Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

  1. oldmulti
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    oldmulti Senior Member

    There was a request for more about Team Scarab boats. Ray Kendrick sells all his plans for $150 Australian. This is for any size of boat from his 16 foot tri to his 32 foot tri. They are well drawn and detailed. Rays plans are a bargain and his boats sail well. He or his offsider Fran Sneesby have personally built almost all the designs so they have been well debugged. There are many of his designs that have been home built and sailing. A few blogs describing the builds are on the web the best being a Canadian who built a Scarab 22. My suggestion if you wish to learn about the structure of these boats buy the Scarab 22 or 650 plan, as they can be built in ply or foam glass and has similar specs to the Scarab 18 et al as Ray has a minimum skin thickness, he likes to go on most of his tri’s. This is really cheap design and build information.

    I thought I would focus on the folding cross beams of the trimarans. The reason is they all have the same basic architecture from the 16 to the 32 foot tris. The dimensions and materials differ for each size but the concept is the same. All the jpegs are from the Team Scarab site and I will not be giving any specific structural details.

    The concept of the cross arms is to build a solid fiberglass (there is minimal foam in some) cross arm structure in a U shape with flanges on either side of the U. The fiberglass basic structure is done in biaxial cloths then unidirectional are laid at the top of the U and along the flanges of the U. Metal cross arm components for folding are fabricated and then with appropriate timber and glass inserts attached to the beam. The main hull has 2 “ring” frames inserted in to handles the inner end if the metal cross beam components for the folding mechanisms. The jpegs give a clearer picture of the components. The first set of jpegs is about the female molds for the fiberglass U sections. The next jpegs are of a glass U section. The next are the ring frames that are in the main hull for the cross arms to attach to. An finally the metal components to connect the cross arms to the main hull. Notice the jpegs are for 18, 22, 650, 32 foot tri’s but have a very similar concept.
     

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  2. oldmulti
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    oldmulti Senior Member

    This came from a 2009 blog where Ian Farrier replied to some people suggesting plywood was an appropriate material for boat building.

    “It is interesting that ply is still being considered for use in 2009, and being a ply fan from way back, I have occasionally revisited to see if it could be used again in some way. Ply is nice to work with, available everywhere, and a very familiar material to everyone. I also considered a multi chine ply option for F-22, even got the CAD software to plot the panels, but after spending a considerable time on this reality won out once more. There are numerous reasons why 95% of the worlds boat builders and designers no longer use plywood. Nice stuff, but it is labour intensive, it rots, and it is heavy. Just no longer competitive against more modern materials.

    Fiberglass foam core certainly costs more, but besides the base foam core panel being 10 - 30% lighter, it can also be two to three times as strong/stiff, so it does not need the extra internal support bulkheads or stringers that ply requires to achieve a similar structural standard. Foam core can also be easily formed into much greater compound curves, which makes it even stronger/stiffer again.

    There is also no need to fiberglass tape all those plywood join seams, which adds even more weight and time. Ply then needs to be epoxy saturated inside, along with a coat of fiberglass and epoxy on the outside to protect it from rot and the weather, all of which adds more cost and weight. Foam core can also use the lower cost and faster to use polyester resin (as used by 90% of the worlds fiberglass boats) with no need for any expensive epoxy. So after spending all those many hours taping seams, adding the necessary extra support, and then fairing in all that seam tape, you end up with a boat that is heavier, can rot, and you only saved maybe 3 - 5% of the overall cost….

    Even worse, when you come to sell it, you have to use 'plywood' in the description, and price is going to have to be dropped by 30 to 40% in order to get any interest. That 3 - 5% initial saving is not looking so good any more. 'Clued up' buyers of trailerable multihulls will also want you to demonstrate just how long it takes to assemble, so not only do you have an expensive plywood boat, you actually have a labour intensive way of going sailing as well. But at least you saved that initial $2000 to $3000.”

    Ian Farrier Farrier Marine
     
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  3. catsketcher
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    catsketcher Senior Member

    Interesting you put that piece from Farrier just after the one on the Scarab, the very design I suggest he is pointing his finger at.

