power trimaran design plans

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Hammerhead 54 Trimaran

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The 60' Power Trimaran 'Penny Wise' Larger Profile | Deck Plan & Interior Sketch Larger Perspective Aft | Larger Perspective Fwd Copyright 2000 - 2006 Michael Kasten The Concept The client stated the following: "I seek a marine mobile home primarily for long-term island/harbor hopping in the Western Pacific and the rivers of China, but capable of occasional long ocean passages.

My wife has three requirements:

• A lounge / house with a view
• Galley with a view
• Emergency sailing capability

Layout: I have few pre-conceived notions about layout but I have done a lot of sketching. The trimaran configuration is so narrow that there are really not all that many options for layout unless it has a wide topside blister or the length exceeds 60 feet, and 60 feet is already a hard boat to manage in port. Too much beam hanging out above the waterline looks funny, and may preclude trucking. I am open to all suggestions including a catamaran that does not break the budget.

The galley should be large enough for everyday living, the head (one is enough) should have a separate shower, not a grate below the toilet. My list includes lots of storage, a well equipped shop, and an office/computer/study area. It would be good if settees were long enough to serve as guest berths in port.

Appearance: I actually like 'spaceships' that look like the ILAN Voyager and Cable & Wireless, but for this vessel I prefer a more traditional looking boat in order to find better acceptance in third world ports. Except for the local fishing boats however, it is not clear what "traditional looking" will actually be for such a craft as this, given that the concept is most certainly not traditional...!"

Sails: There is no need to think too hard about an emergency sail. Indonesian trimarans routinely use rectangular sails on near-vertical yards hung from short masts. They could be awnings with the necessary fittings added. I prefer simple mechanical systems that I can fix myself, except for communications and navigation gear.

Materials: I like unpainted aluminum a la Northwest fishing boats, but given that construction will be in the Philippines, I think materials are confined by cost to sheathed strip planking and/or ply. At a minimum, the boat should be designed to accept routine bashing about in rough ports.  

On the face of it, a somewhat bizarre request; a tough assignment, at best: Come up with a "traditional looking" trimaran...?

Well, the result is certainly that. It would fit in well in any Asian port, and also would not be out of place among trawler yachts in a Western port. It is perhaps somewhat in the Wharram "tradition" though the hull form is quite different from those craft, and is combined with a pilot house that any trawler would be proud of.

Is the result strange? I think not -- it is instead quite a neat little ship!

To fit a sail rig would be quite easy, and if primarily used as an "emergency" get-home affair, it could easily be a Chinese Junk type of schooner to keep the rig low.

For 'get-home' sailing, one might want to have dagger-boards or centerboards arranged to not interfere with the accommodations. Mast placement is always problematic to the interior of a vessel, so in this case, they would probably be deck stepped, and in that way could be made to easily be lowered.

The "Sitting room with a view" is accomplished in the Pilot House, with a wide settee facing forward. A table forward of the settee allows meals to be taken there for a good view of the harbor. Another larger settee and table arrangement below provides privacy for evening dining as well.

The requirement for "a galley with a view" is well met in my view, since the galley is located centrally below the fore deck, and is of a generous size. As for the view, it would be quite good. The galley is forward of the amas, so they would not be an interference. The fore-deck spans the full width of the ship, extending from bulwark to bulwark athwartships, and from the pilot house, all the way forward until the anchor well is reached at the bow. With generous ports within the bulwark outboard of the galley, plus several good sky lights, all requirements are met.

An alternate location for the galley would of course be to lengthen the pilot house, and place the galley within it. In spite of the excellent view from there, this arrangement has multiple disadvantages. It would raise the center of gravity of the main hull due to the longer pilot house; it would locate more weight aft (already pushing the limits); it would place a lot of activity in the piloting area, always a disadvantage; and it would make food preparation underway during the night watch somewhat of a risk to the helmsman's night vision. For a true voyaging vessel, the piloting area is best reserved for just that, and for comfortable lounging while at sea or in harbor.

Below, forward of the settee are a pair of office-like desks with swivel chairs. Below aft, there is a generous shower to starb'd, and a large head to port. Right forward is a very spacious double berth with a big shelf / locker forward of that. Farther forward yet is the fo'c's'l locker to house the anchor rode.

