Multihull Structure Thoughts

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

  1. revintage
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    revintage Senior Member

  2. oldmulti
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    oldmulti Senior Member

    After the exotic’s something simple. Slider is a day sailing cat of 16 x 8.5 foot weighing 500 lbs and displacing 1100 lbs. The sail area is 140 square feet. It has 10 to 1 hulls. She has a deep offset daggerboard in the port hull, and moderately high-aspect kickup rudders. She goes quite well to windward, especially considering her sprit-sloop rig, with its home-made mainsail. Slider can be built with 14 sheets of 6mm or 1/4″ marine ply, plus one sheet of 3/4″ and a half sheet of 1/2″. 4mm ply would do for planking the topsides, if glassed on both sides, but stay with 6mm for decks and hull bottoms. The beams are 10 foot 2X6 (50 x 150 mm), and the mast, which was laminated from 2 x 16 foot white pine 2X4s, then rounded and tapered. The only permanent fasteners were 3/8″ bolts holding beams to the beam webs and rudders to the rudders stocks, U-bolts for the forestay bridle, and various stainless screws used to attach deck and rigging hardware.

    Rigging is 3/32″ 1X19 stainless. There are no turnbuckles– shrouds are tensioned via 1/8″ Dyneema lanyards. The forestay is tensioned in a similar manner, but the lanyard is taken aft to the forebeam, so that the mast can be dropped just by casting the lanyard off a cleat. The plans were being sold on the web but that has appeared to died.
     

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  3. oldmulti
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    oldmulti Senior Member

    A simple little cat that plans are still available for. The Simpson 6.5 Woodwind was designed as a boat to “educate” people into many aspects of boat building. The tube cat is 21 x 13 foot weighting 1200 lbs and displacing 1800 lbs. The boat carries a 220 square foot fractional sloop rig that could come of a big beach cat. The mast is 29 foot 150 x 100 mm section with 5 mm diamonds and 6 mm 1x19 caps and forestay. The hull is a combination of materials. Chine bottom is 6 mm ply, sides are 6 mm or 4.8 mm ply with taped chines and glassed with 200 gsm cloth on the outside. 25 x 15 mm stringers in the hull on 6 mm bulkheads with 35 x 15 mm timber edging. The main beam bulkhead is 9 mm ply. The cabin sides are 6 mm ply with the moulded cabin roof 2 layers of 3 mm ply. The dagger boards are 60 mm WRC shaped foils with 600 gsm biax on the outside. The fore and aft beam are 150 mm aluminium tubes. The main crossbeam supporting the mast is a timber box structure. Sorry I cannot find pictures on the web.
     

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    Last edited: Oct 8, 2019
  4. oldmulti
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    oldmulti Senior Member

    Andrew Simpson designed the Shifter 11 trimaran early in his career. Here is his words. “This 32ft (9.75m) trimaran design emerged in 1975 and was the first to be built in moulded glassfibre rather than the more labour intensive custom techniques we had used previously. The boat could be disassembled for transport overland — an important consideration in the US.

    Of all my multihull designs, this was the one that gave me the most satisfaction. She proved fast and well mannered, handling beautifully and with an exceptionally comfortable motion at sea. Unfortunately, the complexities of the construction made it difficult to compete on price in a market awash with cheaply produced monohulls. Only six Shifters were built before production ceased.” The boat was manufactured in America.

    The tris crossarms and folding system are “interesting”. My best guess is the tri is 32 x 21 foot and based on Simpson Wilde designs of the time probably carried 450 square foot of sail. The crossbeams appear to be 100 x 75 mm box section with 4 or 6 mm walls. A metal thick strap 500 mm below the crossbeam at the main hull connects to the crossbeam at the float. The folding system is simple hinges and it would need a winch and frame to pull the floats into the folded position. The boat folded would be at least 10 foot wide. If anyone knows any further detail can they please comment.
     