    I agree with Farrier to a point. I like ply when it does not need to be supported by stringers - then I switch to something else. So the sides of the cabin on my 7.3 metre folder are ply and do really well but the rounded bottom and deck are foam. The interior and bulkheads are ply and much faster to build than foam or Duflex because foam and Duflex require much painful edge routing, filling and sanding of each external cut edge.

    I woudl agree with Farrier on multi chine construction. It just seems unnecessary with easy foam options and hard lines are much harder to fair than a soft curve, so a multi chine hull will be much harder to build than a round bilge one. I don't get why anyone would build a Scarab. It took me about 2 days to foam up the bottoms of the 7.3 hulls with vertical strip foam. And it was easy to fair with no sharp lines to demonstrate any lack of fairness. Lines are a real bugger when fairing.

    So consider ply, 4-6mm will do well for lightweight interior furniture verticals, and does not need glassing when inside. But don't go multi chine with ply, it will be harder than round bilge in foam or cedar.
     
  4. oldmulti
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    oldmulti Senior Member

    Catsketcher pre-empted my second post on Farriers blog comments. This is from Mike Leneman the designer of the L 7 trimaran. PLEASE read about the L 7 cross beams at the bottom of the response they are very simple and effective for a smaller tri or possibly a cat.

    “Semi-long post....... First, it seems to me there are at least two things to consider when coming up with a hull lay-up strength to take the loads of sailing and puncture resistance. Puncture resistance is almost a constant and doesn't vary much with the size of the boat, where as overall strength needed of the skin does. On small boats, once you have your butt covered as too puncture resistance you pretty much have a skin that will carry the sailing loads.

    Because of this and the good puncture resistance of plywood, small boats are often lighter than built from foam glass. The L-7 (final design) sleeps two adults, has a big cockpit, plenty of float buoyancy, it's 23.5 feet long and 16 ft. wide, has flat tramps, folds horizontally and is faster than an average F-31. Heck it regularly beats the F-25 C locally and has even beaten the Multi 23 in two out of four races on the Berger Series. It weighs 1300 lbs. empty........note that the Sprint weighs a lot more and the F-24 have been weighed in at over 1800 lbs. empty.

    What we use is Okoume, 5 ply, 6mm plywood ( BS 1088 standard) with 3.1 oz. satin weave, aircraft glass on both sides of the plywood with WEST System epoxy. We do not need a "fill coat" with this flat weave glass and can roll primer over the layup the day after we glass with no sanding. We sand the primer and paint with LP.

    For comparison the glass samples we used the layup and foam suggested by Ian Farrier on the F-82, which was the only info we had at the time (it was over 3 years ago)

    Note that I also have a 24 year old, cold moulded 39 ft. catamaran (race cruise with cabin, head, galley etc.) that was second overall on the multihulls on the race to Ensenada and we won on corrected time, it's also a very light boat.

    For one-off boats, backyard built, it's hard to beat the weight and saved time of using plywood. The trick on the L-7 is that the below waterline shape is moulded glass/epoxy and the above waterline shape is bent plywood…”

    Cheers,

    Mike Leneman

    Mike Leneman designed Minette. Launched in 1984, Minette is a demountable, trailerable, 40 ft. catamaran made from cold-molded spruce veneer hulls and a composite and plywood cabin. Four bolts and 2 pins and the boat comes apart. On the racing end, she is the most “winning” multihull in California and at one point held the course record for the Santa Barbara – King Harbor race for over a decade. Today, after 30 years of sailing she is still winning races and going strong……a testament to wood/epoxy construction and advanced design.

