Multihull Structure Thoughts

Farrier folding crossbeams are good but are difficult to construct well. Just one aspect of the beam structure is the attachments to the main hull structure. Just a few photos show an F22 structure and its internal attachments. All of these components need to well built but more importantly aligned so the folding can happen smoothly. Also is an exploded view of Tony Graingers version of a folding beam. Farrier recognized the difficulty of cross beam structures and sold the beams and support infrastructure for many of his designs. If you need to understand the full folding system the Command 10 plans on page 10 of this thread gives details of a wooden version.

 

What makes trimarans fast? A design discussion. https://www.rocketfactorytrimarans.com/rocket-x-37/what-makes-trimarans-fast/ is an interesting read on float design and other aspects of tri design. Good read. A PDF version is available What Makes Trimarans Fast.pdf https://drive.google.com/file/d/0Bx_50rWhaxVzandOOGNJV3k4aE0/view

 

Just to remind you the following resources are available on the web about Searunner Trimarans. The first PDF is a study book with a bit of philosophy. Its an 18 meg download. http://www.tayvaughan.com/places/greatbear/images/SearunnerTrimarans.pdf The second PDF s the full Searunner construction manual which contains many gems especially if you are building in plywood. Unfortunately its 130 meg download. Go to http://outrigmedia.com/outrig/searunnerconstructionmanual/ Painful but you get the manual. Finally there is a Jim Brown note on longevity of boats many good hints. Attached are some Searunner 25 studies.

 

I love the Searunner construction manual, a great read. I have the plans for the 31 but saw a Searunner 31 next to a Twiggy and decided to get a Twiggy rather than build a 31.

 

The Impala 38 was an early full wing deck Crowther trimaran. Several were built and rarely come up for sale. The last one I saw was in WA and the owner had just finished sailing around Australia. He was a happy man. It is relatively fast and can go to windward very well. The latest tris may have fuller floats and daggerboards with more efficient rigs but some of the older designs can be very good fast cruisers. The PDF’s give you the structure, general layout of the boat and Crowther's approach to tris at that stage.

 

A friend bought the one of the Impalas shown above. I got to sail it a bit. I had been on it back in the 80s and thought it was a lovely boat back then. What is strange is that I have been changed by the easy access of modern cats and found her less of a wonderful boat twenty years later.
Lock was a great designer but these early tris have a few issues with ease of use - bunks that have foot wells into beams, hard to access cabin steps and thin cabin soles. Today a modern trimaran would have chamfer panels or hull flare to increase interior room. Also beams are often modified to allow better access forward and aft.
Impala's are still a nice cruising boat but the difference in usability between an 35 year old 38ft tri and 20 year old performance cat is pretty staggering. That all said - I like Impalas a lot and they suit a singlehander who wants a good ocean capable multi, or a family who want a short term cruiser.

 

Great thread Oldmulti, I've been enjoying reading through it. It's funny you should mention that catsketcher, I went onboard a friends classic trimaran a while ago. The old Escapade by Lex Nicol. I found the interior layout pretty nonsensical and hard to put up with by modern trimaran standards. It suffers by comparison to a production boat like a corsair particularly with the full height daggerboard case splitting the saloon like it does with tons of intrusion over the small amount of sole space available. Anyway I still love the boat, don't love the interior. Double shuffle topsides as Kurt draws on his trimaran cruisers or the Farrier or Shuttleworth flare add a lot of living area without adding too much weight.

 

Whilst we are on wooden trimarans I will discuss one of my favorite boats. The boat is still racing and winning at after 49 years. It good period of racing was between 1998 and 2010 were it was beating boats longer than it in 300 miles races. Adagio was design and built by the Gougeon brothers in the late 60’s as an example of wood epoxy construction. The boat is 35 x 24 foot weighing 1200 lbs and displacing 2700 lbs with 640 square foot of sail. There were several versions of the 40 foot high wing mast from a full bent plywood 450 mm long cord version to the latest 300 mm long cord Stressform wing mast. The Stressform wing mast had several layers of 5 oz carbon fibre, 2.5 mm Douglas fir layup for the wing sides with several layers of 100 mm wide 9 oz carbon fibre unidirectional tapes wrapped around a 75 mm PVC mold tube as the forward edge of the wing mast instead of a solid wood leading edge. (Stressform details on page 5 of this tread). The main hull and floats were 6 mm 3 ply Okume tortured ply. The main hull had 12 x 19 mm ribs every 100 mm from bow to the main beam then 19 x 19 mm ribs every 300 mm from the main beam aft. Float gunnel was 2 layers of 22 x 45 mm. The ply float deck had deck beams every 300 mm. The float deck also had 2 deck stringers of 19 x 37 mm. The cross beams were timber with wire water stays. This boat had relatively small floats but adequate for the displacement. You don’t need carbon foam to create a simple light and very fast boat. And yes there is not much real accommodation. The PDF’s were written by Meade Gougeon. The photos will give an idea of the boat. Epoxyworks magazine has some other details.