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  5. oldmulti
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    oldmulti Senior Member

    Off the beach cats can vary in structures a lot. The attached Simpson 16 foot cat is 16 x 8 foot weighs 300 lbs. The hulls are 150 gsm e glass cloth 6 mm WRC and 150 gsm e glass cloth with a 6 mm ply deck. The fore and aft crossbeam tubes are 122 x 86 mm aluminium tubes. The mast is 23.5 foot 105 x 75 mm with 3.2 mm rigging wire. The Viper 5 is 16.5 x 10 foot weighs 300 lbs with 195 square foot of sail. The hull is 200 gsm e glass cloth 5 mm WRC 200 gsm e glass cloth with 4 mm plywood ring frames and stringer reinforcement. 6 mm ply deck. A Dolphin 16 off the beach cat is a plywood racing cat is 16 x 8 foot weighing 350 lbs and 150 square foot of sail that has 6 mm ply chine hull with 18 x 18 mm stringers and gunnel. A 30 mm thick Styrofoam 1200 mm long horizontal panel is in the bow section. The deck is 6 mm ply with 6 mm ply deck beams on edge. The daggercases are 6 mm ply with 25 x 25 mm fore and aft logs on top and bottom. Under beams bulkheads are 6 mm with 50 x 25 mm timber on the top edge. The 2 crossbeams are 50 x 50 mm square aluminium beams. A, B, C class cats vary just as much but can be from plain to very exotic construction. I will do them later.
     

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  6. oldmulti
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    oldmulti Senior Member

    We have spoken about folding cats before but this is the first time I have a study plan of one of the boats. Takeaway is a Roger Simpson design. The cat is 24 x 14.75 foot that folds to 7.75 feet. The boat displaces 2400 lbs and weighs about 1600 lbs and carries 250 square foot of sail. The hulls are strip plank WRC and glass with decks being plywood. The folding cross arms are aluminium frames of 75 x 75 mm box sections for the top bottom and diagonals. When I saw the boat being built on one face of each truss a 3 mm aluminium sheet was welded covering the full face of the truss. I do not know if this was in the original plan. The hinge plates are 15 mm aluminium with 19 or 25 mm stainless steel bolts. The trusses are 500 mm high x 900 mm long. The owners of this boat are very happy with it. Also included are pictures of a Kurt Hughes proposal for a 30 foot folding cat system and a sample of a Russian boats approach.
     

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  7. oldmulti
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    oldmulti Senior Member

    The Breton 7.3 is the cruising non folding version of the Takaway 7.3 folding catamaran. The cat is 24 x 14.75 foot weighing 1600 lbs displacing 2400 lbs and carrying 250 square foot. The minikeels total draft is 2 ft. The boat is strip plank WRC and glass in epoxy hulls below the chine and 6 mm plywood above. 6 mm plywood bulkheads with a 9 mm plywood main beam with top and bottom timber flanges. The underwing is 9 mm plywood. Decks are 6 mm plywood. The hull plywood is covered by 200 gsm e glass. The cat is a compact weekending bridge deck catamaran named because of the "Breton Berth" athwartships in the bridge deck. The hulls contain single berths, the galley and shower/head unit.
     

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  8. oldmulti
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    oldmulti Senior Member

    Simpson Wild started early in trimaran design (mid 1960’s) but they had a very good feel for the concept of tri’s. There emphasis was on good all-round performance especially to windward. The following 2 designs of similar design concept is the 30 foot Calypso and the 23 x 15 foot Shangaans which has a 25 foot mast carrying 180 square foot of sail. The main hull is 7:1. The boats were cold moulded plywood on some stringers and bulkheads, the hulls were probably 2 layers of 3 mm. The cross arms are timber and ply. The photo of the small tri is a boat built to compete in the Round Britian race as Shangaans was refused entry because it was to small. The 26.5 x 18 foot Three Finger Jack carried a 174 square foot main and 196 square foot genoa or a 105 square foot jib on a 31 ft mast. It has dagger boards in the floats. This tri finished 10 th in the 1970 Round Britian race after 17 days within hours of 35 foot plus cats and tri’s. The boat could go upwind in a force 7 and could average 7 to 9 knots under reasonable conditions. The boat was cold moulded and glass covered. The floats had 100% buoyancy. Three Fingered Jack and Shangaans had canvas berths and few shelves for cooking etc but not much else internally but they were simple functional fast fun boats.
     