    After this he designed the L7 trimaran a 23 x 16.5 foot compresses to 8.33 foot wide, weighing 1300 lbs. The mast is aluminium 178 x 95 mm LeFiell OM1C oval section, 33 foot long then replaced by a custom 178 x 76 mm section carrying 370 square foot of sail. He built fiberglass epoxy shoes for the main hull and floats. Attached to the shoes were 6 mm ply hull float sides covered with 100 gsm satin weave cloth on either side and foam glass decks. The bulkheads are plywood. Now the brilliance of this design. The cross arms are off the shelf pultrusion fiberglass I beams and they slide pass one another for folding. The fiberglass pultruded I-beams have the sheer web of the beams cut at the float ends and the caps are bent down to make a nice looking, tapered, outside shape to the beam. The beams look good, they are pre-made, they can't corrode, they are strong, you can paint them any colour you want, and they are inexpensive. Jpegs below are Minette 40 foot cat then the L 7 trimaran, the final Jpeg shows the shape of the fiberglass I beams in the bottom right corner.
     

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  5. oldmulti
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    oldmulti Senior Member

    For those who would like to experiment with foam glass but don’t want to spend a lot money or time the following design may help. The Scarab 12 is a small trimaran 12 x 10.5 foot which folds to 8 foot with a 17 foot mast carrying 80 square foot of sail in the main and jib. This boat is an entry day sailor that would be a fun machine. The tri is built from 6 mm foam with 200 gsm plain weave cloth on either side in polyester resin. The hulls are roundish bilge. There is choice of cross beams. Either aluminium tubes that slide inside one another to reduce beam or there was a model that had molded folding beams. The jpegs are of the yellow Scarab 12. The jpegs of the green and white tri is a Scarab 10 built by Fran Sneesby which has a similar structure and beams. Plan price is $100 Australian.
     

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  6. garydierking
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    garydierking Senior Member

  7. oldmulti
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    oldmulti Senior Member

    Rod MacAlpine-Downie produced many interesting designs in the 60’s through to the 80’s. They all had a few characteristics. If they were cruisers, they had internal space secondly, they could sail. They may not be outright speed machines but they could as one reviewer said “this block of flats could get along quite well”. So we will discuss 2 associated designs. The Arapaho and the Cherokee.

    The Arapaho was a home build design 36.5 x 16 foot full bridge deck cat that displaced 8660 lbs at LWL. The masthead rig has about 600 square foot of sail. The hulls have a length to beam of 10:1 and a prismatic coefficient of .608. A prismatic coefficient is the measurement of how full ended a hull is. A PC of 0.6 generally means the hull has full ends meaning less pitching and a higher speed capability. A PC of 0.55 means finer ends which can pitch more and less speed potential. This is a whole topic which we will discuss in another item. The cat is an enlarged version of the 30 foot Iroquois cat. The hulls are 4 x 3 mm veneers for a 12 mm thickness. The rest of the boat is mainly 9 mm plywood. All the stiffeners, deck beams etc are spruce or equivalent. The mast beam is a mild steel triangular steel box structure as in the Iroquois. The cat is held together by several full with bulkheads and a rear box beam aft of the cockpit. This cat is relatively light for its size and type with relatively good sail area.

    The Cherokee was a similar sized design but was a production cat which has more accommodation, weight and displacement but very much of the same concept. The Cherokee is 35 x 16.5 foot displacing between 11000 to 13600 lbs and carrying 615 square foot of sail area. The hulls have a length to beam ratio of 7.8:1. The cat is mainly solid glass with a balsa glass sandwich deck and cabin structure. The Cherokee beam may be narrow but the displacement ensures stability. This is a floating block of flats that does sail well according to reviews I have read. The jpegs give an idea of the interior.
     

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  8. peterAustralia
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    peterAustralia Senior Member

    A short query about the slope of transoms in modern multihulls. It seems almost all slope forward, I assume this is to reduce weight/windage for a given waterline length? Just wondering if there is a downside in a following sea with waves breaking over the stern, perhaps in this situation a transom sloping the other way would be preferable? Perhaps this is a rare event because the speed of a multi downwind reduces the apparent wave speed. Was thinking along the lines of Wharram and how he slopes the angle of the transom on his boats aft.

    I.F. You are probably right about foam core better than plywood. For the uninitiated building a ply boat seems straight forward, yes I can do that. Building in foam core seems hard and daunting, I dont think it is like that in reality, having recently seen a couple videos and it does not look super complicated, but for many that have not built with foam it can look hard from the outside

    As per everyone else, excellent thread, thankyou.
     