In order to achieve any sort of meaningful 'shop-space' the vessel would either have to become longer, or one of the interior spaces would have to serve multiple purposes. For example, if it could be tolerated to locate the shower within the head compartment to port, one could then dedicate the starb'd compartment to being a shop-space, or possibly an office, or even an extra guest cabin...

Naturally, in such a long and narrow living space, the width of sole is very restricted. One can raise up the sole in the outboard areas if it is needed, and then arrange for comfortable seating for whatever task is contemplated, whether this be a shop area, shower, office, or what have you.

On the Exterior, one could arrange a nice wrap around seat at the stern and maybe a BBQ / bar on the aft face of the pilot house on either side of the 'stack.' There is plenty of foredeck area for sun bathing, and with a mast and boom there, one can easily arrange an awning for a bit of shade...

Though not illustrated in the above-linked drawings, there could be a 'trampoline' net arranged between the main hull and the amas, extending from the forward beam to the aft beam... a great place to hang out under way...

The amas themselves are quite small, and should be kept water tight with a secure hatch. Each could become a small 'kid's cabin' and they would have their own 'private vessel' to command... Alternately, though it is best to keep them as light as possible, they could be used for light weight storage, say for fenders, lines, sails, etc.  

Materials of Construction

Given the type of vessel in question we can definitively say a few things about the materials of construction:

• Building a vessel of this type in steel would be completely out of the question.
• Aluminum alloy would be an excellent choice for the hulls, the decks, the pilot house structures, and for the cross beams. If built in aluminum and NC cut, the building of the hulls would be very quickly achieved.
• Plywood is certainly a quick method of construction, and if covered using a composite resin / laminate, there would be very good strength, lightness of structure, and ease of general maintenance.
• Cored FRP construction does offer several advantages, among them strength, longevity and freedom from general maintenance. Naturally, FRP construction of any type will be much more costly in terms of materials than any other type, particularly if construction were to be in a remote part of the world.
• In order to achieve the best laminate quality, we prefer vinyl ester resin or epoxy resin. With these, the logical fabric would be Kevlar or another high strength fiber such as E-glass or S-glass. There is little point with a cruising vessel to use high-cost exotic fibers however such as carbon fiber. While light and stiff, carbon fiber is quite expensive, thus it is ordinarily ruled out for this kind of boat.

A very light weight interior would be quite important as well. For this, we specify the use of honeycomb panels for the joinery flats wherever it is practical and easy to do so. One such material brand is Nida-Core, possibly the most cost effective among the HC panels. Nida Core panels have thin plywood skins bonded to a phenolic resin honeycomb core. This produces very stiff and light weight interior joinery with more or less the same labor cost as with standard plywood construction.

Ideally the struts / cross-beams would be laminated wood, and would be tapered. If the hulls were all plywood, construction would be very fast. In that case we could detail all the sheets for NC Cutting by a 2-axis router. For maximum construction speed and the least number of man-hours we could make use of the "stitch and glue" method of hull construction. This would allow eliminating many of the heavy carlins and chine logs common to traditional plywood construction. Each hull could be quickly built upside down until the deck is reached, then turned over.

If built in cored glass, the mould would be simple to build using flat pressed board panels, and would of course be built upright. To detail the mould for NC cutting would be quite easy... we would simply put the 'structure' of the mould on the outside of the hull surfaces rather than on the inside...! The result would be a 'mould kit' which would quickly snap together.  

The hull is a simple single chine type in order to provide the ultimate flexibility in choice of construction materials. The general shape is a V-bottom on all hulls, using a shape somewhat reminiscent of the US east coast sharpies. The transoms are all arranged to be similar to the sharpie's fantail stern, and with a brief little "kick" upward at the stern. The prominent bow is a tribute to the possibility that this may be an Asian built vessel. With a junk rig, all the more fitting.

Of course the amas (the outer hulls) would not be deep in the water. Opinion varies quite widely on this, with some even advocating that amas be completely clear of the water with the vessel upright. In my view, the amas' hulls should just "kiss" the water when at rest with the vessel in an average load condition. This would immerse the keels on each ama, so they would never be entirely free of the water unless a sail rig were provided and the vessel were under a press of sail.

A monohull or a power catamaran can be designed to carry enormous cargo, however a trimaran cannot tolerate being overly burdened with the accretion of extra weight (most cruising vessels seem to suffer from this...!). Vigilance will therefore be required in order to preserve the inherently excellent performance that such a vessel has to offer.