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    Last edited: Oct 10, 2019 at 8:14 PM
  9. Dolfiman
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  10. oldmulti
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    oldmulti Senior Member

    John Marples DC 3 trimaran won a design competition in 2011. The tri is 27 x 17.25 foot weighs 2400 lbs, displaces 3000 lbs and carries 376 square foot of sail. The hulls use constant camber technology to build the hulls. The hulls are 3 layers of 3 mm ply or 3 mm WRC veneer. The main hull decks are also 3 layers of 3mm ply. The float decks are 12 mm ply. The float deck also has multiple layers of 12 mm ply disc on the float deck at the pivot points of the cross arm structure as well as a strong bulkhead supporting the pivot bolt structure. The internal furniture adds to the boat’s strength with minimal amounts of stringers. All bulkheads are plywood. The forward folding component of the cross arms are 300 x 75 mm thick timber (Douglas Fir or equivalent) and the rear cross arms are 225 x 75 mm timber. The cross arms have water stays under them. The mainhull support structure for the folding cross arms are 2 layers of 75 mm timber. The cross arm timber would be best if it is multiple lamination's to build up to the 75 mm or you will have to get very clear timber that is knot free and has no fractures. All beams are West and covered with epoxy glass for protection. Marples expected 7 knots to windward and 16 knots power reaching. John Marples knows how to design a strong boat that can sail as required. His folding cross arms have worked on designs from 13 foot to 30 foot in many sea conditions and they are very cheap compared to alternative folding system even though they are a little heavier. Build time is also very fast compared to other folding systems.
     

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  11. oldmulti
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    oldmulti Senior Member

    A small series on detail of connections of one component to another. In the old days everything was joined using CSM. EG The buccaneer 40 tri connected bulkheads to hull sides with 2 layers 450 gram CSM with an epoxy bead under the edge of the bulkhead. The CSM layer is heavier than the inner skin of a Bucc 40. Once biax cloths were starting to be used, a bulkhead to hull connection on a 40 foot cat was 2 layers of 350 gsm biax laid at 45/45. This is basically the same weight as the inner skin of the hull. There is a bead of epoxy under the bulkhead edge which has rounded edges. The glass taping the bulkhead to the hull should be at least 4 times the thickness of the bulkhead on either face being attached to. The minimum width should be 75 mm on each face attached to the BH or hull with a second layer of 60 mm on either face to create a taper at the edge. For a bulkhead tapes should be on both sides and all around the full edges of the bulkhead. Treat a plywood bulkhead the same way you would a foam glass bulkheads. This only applies to secondary bulkheads, not main structural cross beam bulkheads to hull joints.

    A deck to hull joint has the same glass weight of biax on the hull on the inside hull glass weight. The outside of the deck hull joint is the same as the outside glass weight. Again the biax should be at 45/45 degrees. The tapes should be at least 4 times the thickness of the deck or a minimum of 100 mm on either the deck or hull side especially if the deck edge is rounded. If there is a timber insert in the hull at the gunnel level the tapes should extend past the timber insert. Next we will discuss ribs.
     

