searunner trimaran

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Bringing the second golden age of sail

Spice, SeaRunner 37 Cruising Trimaran

Instead of going on to college as all his friends did, Tim graduated a year early from high school and spent his senior year building a 25-foot SeaRunner trimaran designed by renowned multihull designer Jim Brown, in a friend’s driveway in Palo Alto, California. When his friends were having their graduation night party, Tim was 20 miles off Point Sur (near Monterey) headed south around the world in that little boat, with $100 in his pocket.

He soon found out that both the boat and the amount of money were way too small! Tim stopped in Santa Barbara, sold the 25-footer, and built a 37-foot version of the same boat. This was a SeaRunner 37 trimaran designed by the renowned multihull designer Jim Brown, and named Spice . She had big comfortable DRY bunks, a nice galley (kitchen to you non-boaties), and even a SHOWER!

He had Spice for 6 years, and sailed her to the Marquesas (near Tahiti) in 1976, then spent a year cruising the Hawaiian Islands in 1977 before settling down in Hawaii (to build a bigger boat) and selling her.

There’s an interesting story about the following photograph that Tim took of Spice while she was in the Marquesas:

First, cruiser’s parties are always held on the boat that has the most deck area. Spice wasn’t always the longest boat in the harbor, but at 22 feet wide, she had more deck area than most 60-footers, AND had the biggest “boom tent” for shade in the whole anchorage, so she had more than her fair share of parties.

Second, Tim took this photo with a Nikonos V waterproof camera: no light meter, no through-the-lens focusing, and none of the other modern camera features that make for great photos these days. Luck.

Third, Tim had already had three rum punches when he climbed to the top of the 50-foot mast where he took this photo, with no safety harness on (hey, we’re all young and bulletproof once!). Again, Luck.

The life sounds perfect, right? What Tim found, though, was that he had to stop every six months or so when he ran out of money and pick up work to replenish the cruising kitty. This meant getting a job, and living aboard in a stinky polluted harbor near a big city, and was never much fun.

Tim had learned to catch fish aboard Spice in the Marquesas, and that seemed like a good way to make money without having to stop and get a job. But Spice had a fault: with no fish hold, no refrigeration, no ice-maker, and nowhere to buy ice out in the small islands he loved visiting, Spice had to fish close to the harbor and get the fish back within an hour or so, before they started to spoil.

So Tim started dreaming of a trimaran that was designed from the ground up to be a fishing boat; with refrigeration, bait wells, and all the “stuff” that goes with commercial fishing boats. His idea was to keep cruising and feed people along the way; earning a living at the same time.

He couldn’t find anyone with a practical sailing fishing boat design in the size and type of vessel he wanted, so ended up designing his own 56-foot sailing fishing trimaran, Tropic Bird , then building her himself in 1977-1978.

You will find that story on the Tropic Bird page.

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Is anyone building a Searunner 31?

Discussion in ' Boatbuilding ' started by YMan , May 1, 2019 .

YMan New Member

I just found this site as I never considered building a sailboat before but am recently interested in the Searunners. Reading Jim Brown's books have given me confidence and inspiration. My plan has always been to retire into a traditional small cruising sailboat (Pacific Seacraft, Island Packet, Pearson Vanguard, etc). Downside is they are kind of slow; slower than my Cal 29 or especially the ultralights, and I would enjoy a lively boat for local sailing and fast passages. I started looking at multihulls, F28 and F31 in particular, but they aren't designed or suited to ocean crossings though they look like great fun. They did get me fired up about trimarans. Enter Searunner. It seems to me one of the smartest designed small cruisers around - as long as your idea of cruising doesn't require big electrical loads, diesel engines and modern conveniences. Some of those things interest me, but I consider worth sacrificing for fast, fun and safe cruising. Othe reasons I'm thinking of building one: 1. The design is simple and proven (designed for backyard builders). 2. The plans with full size patterns and a 400 pg construction manual are still available. 3. Materials are still available and overall cost to build and outfit seems to be about 3X-5X a used Searunner ($50-75K). 4. Most used Searunners were built in the 70s so the used market is thinning out and is 50 years old (like me). 5. A DIY build is the only way I would ever afford a new boat capable of world cruising (new hull, sails, rigging, wiring, etc) 6. John Marples who is a multihull designer and infinitely familiar with Searunners, and the Outrig.com folks are still around for support. 7. Building my own boat capable of crossing oceans in this day of outsourcing and buying from China is very appealing to me. Made in America. 8. Though it's an old design, many of us are still sailing plastic classics from the 60s and 70s and love them. The Searunner seems as comparable today for the cruising minded sailor and is faster. So, with what I believe are reasonable considerations and great benefits, I wonder why I haven't heard about others doing the same or seen new plywood & glass DIY multihulls in the marinas or oceans where I've traveled. And though I don't claim to be an expert on this subject, I'm wrapping up a 30 year combined navy and Coast Guard career, and I've seen a few marinas and spent a fair bit of time on the water. Am I missing something in my reasoning? Is there a hidden DIY boatbuilding scene in the US I don't know about? Or am I just an anachronism?  