 

For those who need a good read "The Gougeon brothers on boat construction" book is available for free in PDF format at Gougeon Brothers on Boat Construction book - WEST SYSTEM Epoxy https://www.westsystem.com/the-gougeon-brothers-on-boat-construction/ This is a classic on epoxy wood boat construction with an enormous amount of detail. It is a 9 meg download.

 

Kelsall went through many evolutionary phases in his designs. Originally it was foam glass then a plywood timber period for some designs and now mainly foam glass again. During his timber ply stage he produced some very neat fat hulled cats that had good performance, internal space and were fun. They went under several names but were basically the same boats. The Tonga Trail (or Kelsall trail, Kelsall Classic Cat) came in 2 main sizes of 22 foot and 27 foot. The boats were 2 tube hulls with plywood timber cross beams. The boat I will talk about is the Tonga Trail. The cat was 22 x 13 foot weighing 1400 lbs with 220 square foot of sail. The hull was 6 mm ply chine hull on 19 x 19 mm stringers on 150 to 200 mm centre lines. Chines are 19 x 55 mm. Some bulkheads 6 mm ply with timber edging. Crossbeam bulkheads are 9 mm ply with timber edging. Decks, cabin roof and cab sides are 6 mm ply. The wing deck floor is 9 mm ply with underwing stringers. Cross beams are full width and bolted on with 9 mm stainless steel bolts. The main and rear beams 240 x 120 mm max tapering to 120 x 120 mm at ends with 9 mm ply web faces and top and bottom timber flanges of 25 x 100 mm. The fore beam is 115 x 70 mm with 4.8 mm bird striker wire. The mast is 8.75 meter 115 x 70 mm with a single set of diamonds. Most rigging wire is 4 mm. I have day sailed on one. It was well balanced, relatively fast in lighter winds and tacked well. Its internal space is good and could accommodate a couple for a week cruise with decent berths and space for cooking etc. More room than a Tiki 26. The Tonga Trail is a good little boat. The PDF has a study plan.

 

This is an entry I wrote on another thread. It was in response to a guy who was dreaming big. I will give you the structure of the Piver Empress trimaran. The boat is 64 x 32 foot weighing 44000 lbs carrying 1500 square foot of sail in a ketch rig. This boat would displace 60,000 lbs. Piver sold home build plans and several were built by professional boat builders. No home builts that I know of. This boat was a charter machine with huge internal space 18 berths and 2 floors in the main hull etc. Float and main hull 15 mm ply sides 18 mm ply bottom. 30 x 140 frames on 12 mm ply at 70 to 950 mm centre lines. Stringers 30 x 140 mm on flat in main hull Float stringers 25 x 95 mm flat at 400 mm centre lines. Decks 12 mm ply with deck beams and stringers of 30 x 140 mm. Cabin roof and sides 15 mm ply with frames and stringers. Underwing 18 mm ply wood. The crossbeams are box plywood 950 mm high by 850 wide. The top and bottom are 3 x 18 mm plywood. The webs are 2 x 18 mm ply. Rudder shaft 1.5 meters long by 65 mm solid stainless steel. This boat is not fast. Or you could build a Cross 80 footer, better design, faster boat if one was ever finished. It only took 18 sheets of 9 mm ply per bulkhead. These boats are massive and consume large amounts of material. For an Empress you would be using at least 500 sheets of 12 mm plus plywood and tons of timber let alone the several 44 gallon drums of glue etc. Anything is possible with enough money and time (20,000 plus hours or 10 years full time).

 

Catsketcher on another thread said that the hull maybe designed to handle global forces but if it is a racer may have a limit fatigue life built in. I agree with this statement and will use some examples. First in hull design you have 4 competing issues, strength (global forces), weight, longevity (fatigue life) and knock resistance (you have all hit a wharf!). When you design a racer the 2 primary forces you are designing for are global forces and weight reduction. When you design a cruiser, the primary forces are global strength, longevity and knock resistance. Weight is of interest but not primary.

Richard Woods has 3 boats in the 24 – 25 foot range. Each designed for a purpose. The Strider 24 is a general use boat which can be lightened to have a “race” version. The Gwahir an outright micro multihull racer and Merlin a half way house between Strider and Gwahir. Striders have a basic structure and depending on use has 4 mm ply skin unglassed (except for deck) above the chine line in race mode. In cruise version 5 mm ply with 200 gsm glass all over. And if you cannot get 5 mm ply use 6 mm ply. The Gwahir is 4 mm ply all over with no glass. The Merlin uses 400 gsm 8 mm wrc 400 gsm in epoxy for hull.

The Strider weighs between 1000 and 1200 lbs, Gwahir weighs 900 lbs and Merlin weighs 1400 lbs. Very similar size of cats with similar accommodation but intended purposes are different. The Gwahir and race version of a Strider push the edges and have limited knock resistance. The cruise version of Strider and Merlin are conservative, will last well and handle knocks. Please understand Richard Woods is a very good designer and race versions of Striders and some Gwahir’s have raced and lasted 30 years. So we have a weight difference of up to 500 lbs or 35% between very similar sized boats designed for differing functions. Each has sufficient global strength but their longevity and knock resistance will differ. This issue becomes very important when designing foam glass boats.

