiecki design

By Stephen September 24, 2016 in Payware Airports and Scenery Reviews

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7 months later...

Thank you for this very thorough (as always) review. I just bought it (it's on sale) and have only one disappointment so far: Red Square has no ILS or any landing aids at all for that matter. And what a nightmare of an approach! Also I was hoping the package would include an add-on that gives my c172 a big cup holder for my Stoli. Otherwise the scenery is gorgeous. What a country!

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Drzewiecki Design's Moscow City for X-Plane released

Drzewiecki Design just released their latest scenery: Moscow City for X-Plane. You can now fly in the capital of the biggest country in the world.

Beautiful buildings that look like they were taken from fairy tales, a huge city, over half a dozen airports and Vladimir Putin. What else do you need?

Extremely detailed model of Moscow metropolitan area in Russia
Almost 2000 custom-made buildings and other objects, all high quality, FPS-friendly and with night textures
Whole Moscow center done in 3D as well as all other important landmarks - museums, palaces, skyscrapers, towers, bridges, railway stations, Zara stores...
Trains, ships, 3D people, cars, airport vehicles, static aircraft - anything you can imagine
About 4000 sq.km of photoreal 0,5-1m/pix terrain with autogen
Sceneries of all surrounding airports including UUWW Vnukovo, UUDD Domodedovo, UUBW Zhukovski, UUMO Ostafyevo, UUBM Myachkovo and UUMB Kubinka, with all airport buildings, detailed layouts, people, airport vehicles and more
Very detailed Kremlin model with newly constructed heliport

Drzewiecki Design Moscow City for X-Plane

Please note: the link above was either provided by the author of the product, the store that sent us the review copy or a personal choice of the article writer. It does not, in any way, indicate an endorsement of HeliSimmer.com to the website hereby linked.

More about this scenery

Characteristics.

About Sérgio Costa

DreamFoil Creations shows blade slap research for their X-Plane helicopters

Review: Society Islands XP - Tahiti & Windward Islands for X-Plane

Maps2XPlane released Society Islands XP - Tahiti & Windward Islands for X-Plane 11/12

New screenshots of the X-Trident AW109 for X-Plane 11/12

WAIT! Before you leave...

IMAGES

Sailing trimaran and catamaran design : Owen Clarke Design
First look: Rapido 50 folding trimaran
TR36 Performance Trimaran
Ama to beam connection on trimaran
TR36 Performance Trimaran
AM27 Trimaran Folding Ama Trailerable 2007 Boats for Sale & Yachts