fallguy Senior Member

I am building a Wood's Skoota 32 powercat. It was a two year plan, but hip failed and that added time to it. New Hip is better. I think I can finish in 3 years.  

rwatson Senior Member

In my humble opinion "The design is simple and proven (designed for backyard builders)." does not equal "it's an old design, .... Searunners were built in the 70s" The ease of building game, and designing game has moved way ahead in that time. This "homemade quality" and "old rugged design" philosophy is a bit overrated. I wouldn't build/buy a 1970 car either. A Searunner is no stronger than modern designs, but it is a lot heavier and lower performance with less room. Searunner Multihulls - Searunner 31 Trimaran http://www.searunner.com/index.php/searunner-trimarans/searunner-25/16-our-designs/searunner/34-searunner-31 Sure, you don't need speed in cruising, but if you can turn a 4 day passage into 3 days in fine weather, why wouldn't you ? I would be looking at places like M90 Trimaran | Build Your Own Trimaran https://trimarankit.com/m-90-stb-screecher  

RWatson: Thanks for the info. I'm glad to see there are new DIY plywood trimaran designs out there. I hope they are doing well at least in AUS. I looked over the M90 and would still prefer the SR31 for my interests. 1. The SR31 is about 30% heavier but is almost a meter longer. In the monohull world that is pretty equivalent for the length difference. Also, I think the liveable space is much larger and the SR31 almas are larger and provide some stowage, both of which account for added weight - the M90 has a very long, shallow stern cockpit more suited to racing and weekending in my view. 2. I much prefer the split cabin with captains berth and galley in the stern. The M90 v-berth doesn't suit me where the SR31 makes better use of the bow for sail stowage where sails can be handled thru the forward hatch. In military and commercial vessels no one but the lowliest sailor sleeps in the bow where pitching and pounding are the worst. Jim Brown puts the crew amidships where its comfortable and the captain in the stern where it's primo. 3. I would never want my galley next to my head. 4. The SR31 A-frames make incredibly strong support structures for the almas - like bridge trusses. I'm not sure about the M90. I'd be interested to know if anyone has done any significant cruising or ocean crossing in an M90. 5. I would trust the SR31 for good protection from heavy following seas. The M90 cockpit would certainly drain quickly but the crew would be at one with the sea (again more of a racing design).  

Ashantha Peiris New Member

Hello Mr Yman. We are qualified design and manufacturing team in FRP Boat Construction.we can design and manufacturing your boat as per your requirement . For More information please email to me. Email : [email protected]  

santacruz58 Senior Member

Hello Yman I don't know if you still follow this thread but I will comment anyway. I agree with your assessment of the sr31 for the most part. I have been looking at the sr31 as a cruising boat for my retirement. I don't plan on building one but would look for a more recent build that is in good shape. I have cruised in the past including ocean passages and I know how I would use the sr31. I like the eating and dining quarters separate from the sleeping and head area. In my research of this design the average speed on a passage is between 5 and 7 knots. A respectable speed, certainly faster than my last cruising boat. Standing head room is important to me as well as lots of usable storage space, thou you have to be very careful about how much weight is carried aboard. Kurt Hughes has recently posted on his blog that he has done an update on the sr31 on request of Wooden boat magazine. The design is definitely more modern with the nice hull shapes that he is known for. If I was going to build a new trimaran this size I would be knocking on his door to finish the design. All the best in your search.  