Opinion is also divided on the best fore and aft location for the amas. In this case, the amas have been located quite far aft so that they can provide added buoyancy where it is needed most, due to the weight of engines and PH, etc. The aft location will help minimize any tendency to trip, and will provide the maximum in terms of tracking ability. In the aft location, the amas will also help the stern of the vessel resist squatting at speed.

Naturally, if sailing ability were to receive greater emphasis the amas would be located farther forward. The amas would not be moved beyond amidships however, as that would introduce the potential for tripping as well as possible steering anomalies.

The amas need to be as light as possible, so fuel and water can not be located there. Still, the amas need not be empty... The amas are 26' long by 3' 9" wide, so a perfectly comfortable berth could be placed into each ama with room to spare, making each one a little microcosm for young sailors... A little water tight combo house-companionway for observing the ocean... very nice indeed!

I have modeled the hull and house as drawn, and I've also modeled the house with considerably less "saddle" shape. Please see the links above for images of the hull model.  

Power & Range

The trimaran allows the use of an efficient single engine and there is no need for roll attenuation devices. A long slender hull is the very ultimate in terms of efficiency.

Power would be via a diesel engine located below the Pilot House in the main hull. Overall, all systems must be kept as light as possible. Therefore, one can presume there would be a single engine of modest size to adequately drive the boat; that there would be no generator per se, and instead a very adequate 12v DC system would serve electrical needs; that tankage would be kept to the minimum; and that construction materials would be as light as possible.

If very long range were a requirement, then emphasis could be placed on the vessel's sailing ability, for which purpose as mentioned the Chinese Junk rig would be an ideal choice. If that were chosen, the rig would likely be similar to that shown on our 50' Renegade . A sail rig would offer the ultimate in terms of range, get-home power, and the fun of sailing...

For the Penny Wise, if built very lightly as intended (around 26k to 28k lb. total in the light load condition), power could be in the range of around 65 - 70 hp, and that would attain a speed of around 10 knots depending on the load condition.

However due to being very light, narrow and long the hull is capable of exceeding that speed by quite a lot. In optimum conditions 15 knots would be possible, but ... that would require 165 hp...! On a more rational note, we'd prefer a maximum of around 95 - 100 hp total, sufficient for 12 knots depending on load condition.

With 400 USG and a 15% reserve, range would be around 3,000 NM at 8 knots. That's enough to reach Hawaii from any West Coast port - and then some.

Now that's what I call economy and good speed...!  

The 'design-study' budget for this vessel was limited in the extreme, therefore the vessel's name (and its implied meaning). As a result, the 'Penny Wise' remains 'Pound Foolish.' In other words, the concept design presently awaits a more qualified owner to pursue the remaining drawings, design calcs, propulsion and resistance calcs, etc.

Even so, I believe the concept of the Penny Wise has excellent merit, especially for long range cruising. A few of the reasons for that opinion are as follows:

Monohull power passagemakers expend terrific expense on roll damping, which in this case would just not be an issue. Relative to active stabies, the amas would be silent and efficient. Relative to paravanes, the amas would be very simple, would require no tending, would provide extra space and extra buoyancy, and would not present nearly the amount of drag induced by paravanes.

There have been some very impressive voyages in power trimarans of late. The whole concept has engendered a completely new vessel type: the ILAN (Incredibly Long And Narrow). An internet search for ILAN will turn up quite a few interesting precedents for such a vessel, and should illustrate the practicality of the concept. With an ILAN type however, the hull materials must be light weight in the extreme, thus they require quite an extreme budget to build...!

The adventure with this particular assignment has been:

• To create the best combination of economy and efficiency
• To somehow nail down a style that gives the vessel a "traditional" aspect
• To create a type that lends itself to alternate building methods
• To provide an aesthetic that will have the best chance to "fit-in" regardless of where the vessel may be found, whether that may be in the South China Sea, or the South of France...

All of those design requests have been admirably met in the Penny Wise...

Similar Designs...?

We have developed various prototype designs for multi-hull craft, including catamarans and proas. We have also developed a series of mono-hull vessels which are quite similar to the Penny Wise. They share quite a lot of design traits with the Penny Wise ...