  9. oldmulti
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    oldmulti Senior Member

    PeterAustralia. First question, the sloping transoms can be for aesthetics, weight reduction etc but another reason is more important and relates to the comment about following seas. Multihulls for good upwind performance need reduced pitching. Full ends provide reduced pitching. But if you have a very full buoyant stern and a following sea lifts the stern up you can nose dive. A way to solve this is to have a wide low buoyancy stern. A stern step is a start but if you slope the stern forward to the deck you reduce any buoyancy build up. And finally if the wide stern is shaped correctly the wave landing on it will help keep the nose up. Colin Archer et al claimed waves could slide past pointy sterns downwind, this applies to slow heavy (mainly monohull) boats. Modern multihulls can run downwind under control very well, even in storm conditions. If conditions become critical multi’s deploy a parachute of the bow and sit there.

    I have built plywood and foam glass cats. I can assure you a plywood boat can be easier up to the point of having to reinforce the ply with stringers, frames, keels etc. Finding good quality ply and timber at a reasonable price is also getting harder. Your local Bunnings may have BS 1008 marine ply but is it produced to the BS 1008 standard applying in Britain or is it manufactured by the Indonesian BS 1008 company that puts its brand name on a lower quality of plywood. Long lengths of clear grain Douglas Fir (oregan) etc are hard to find then you have to apply an epoxy covering for a longer life span.

    Foam glass comes in 2 versions. Old style where you built a timber mold then laid your foam on top then laid you glass then you faired the hull then gelcoated or painted the hull then took out the timber frame then glassed the inside. You in effect fair (smooth) the hull components 3 times. This is painfull compared to a plywood hull. Modern foam glass is different. You get a big flat surface, wax it, put a gelcoat layer if wanted place down glass (dry) put on foam put a vacuum bag over it turn on a pump to pull the air out and pull resin into the part wait 2 hours and you have a side of a boat hull done fully faired ready to place into a frame along with the other flat panels. Then you lay an inside skin in it all and you have a hull. This description is much simplified. Look up flat panel boat building, vacuum bagging and resin infusion for more detail. In short modern foam glass can be a shorter build time than plywood once you are experienced. Do practice runs etc. or build a small foam boat to get a feel. But if you are a natural with wood, stay with a ply and wood build. Hope this helps.
     
  10. catsketcher
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    catsketcher Senior Member

    Peter

    A dinghy is almost always easier to build in ply, when the stringers are needed it becomes more of a pain.

    Sterns slope for a number of reasons. Most rudders today are underhung so you do not need a vertical transom to mount a rudder on. There is a alos the amazing concept of steps - steps on a cat are so great that they are a huge selling and safety feature. I can get on board my 38ft cat easily from the water. Most mono sailors can't.

    There is some very tricky stuff about wide sterns and cats that low a cutaway transom can help with. As Old Multi mentioned a wide stern can be okay but it is problematic when the hull is pressed down - it pushes the bow down. Lessening the above water volume can get you a little of both worlds, especially in a following and quartering sea.
     
  11. oldmulti
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    oldmulti Senior Member

    Here is an Atlantic Proa that Newick designed in 1982 after the 1980 Ostar where several proas failed to finish. He wanted to design a proa that could be a good performer in short handed events and was accessable to the common man. The proa was 44 x 23 foot, displaced 4000 lbs and carried a total of 650 to 790 square foot of sail on it’s 2 wing mast schooner rig. This design followed on from Newicks 1979 design for Nick Clifton’s Azulao 11 Atlantic Proa which is 42 x 20 foot weighed 3000 lbs and carried 630 square foot of sail in a schooner rig. Notice the 44 footer has more beam, is heavier in displacement and carries more sail area. Also notice the slightly fuller hull shape in cross section and in the ends on the 44 footer compared to Azulao 11. This may have been to get better speed in a seaway due to less pitching.