I sort of stumbled back into this thread while searching for SR31 info again. I think your speed should be a little better on a passage. Jim Brown quotes about 1.5X a comparable monohull into the wind and better on a reach. I've read several accounts of hitting approx. 15 knots and think between 7-9 kts on avg is doable. Since I started this thread I had a chance to look over a new build of a John Marples design DC-3 which is a trailerable 27' trimaran using his and Jim Brown's "constant camber" process, which allows a more open interior. I like the design a lot though it is quite a bit smaller than the SR31. It might work well as a single-handed cruising boat, but can't carry enough weight for more than 2 for any duration. The ability to save on slip fees and trailer all over the country is pretty appealing and my build space is a little small for the SR31, so I purchased the plans and am studying and preparing my home shop. In the meantime, I still have my Cal 29 to stay on the water and plan for the future. I do still keep an eye out for used Searunners - it could save me 3-5 years of building.  

HeyGuy New Member

YMan I have been rebuilding a 1976 SR31 A-frame , very strong solid boat, no dry rot, on custom built trailer. It has a 9,9 Yamaha out board, New folboat dingy w/2.5 motor, all riggings, sails- main, genoa and jib, custom mast step for raising and lowering aluminum mast and more. Electronics are out of date. Just needs hulls resurfaced and gel-coat. If you are still interested in getting into a SR31 let me know.  

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joe weaver New Member

I am interested in your 31 under restoration. Or any other searunner 31 for sale. Where is boat located? Email me please [email protected]  

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Seaclipper 16 OC

A FAMILY BOAT FOR ALL SEASONS

The Seaclipper 16 Open Cockpit folding trimaran is designed for amateur construction from mostly ¼″ (6mm) plywood with epoxy adhesives and coatings. It can be built in less than 6 months by an individual or as a family project.The plans are highly detailed and easy to follow with full-size patterns for many hull parts provided to facilitate the process. The mast and sails are scavenged from a Hobie 14 beach catamaran , which are inexpensive and available used or new. 

This boat can satisfy many different needs within a family. Its large cockpit and 11′ beam provide ample space for four people to sail comfortably , with compartments enough to stow the gear needed for family outings on lakes or along the coast. With a full crew, the 16 OC is a stable and lively daysailer—an ideal place for novices to learn sailing technique. Add a spinnaker and develop your skills further.

Sailed solo, the 16 OC is exciting to sail, given a steady breeze, speeds of 10 knots are likely. For intrepid sailors, the boat can be outfitted for short solo expeditions. It has plenty of compartments to stow provisions and extra gear. A small camping tent will fit over the cockpit and side deck. Add a side-mounted outboard (up to 4-hp) to provide auxiliary power.

A flat-wing folding system quickly reduces the beam from 11′ to under 8′ for legal trailering on the highway. The akas (crossbeams) are made from standard lumber materials, allows the boat to live on its trailer and be launched and ready to sail in less than an hour. One can even reduce the beam when afloat with the mast up, for instance in order to fit in a marina slip. Plans are included for building a custom trailer or for modifying a standard boat trailer to safely support all three hulls.

The Seaclipper 16 OC has two steering options : the standard tiller steering for conventional seating in the cockpit, and pedal steering to be used with sailing solo while seated on the cockpit floor. The rudder has a unique kick-up transom design with control lines leading to the cockpit. Weeds or debris on the rudder blade can be quickly cleared by pivoting the rudder up momentarily while sailing. The boat can be sailed onto the beach by releasing the rudder and pulling up the daggerboard. With the rudder and daggerboard partially up, one can explore the shallows to about 18″ of depth.

The Seaclipper 16 Open Cockpit  is designed for sailing fun. Simple to build, easy to sail, on a very modest budget. Build a trailer from the plans, or buy one and build the bunks to support the boat. We encourage purchasing used outfitting gear; mast, sails, blocks and other fittings to reduce cost. Paint your boat a wild color and go sailing with your family soon.

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• Seaclipper 16 Open Cockpit

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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