Other designs in the "Peregrine / Renegade / Moxie" Family:

36' Molly | 43' Moxie | 49' Quinn | 50' Renegade 61' Peregrine | 82' Peregrine | 100' Amazon | 164' Peregrine 60' Pennywise Trimaran | 70' Peregrine-on-Thames

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32m Power Trimaran

Go anywhere, anytime..

The Shuttleworth 32M redefines long distance voyaging yachts of this size range. Her rugged 'explorer' style exterior maximises interior volume to provide spacious and comfortable accommodation whilst still maintaining unrivalled seakeeping, stability, and efficiency.

Powered by a single 1150hp C18 caterpillar engine with an additional 110hp engine in each outrigger for manoeuvring and back up propulsion, she has a top speed of 26 knots and a range at 12 knots in excess of 6,000nm.

Using state of the art composite engineering we have achieved the light weight structure required for exceptional fuel efficiency and performance, as well as the strength and rigidity suitable for heavy weather ocean capability. Ocean voyages can be undertaken with confidence, at high speed and in comfort, allowing you to go anywhere, anytime.

• 6,000nm range at 12 knots
• 26 knots top speed
• Exceptional seakeeping and stability
• Unrivalled fuel efficiency
• Spacious accommodation

Award Winning Design

Adastra covered more than 60,000 miles during her first 7 years at sea and has proven herself to be a very comfortable and exceptionally fuel efficient ocean going vessel. She can cruise at 17 to 20 knots in most conditions, and has weathered 50 knot winds and seas up to 4 metres. She is incredibly stable and comfortable at rest and rolling is virtually eliminated by the outriggers.

To find out more about the award winning naval architecture used in these designs read the article "Adastra - The search for a seakindly fuel efficient vessel". Read more.

Accommodation Layouts

The layout of the Shuttleworth 32M has been designed to optimise ease of operation and maintenance allowing the vessel to be cruised by the owners alone or with a minimum crew. Engines, generators and other systems are installed for ease of access and ample workshop space is provided. Dinghys and toys have launching and retrieval systems that are reliable, simple and quick to use. The living space, which has been carefully considered to make the vessel comfortable to live aboard for long periods of time is open plan but practical as it can be compartmenatilsed when required. Massive storage space is provided in the wings for supplies, spares and emergency gear.

The flybridge has a large shaded seating area surounded by low coamings to allow for expansive views, as well as a BBQ and helm station. When the tender is launched the aft deck provides a spacious area for sun loungers.

The main deck houses the saloon, dining area, kitchen, breakfast bar, helm station and chart table with forward viewing seats. To increase visibility at night the helm station can be separated from the main living area with a retractable screen. Stairs through the forward windscreen lead to a teak covered foredeck with movable deck chairs and a sun bathing area which can be covered by an awning. The aft deck has a rope handling bench and storage space for kayaks as well as a seating area, BBQ and lazarette. Steps onto the wings lead to teak covered side decks, large storage lockers on the wings and steps that access the outriggers, both of which house engines and generators.

32m Technical data

• Length Overall 32.9m
• Waterline Length 32m
• Hull Construction Composite (Glass/Aramid/Foam)
• Deck Construction Composite (Carbon/Nomex)
• Performance
• Speed cruising 12 knots
• Range at 12 knots 6,000nm
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• Owner and Guests 4
• Tenders 4.9m and 3.1m RIBS
• Main engine 1 x Caterpillar C18 1150 hp @ 2300 rpm
• Outrigger engines 2 x Yanmar 110hp @ 3200 rpm
• Generators 3 x 36 kw
• Exterior Styling Shuttleworth Design Ltd
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Shuttleworth Design Ltd Gurnard Isle of Wight UK

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210 SUNREEF POWER TRIMARAN

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ASK ABOUT THIS YACHT

The 210 Sunreef Power Trimaran is the avant-garde of superyacht design and the ultimate expression of engineering expertise. An ambitious 210ft three-hulled concept, it fully utilizes the space within and between the hulls to reach a living area of nearly 1000m². The vessel’s outdoor social area features an immense flybridge, a cockpit opening onto the saloon as well as terraces at the front and aft.

The power trimaran’s upper deck features an aft terrace giving access to a grand dining room, an internal navigation station, a gym space, and the captain’s cabin. The main deck stretches over 510m² of entirely customizable surface. The saloon features a main lounging space placed amidships and four relaxing scenic areas with floor to ceiling glass windows.