    The later 44 foot proa design was built using constant camber hull shapes of 4 plies of 2.5 mm WRC or spruce covered with 330 gsm cloth and epoxy resin. There is a keel, gunnel and single stringer at the waterline of each hull (which are identical in shape). The accommodation pod is constructed of 6 mm ply on the outside 12 mm vertacell and 3 mm ply on the inside. The crossarms and masts are timber covered with glass. As you can see at least there is 2 “berths” in this boat and some storage. Not luxury but functional. I know a smaller version (32 foot) was built and raced lasting quite a few years. I cannot find any reference to the 44 foot being built or raced. I am still trying to find the study print I have of Newicks 60 foot Atlantic Proa intended to be a cruiser racer. It has some real accommodation and would have been fast.
     

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  12. oldmulti
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    oldmulti Senior Member

    Tortured plywood design are a love hate relationship for me. The love is if well done they can produce some very fast light weight boats. The hate is it can be a fast build technique if you have the right materials and you are a builder who can do things by eye and adapt as you construct a hull. If you have the wrong materials, design or attitude it can go wrong very quickly. As Ian Farrier said in his first published article about the Trailertri 18 he said floats would be tortured ply. He built 3 tortured float shells, one exploded as it was being folded, he then redesigned the floats to sheet ply. So we will do a small series on tortured ply. First item will be beach boats, then we will talk about larger cabin boats. Gary B please correct me if I get anything wrong about your boats.

    It all started when a guy called Davis folded up a canoe from a flat sheet of plywood. These boats built from the same plan ranged from excellent handling to a problem if the builder choose the wrong ply and did not adjust things as they built. Next came the most famous tortured ply catamaran, the Tornado designed in 1966 by Rodney March. The design is still racing 50 years later and has been built in many materials but the tortured ply boats are still semi competitive. At the same time John Mazzotti designed the 18 foot Unicorn cat which uses the same tortured ply approach. The Tornado Unicorn description PDF describes the 2 boats and the build approach. The Tornado plan build instructions PDF describes exactly how to build a Tornado with details of the hull cross beams etc. 4.5 mm ply cut to a very specific shape stitched together at the keel line by copper wire then glassed at the keel line. The hull is then forced into a very specific shaped deck flange which compounds the ply into a hull shape. A deck and appropriate reinforcement you have a hull. Good.

    Now the down side, you do lose some control over hull shape, the cost of a good quality plywood hull is not that much cheaper than other materials in the total cost of a build, the hull material thickness is 3 to 5 mm in smaller boats which means that they need to be treated carefully in use.

    There have been many tortured ply designs since in beach cats, trimarans, foilers, proas and a few monohulls. To give you an idea Gary Bagient has a 6 meter tri/foiler built of 3 mm ply with a light glass outside. Groucho is 30 years old. Gary also built a 18 x 7.33 foot monohull day sailor in 4 mm tortured plywood. There was a few Sydney 18 foot racing dinghies built of 4.8 mm ply with a few darts. The Taipan 16 and 18 foot, Blade 16 foot beach cats Building a Blade F16 :: Catamaran Sailboats at TheBeachcats.com https://www.thebeachcats.com/pictures/?g2_itemId=11955 etc are other examples.

    But the most fun tortured ply boat, done on the cheap, is a 19 foot proa with asymmetric hulls built from 7 mm 3 ply pinus radiata plywood. Te Wheke at 19 feet will take 7 sheets, including ama. 2 litres of polyester resin $38 per litre, about 6 tubes "no more nails" and 1 bottle gorilla glue (polyurethane) . 200 screws (or more) stainless steel square drive. The compound ply worked on the asymmetric hull. The hardest part was keeping the bow pieces from exploding out. Had to build a stop on the end of the jig as copper wire stitching just gave way. The stems were going to be vertical but the pressures involved gave a slightly curved stem by the time it was finished.

    The builder on the next one would place a shaped stem piece glued and screwed to leeward side, before placing the bulkheads and tortured windward sides in place. He had trouble inside in the very narrow ends getting resin and tape in place. With this in place would assist fixing down windward side. Te Wheke uses bamboo cross beams and a simple sail setup. This 19 foot boat cost under $1000 dollars by using cheap materials. A fun day sailor proving if you are willing to experiment tortured ply can be used on many designs. The jpegs and PDF’s will give a clearer idea. The final PDF is Russell Brown's experimental model build of a tortured plywood model, informative. Also Gudgeons boat building book has several pages on building tortured plywood designs.
     