COMMENTS

Longer Amas and Increased Beam
Most hull designs to be built of plywood, from the 50's right up to the end of the century, attempted to replicate the round bilge hulls of carvel, moulded ply or fiberglass construction, by using multiple chines. The Brown, Crowther and Cross boats were early examples of that while the Scarab's are more recent ones.
Small Trimaran; Ama design?
#1 Joined: Oct 2007 Posts: 4,604 Likes: 177, Points: 63, Legacy Rep: 2484 Location: Colonial "Sick Africa" Fanie Fanie Hi Science, The same as a deep ama, a flat surface one will also have a lot of surface cantact. The right ratio between depth and width should give optimal.
Longer Amas and Increased Beam
Answer: This is perhaps the most frequent question I've received over the last 10 years of running this Q&A service. Here is some personal advice on the subject. First, the owner must accept that the performance of the final boat will not match that of a custom-designed trimaran.
Folding Multihulls
A Farrier 33R trimaran with amas folded and secured on a road-legal trailer exem- plifies the advantages of adjustable- beam multihulls, which are easy to trans- port and store out of the water. "I'll be surprised if you can find space in the harbor for that thing," I heard him say as my new 37 ' (11.3m) trimaran was about to be launched.
Longer Amas and Increased Beam
So when designing the ama structure, you'd need to imagine two heavy individuals standing on each and every sqft of the ama surface to get a realistic image of what the ama structure must resist. Quite a number of trimarans with low buoyancy amas have had one pressed deep under, only to implode and disintegrate.
Trimaran amas using extruded polystyrene with epoxy
#1 Joined: Nov 2016 Posts: 414 Likes: 101, Points: 43, Legacy Rep: 10 Location: Sweden revintage Senior Member Hey Thomas, don't overdo it. Buy an old windsurfing short board and split it in half, should not cost more than 50 bucks. Epoxy the cut areas. Then go for 2" alu tubing as beam, max 8 ft long.
Multihull Capsize Risk Check
The amas are generally much finer, designed for low resistance when sailing deeply immersed to windward. As a result, trimarans are more susceptible to broach and capsize when broad reaching at high speed or when caught on the beam by a large breaking wave. In the first case, the boat is sailing fast and overtaking waves.
Building Narrow Amas
Building Narrow Amas Building a Narrow AMA has its Unique Problems Being as there are now at least three new designs out there using the simple 'flat bottom, deep vee' shape that I recommend on my website for 'ease-of-build but efficiency through the water', I think it's worth discussing some of the construction issues for all concerned.
Erin
The prototype Antrim 30+ trimaran "Erin" was built for Dr. Dan Buhler and launched on December 9, 1993. Easy handling and high performance sailing are features of this high quality and ocean rugged racer/cruiser. High buoyancy and dynamic lift from the unique ama design provide an unusual level of safety and control.
Trimaran Ama Foils
What about putting foils in the amas? QUESTION: I have noticed a few small trimarans with daggerboards in each ama. What are the pros and cons of this arrangement compared to one in the center hull? ANSWER: It really depends on the type of trimaran we're considering.
Trimaran Turning Radius: Ama Position
Trimaran Turning Radius: Ama Position Discussion in ' Multihulls ' started by Doug Lord, Jun 27, 2012 . Page 1 of 5 1 2 3 4 5 Next > Joined: May 2009 Posts: 16,679 Likes: 349, Points: 93, Legacy Rep: 1362 Location: Cocoa, Florida Doug Lord Flight Ready
Small Trimaran Design
welcomed here Copyright Use / Legal Disclaimers Small Trimarans Report Back in 2010, sailor/naval architect Mike Waters published a 22-page report covering 20 small trimarans. It includes charts, graphs, photos, and critical objective reporting on many of them. Read more… Review of nine Small Trimarans
Small Trimaran; Ama design?
Posts: 3,358 Likes: 497, Points: 83, Legacy Rep: 1279 Location: Lakeland Fl USA messabout Senior Member Planeing amas mgiht work on a high speed boat, but the canoe is definitely not in the planeing realm. So if you design the amas with a planeing planform you will be shooting yourself in the foot. messabout, May 28, 2010 #17
Outriggers on canoes and sailboat- Proas, Trimarans, even Catamarans
Ama volume, volume distribution, aesthetics - no rules but common sense bounds. Relationship between main hull volume and centres of ama volume is a big area. if the ama volume is too far aft the bow of the main hull will be forced deeper into the water. Not good for steering - also the trim pushes the bow of the ama under.
A Comprehensive Guide & Introduction to Trimaran Yachts
This design allows for smooth sailing and comfortable overnight cruising, making it a great choice for those looking to spend a weekend or longer on the water. The aluminum construction also ensures durability and low maintenance, making it a cost-effective choice for sailboat enthusiasts.
LEEN 56 Power Trimaran Boat Review: A Bluewater Cruiser
NEEL sailing trimarans changed that up a few years ago by introducing a significant amount of living space across a larger main hull and wider amas capable of accommodating full-sized cabins. The powertri has adopted this design and it offers an astounding amount of room. Above: The salon onboard the 2022 LEEM 56 TRIMARAN.
Trimaran Folding Systems
More Trimaran Folding Systems — Part 3 Hinge and Latch system (Gull-Wing) This system works well on small trimarans and is employed on the Cross 18, Discovery 20/21 and the W17.. On the older Cross 18, this hinge system is created with vertical metal plates bolted to the sides of relatively small beams, and mating plates are sandwiched together with a pivot bolt.
Trimaran
A trimaran (or double-outrigger) is a multihull boat that comprises a main hull and two smaller outrigger hulls (or "floats") which are attached to the main hull with lateral beams. Most modern trimarans are sailing yachts designed for recreation or racing; others are ferries or warships.
Trimaran Plans
Click for complete details TRI-STAR 26 MT Study Plans $40.00 Add To Cart TRI-STAR 26 MT Plans $550.00 Add To Cart TRI-STAR 27-9 Click for complete details TRI-STAR 27 Study Plans $40.00 Add To Cart TRI-STAR 27 Plans $600.00 Add To Cart TRI-STAR 31 Click for complete details TRI-STAR 31 Study Plans $40.00 Add To Cart
Drzewiecki Design
MOSCOW CITY X Moscow City X is a very detailed model of the Moscow metropolitan area in Russia, together with lite sceneries of 7 airports, many heliports and thousands of buildings. It took almost a year to complete and it includes countless fantastic features. The FPS/VAS-friendly design and advanced optimization techniques make the product's performance as satisfactory as possible.
Scenery Review : Moscow City XP by Drzewiecki Design
Now Drzewiecki Design has released "Moscow City" a scenery package to compliment their UUEE Sheremetyevo, and boy how much a difference this release has done to this totally boring area. To get the full first impression effect I flew JARDesign's A320neo from EFHK Vatnaa, Helsinki to UUEE Sheremetyevo which is flight Finnair AY153 which is a ...
Drzewiecki Design's Moscow City for X-Plane released
Drzewiecki Design just released their latest scenery: Moscow City for X-Plane. You can now fly in the capital of the biggest country in the world. Beautiful buildings that look like they were taken from fairy tales, a huge city, over half a dozen airports and Vladimir Putin.
Drzewiecki Design "Moscow City" Released
Drzewiecki Design is pleased to inform you that they have released Moscow City X. Moscow City is without doubt a very detailed model of Moscow metropolitan area in Russia, together with lite sceneries of 7 airports, many heliports and thousands of buildings. It took almost a year to complete and it includes countless fantastic features. The FPS/VAS-friendly design and advanced optimization ...

Professional BoatBuilder Magazine

Folding Systems

Catamarans and Proas

Take-Apart Aka Systems

Telescoping Aka Systems

Simple Horizontal Hinge Systems

Complex Aka Hinge Systems

Swing-Wing Systems

Folding Multihulls with Flat Swing-Wing Akas

Complex Swing-Wing Systems

Conclusions

Nomenclature

The Crossbeam (Aka) Structure

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Beam and Stability

Drogues and Chutes

Capsize Case Studies

Techtronics 35 catamaran, John Shuttleworth design

PDQ 32 Catamaran

Sustaining speed with wider tacking angles will help overcome leeway.

“Tune” the Mast

Check Sheeting Angles

Use your Tell-Tales

Keep Your Bottom Clean

Reef Wisdom

Don’t Pinch

Chris White Custom 57

Gemini 105mC

38-foot Roger Simpson Design

15 meter Marsaudon Ts

Multihulls We’ve Sailed

Stiletto Catamaran

Corsair F-24 MK I trimaran

Without proper testing, calculating stability yields only a rough picture.

Stability by the Numbers

Multihulls by the Numbers

Capsize by Wind Alone

Sailing Windward

Sailing Downwind

Conclusions

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