The optional layout comprises six guest cabins, all fitted with large double beds, each facing retractable private balconies. The primary tender for up to 12 is stowed in the foredeck along with the crew dinghy. Placed forward is the stately master suite with panoramic widows, twin dressing rooms, an office and access to a private front terrace.

The lower level of the 210 Sunreef Power Trimaran is entirely devoted to the crew. Accessed through the water toys garage storing two jets skis and sea bobs, the central hull houses a fully equipped galley, dining and sleeping quarters, a laundry and large storage areas. All three levels of the trimaran are connected via an elevator and a spiral staircase.

For more information about the 210 Sunreef Power Trimaran, reach out to our customer representatives today.

SPECIFICATION

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Advanced composites

Construction

64 m / 210 Ft

Length overall

23.5 m / 77.1 Ft

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1.6 m / 5.41 Ft

Displacement

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Power trimaran world cruiser

Discussion in ' Multihulls ' started by teamcallander , May 13, 2017 .

• long distance cruising
• power trimaran

teamcallander Junior Member

Hi there A few years back I posted some design ideas for an 80 ft power trimaran on the forums here and received a lot of very good feedback from the members which I really appreciated. It helped me to refine what was important and to get read of a bunch of stuff that wasn't. As a result I went back to the drawing board and also talked to some industry folk about what customers want and need in a boat designed to be a world cruiser. My background is in hydrogen fuel cell and renewable energy system design but in order to really make the most of those in a boat you really need to start with something different from what the mass production market currently provides. This is my first attempt at transferring my design ideas into a CAD program - I used Rhino and Flamingo - and I'm pretty happy with the result but in reality everything can be improved on. But enough of the preamble, here are the specs: - 10:1 round bilge, canoe bottom power trimaran of 98ft length and 30ft overall beam - max draft 4ft - 35 tons light ship - 2000 gallons of fuel - accommodation for 10 in 5 ensuite guest cabins - separate entrance crew quarters with accommodation for 4 in 2 cabins plus kitchen and bathroom - 17ft dinghy garage with rear door onto swim platform - all the usual amenities on the main deck and flying bridge Anyway, I have a attached a short (six page) overview of the design with some renders if you want to know more about it. Thanks and very much looking forward to your feedback!  

Attached Files:

Long Range Power Trimaran Concept Paper.pdf

Forgot to attach the side view...  

Ok, so I don't think I was specific enough in my first post. The following points are what I am most interested in hearing about in the context of this design: - thoughts on twin Yanmars Vs single Cat 12 ACERT for propulsion - thoughts on fiberglass Vs aluminum construction for a vessel of this size - would you prefer it to be bigger, smaller or is it just about right? - exterior styling thumbs up or thumbs down? Cheers  

ChrisPie New Member

teamcallander said: ↑ Ok, so I don't think I was specific enough in my first post. The following points are what I am most interested in hearing about in the context of this design: - thoughts on twin Yanmars Vs single Cat 12 ACERT for propulsion - thoughts on fiberglass Vs aluminum construction for a vessel of this size - would you prefer it to be bigger, smaller or is it just about right? - exterior styling thumbs up or thumbs down? Cheers Click to expand...

ChrisPie said: ↑ Hi. Have you settles on the placement of the amas? You can design them to be riding on the bow waves of the main hull creating lift and reducing drag. This only applies at one particular speed. Compared to other designs I have seen your design looks a little narrow to achieve this. I would love to understand how this is calculated? Click to expand...

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AquaDive New Member

Hi Teamcallander, I like you concept, low fuel etc (el. motor / fuel-cell)..if you don't mind could you contact me on [email protected]  

Want to engineer a small solar powered trimaran. Seeking advice.

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SENSORI POWER CATS

We've updated our Sensori power cat range with newly developed low drag semi displacement hulls, fresh styling and a new range of layout options. Spec's, layouts and more details coming thru November 2023.

Sensori is our range of power catamarans designed for long range, long term cruising with efficient hull forms and a range of powering options

Powering options include Inboard diesels with shaft or sail drive, outboard diesel engines, and diesel electric systems.

There are two styles of accommodation layout for cruising. In the larger size range (58'/17.6m +) you have the option of the master suite layout. All of the designs offer the possibility of two, three or four cabins in the hulls with additional guest accommodation in the saloon.

Any of the designs on these pages can be converted to day charter or passenger transfer vessels.

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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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