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  13. oldmulti
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    oldmulti Senior Member

    Following the beach cat tortured ply success many people decided to try the approach on larger boats. I was going to talk about some 28 foot tortured ply cats but I will take a slight detour to Gary Baigent, a NZ guy who thinks an average day is build a boat, assist in writing a book, do a magazine article, keep up with a blog, do 8 hours of work and then get bored with all his free time. Gary thinks a cruiser is a 36 x 37 foot trimaran with foils and wing mast that weighs 1500 lbs ready to sail. Yes, I said 1500 lbs. The construction is tortured 4 mm ply with a light glass layer over. This boat is over 30 years old and its main hull came from a Bamboo Bomber (Supplejack more on this boat in a later post) cat design that got wrecked on a beach. Now Gary though this boat was overweight so he decided to build a lightweight tri foiler El Cid (now Sid) of 27 x 26 foot that weighs ready to sail 530 lbs. The boats design and build progress is on Alternative to marvelous Buccaneer 24 https://www.boatdesign.net/threads/alternative-to-marvelous-buccaneer-24.32382/

    Sid is basically built as follows according to Gary “I make a simple strongback for building upside down - and for a narrow multihull that can be just one long plank with some crosspieces in the hull's widest mid section areas) then fit the three or four main bulkheads/ringframes, plus the stem and transom to the strongback, then run the keelson, gunwhales and stringers (plantation white cedar in NZ) over and around the heads/frames, these set in and glued into slots cut into the frames. So there is quickly the skeletal shape of your hull. Working on your own makes it difficult to handle the full length, scarfed 3mm klinki hull sides - so I just work a couple of opposing lengths/sheets/sides at a time, that is, glue and staple to the keelson and when it is cured, bend each length round the bulkheads/frames and glue/staple to the gunwhales. Oh, yes, I coat the inside ply with epoxy just before I start bending it, then cove the joints with thickened glue. Working with two or three people you can do the two complete hull length sides at one time ... but you have to work fast and accurately. But on your own, just a couple at a time, works okay. Each ply length is already scarfed both ends of course, so for the next length scarf connection, I do a dry run just stapling, no glue, and pencil mark the correct positions- then glue/staple to keelson etc. coat the interior and move on. After all this is completed and cured, cut the excess ply from the roughly shaped panel sides to the gunwhales. Although this method is not as magical as true tensioned ply building, like the Tornadoes were/still are done, it is sort of halfway between and more suited to one person working on his own. Of course it is a lot easier with two or more people.”

    These boats sail well and have been progressively developed in structure and rig over time. They all sail faster than the wind in many conditions. More to come in the next post.
     

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  14. oldmulti
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    oldmulti Senior Member

    Gary Bagient made further comment on Sid 3mm plywood hull is mostly sheathed in 200gsm box weave glass sheathing and uni directional carbon in high load areas. Just 200gsm box weave, glass cloth - but there is uni-directional carbon in loaded interior areas, like mast base/wing beam connection on the double ring box frame, forestay bulkhead, rudder box and cockpit loaded areas, above and below decks. Some parts of the wing crossarm have 4 mm ply in it. Sid is lighter (maybe all up 240kgs) than Groucho's 650kgs - but Groucho is a monster, longer, has a high Bruce Number.

    Gary then built the 18 x 7.33 foot Cox's Bay Skimmer (monohull) was built in tensioned 4mm ply over frames and a backbone. The skimmer has 2 wing masts that are only 6.5 m tall, around 400mm chord, but obviously enough windage to haul the whole lot over. He also built a 8 foot dingy from 2.8 mm tortured ply for an older lady, it weighed 30 lbs.