In a separate post I will give further structural details of the 3 boats as their base structures and cross beams are very similar.

 

Strider hull construction are very similar to Gwahir. Cross beams are the same for Strider, Gwahir, Melin with a deep main beam variation available or aluminium beams. Merlin hulls are strip plank WRC with 400 gsm either side or if you do want any “movement” (Woods words) in WRC timber 600 gsm either side. Merlin hull construction can be used in Striders. The following is the Materials List:

• PLYWOOD Use best quality gaboon throughout.
  • Hull skins keel-stringer2 x 3 mm ply 11 sheets
  • topsides 5 or 6 mm ply 6 sheets (4 mm if racer, decks must be sheathed)
  • Bulkheads 6 mm ply 3 sheets
  • Daggerboard and beamboxes 6 mm ply 2 sheets
  • 12 mm ply 1/2 sheet
  • Decking 5 or 6 mm ply 8 sheets
  • Rudders & daggerboards 12 mm ply 2 sheets
  • Platform 9 mm ply 1 sheet
  • Beams 6 mm ply 3 sheets

• TIMBER: Use douglas fir, sitka spruce, yellow cedar or similar unless noted
  Note sizes are nominal and should be "planed all round"(PAR) EG 1 " x 1 " = 20 x 20mm
  • 1 " x 1 " 70m, 1 1/2" x 3/4" 120m 2" x 1" 125m, 1 1/2" x 1 1/2" 12m, 3" x 1 1/2" 2m, 3" x 3/4" 10m, 2" x 2" 3m, 3" x 1" 4m.
  • 1 1/2" x 3/4" 15m (for outergunwale - hardwood)
  • 2" x 1 1/2" 4 off 4100mm plus 15m (beams)
  • 1 " x 1 1/2" 8 off 4100mm plus 10m (beams)
  • 4" x 1" 3m (inner stems)
  • 4" x 2" 2m (outer stems use cedar, can be laminated from smaller pieces)

• GLUE:
  • minimum 10 kg epoxy and 20 kg resorcinol or 5 kg polyurethene glue eg Balcotan
• GLASS
  • 1 sq m 600 g Woven Roving or biaxial
  • 15 metres 100 mm wide glass tape
  • 15 sq m minimum of 300 g glass cloth + 5 kg epoxy. 20 sq m extra cloth is required if decks are sheathed. NB glass must be compatible with epoxy
• FILLERS
  • 500 g collodial silica & 1000 g Microballoons (min

• FASTENINGS:
  • approx 5000 12 mm staples
  • 500 g 3/4" barbed ring nails eg "gripfast" nails
  • 500 3/4" no. 6 brass CS screws
  • 100 1" no. 8 brass CS screws

Note: Strider can also be built using strip plank cedar hulls and ply decks

 

I love this thread. The difference in laminate is pretty interesting for me. The Crowther and Nicol racers were light. I think the most force I have put on my boats is when I hit them with the dinghy or put them on the hard. Richard is a designer, builder and sailor - the type of designer I admire (Like Crowther, Chamberlin and Pescott). I like the idea of very solid beams and a nice solid laminate. What is interesting is that a 400gm laminate is pretty common even in boats much bigger than the Woods cat. My 38 footer has 600 db outside and 400 uni inside. So pretty much the same laminate as these small cats.

My take on it, is that there are base loads, like dinghies and wharves that need a certain amount of puncture resistance. 600gm over WRC seems standard and not prone to failure. So small cats will be proportionally heavier than big ones because you need about 600gm outside to stop making dings in your paint job.

I am not sure of the no glass idea. I have not had success with no glass over ply. It always checks on me so I glass all ply nowadays.

cheers

Phil

 

Quinto 2 is now a production catamaran from Poland. The cat started as a home build but was considered good enough to go into production. The prototype is 26 x 15 foot weighing 1200 lbs displacing 1800 lbs at half load with 320 square foot of sail. The hull are 6 mm ply tortured ply sides connected to a 9 mm flat bottom. The decks are 6 mm ply. The forward and aft cross beams are 4.2 meter 9 mm ply boxes 125 x 80 mm. Each corner has a 20 x 30 mm batten in it. There are plywood bulkheads in the beam about every 500 mm. The mast beam is stronger. The production version of the boat has 600 gsm biax 10 mm 80 kg/cubic meter foam 600 gsm biax. The bottom has 900 gsm biax outside 15 mm foam 600 gsm biax inside. The deck house(s) are 15 mm foam cores.
Катамаран QUINTO 2 - Деревянное судостроение - Кают-Компания "Катера и Яхты" https://forum.katera.ru/index.php?/topic/44186-katamaran-quinto-2/

Катамаран QUINTO 2 https://vk.com/album-29709755_184087222