    Gary was also during this period asked by a friend to design him a trimaran. Three Devils was the result. It is an alternative to the Buccaneer 24, not an extension of Sid. Three Devils being larger (but not by much) main hull and with conventional beams, long floats layout, conventional wing mast/soft sail rig; foils in the long floats; still 26 x 26 foot (length and beam) built in 4 mm tensioned ply due to less curvature in its sides. He was aiming for a fast but more comfortable boat and constructed in basic materials, usual story. The wing mast is 11.5 -12 metres (measured from mast base) for a high aspect ratio main, I think that also is too high for the rule; there will be no motor. If there is a minimum weight for the 8.5's, Three Devils will be, for sure, way below (less than half) the average 8.5m racing class figures. I would figure 600 lbs overall platform weight. The jpegs of the hulls are below with the main hull being modified later to provide more accommodation. This would have added at least 100 lbs to an overall weight of 700 lbs. Gary’s comment, it was now a 3 person boat, I person in the cockpit and 2 people sleeping below! Hmm a potential cruiser?

    He also had a request from a French guy for a Round Britain Racer trimaran 36 x 36 foot. Gary wanted it to be light weight so it was proposed to be done in carbon fire and foam.

    All I can say is Gary version of a heavy boat is everyone else’s impossible dream of light weight. 26 foot boats that can actual sail and weigh under 800 lbs including crew is spectacular. This is C class catamaran all up weight territory. The boats are bay racers but Groucho has lasted 30 years and has been sitting on moorings for a lot of that time. Gary has proven that you can build a relatively light simple boat out of simple materials that will provide you a lot of fun.
     

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  15. oldmulti
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    oldmulti Senior Member

    We will now talk about larger tortured ply catamarans. Malcom Tennant did several designs but the first real success was the Bamboo Bomber 32 x 18 foot open wing deck cat that displaced 2400 lbs and carried 556 square foot of sail on a wing mast rig. Gary Bagient built one called Supplejack. Gary’s cat had a 4.5 mm tortured ply hull with the outside lightly glassed and some high load areas inside as well. In late 1970s building the cat he didn't know about carbon, or he couldn't obtain it. However he used uni directional glass tows for chainplates; that was radical in those days, no metal. Gary initially built the beams as specified but moved them from inside the hull to on top of the decks when the boats speed and nose dipping tendencies were realised. This was a fast boat. Tennant went on to design other tortured ply cats which resulted in Redshift a 32 x 20.66 open wing deck cat that displaces 3300 lbs and carries 810 square foot of sail. Depending on the source it either has a 4.8 mm or 6 mm tortured plywood hull. I back 4 .8 mm with a light glass cover. Illegal Alien is a Redshift design and it is fast.

    During this time the 8.5 meter (28 foot) multihull racing class was developing in NZ. Many people wanted to jopin the fun but could not, would not build high tech boats due to cost. Result many saw what had been done by Tennant tortured ply cats and decided they would go that path. John Tetzlaff a spar maker built “This way up” a 30 x 20 foot open wing deck cat that weighed 1700 lbs and carried 500 square foot of sail. It had a 5 mm gaboon tortured plywood hull cover with 200 gsm cloth. After the keel is glassed the ply was tortured and 3 permanent bulkheads installed with a timber gunnel and stringer at about the waterline. Some foam ribs were installed. The deck was strip plank WRC.

    When the 8.5 meter class was introduced John decided to build a new boat to fit the rules. Attitude was the result a 28 x 17 foot cat weighing 2000 lbs displacing 2750 lbs. carrying 450 square foot of sail in the main and fore triangle. The hulls are 4 mm gaboon plywood covered with 300 gsm cloth. There is a cedar gunnel and a stringer mid way up the hull fore and aft. A foam glass ring frame is paced about every meter inside the hull. The decks are foam glass. The cross beams are aluminium. The mast is 155 x 100 mm. Attitude is not only a bay racer but has like Redshift completed the Coastal Classic race on several occasions. These boats can take rougher conditions but they are not ocean crossing cruisers.

    The jpegs give an idea of Supplejack and Redshift. The PDF gives an excellent description and photo’s of Attitude. Next the final in this little series with a few larger boats and a designer/surveyors thoughts on tortured ply.
     

    Attached Files:

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