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  12. oldmulti
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    oldmulti Senior Member

    Hull ribs, underwing stringers, deck beams etc are all basically the same structure for a glass boat. They are attached to a major component to provide stiffness to a larger panel. Ribs on old 40 foot designs were polyurethane foam with 2 layers of 450 gram CSM covering the foam onto the hull etc side. The top of the rib had a 600 gsm woven roving “cap” which covered the top and extended down the side of the rib maybe 25%. The next generation of 40 foot boat ribs were again polyurethane foam with 450 gsm biax covering the full rib and onto the panel. The rib then had a 450 gsm unidirectional “cap” running the length of the rib. This again could be done in vinylester. Finally ribs have evolved to Divinycell foam with 450 gsm covering the full rib onto the panel. The rib then has a 400 gsm unidirectional carbon fibre cap running the length of the rib. Vinylester or epoxy, no polyester. According to one study I read from a commercial builder the economics of carbon fibre capped ribs were about equal to s glass but lighter and stronger. In all cases the foam former for the rib needs a neat epoxy fillet where itn attaches to the underling panel and rounded top edges. Attached is a generic rib diagram and fiberglass rib used on a big power boat.
     

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  13. oldmulti
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    oldmulti Senior Member

    This is a new tri design from Schionning. The tri is 49.2 x 35.4 foot displacing 15600 lbs carrying a 4000 lbs payload. The 61 foot mast carries 1300 square foot of sail. The mainhull has a 10.6:1 length to beam and the boats Bruce number is 1.43. Translation, a potentially very fast cruiser. The reason for the design being shown here is some of the graphic’s which show the components required to build such a boat. Example, the full ring frame every 3.25 feet in the float hull. The full bulkhead’s required at the float connection points. The structural integration of the mainsheet track acting as additional support to the crossarm structure. Even the floors, bunk tops etc add to the structural integrity. This boat would be a very large, 5 year plus, project for a very dedicated home builder who had access to a very good building space and temperature control. The whole boat is foam glass and would have layups like (pure guess here) 1150 gsm triax 19 mm corecell 756 biax inside in vinylester for hulls. The crossarm structure would be (again a guess) EG 75 x 75 mm unidirectional s glass or carbon flanges top and bottom of a foam glass web for the full width of the boat etc. Cross arms like this have to be very carefully built to maximize strength and take time to do well. Do not underestimate the work here but a nice design. The PDF is Schionnings advertising brochure. It starts talking about cats half way through but has many good computer images.
     

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  14. oldmulti
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    oldmulti Senior Member

    The Solitaire 1490 was intended to be a production cat but now is available as a home design. The cat is designed by Schoinning and is 49 x 26 foot with a 60 foot mast and 1400 square foot of sail. The boat displaces 26,456 lbs and can carry 7,840 lbs. Schoinning claims it’s a performance catamaran with its 10:1 hulls etc. But his Arrow series of flat panels cats, the Arrow 1500 is 49 x 26 foot carries 1370 square foot on a 62 foot mast but only displaces 19,000 lbs and carries 6270 lbs. The Arrow is 39% lighter than the Solitaire 1490 but carries basically the same rig and similar mast height. The chine hull Arrow 1500 will be faster due to slimmer hulls with lighter weight, also probably cheaper and easier to build. Again the diagrams for both designs give you a good feel for the components required to build such large boats. The PDF’s have more detail. In a short while I will give details of a 50 foot cat structure which will give a feel for the what would be required to build something like this but expect $400,000 Australian and 10,000 plus hours. Finally even with the same designer, analysis all the options available to you and understand your real needs. The 2 boats here each suit a purpose. One more orientated to cruising the other more to performance cruising and both way faster than a big French cat that would have 7.5:1 hulls and 45,000 lbs displacement.
     

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  15. oldmulti
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    oldmulti Senior Member

    Lets have a look at hull panel options for small multihulls. This is general and assumes the boats are cruiser racers of 23 to 30 foot of about 1500 to 4500 lbs displacement. We will do a foam glass, Durakore and Strip plank cedar version of hull weight. There is a light weight or smaller boat layup (1) and a heavier weight larger boat layup version (2). The foam is PVC Corecell, Airex etc. The glass is e glass in all cases. The resin with WRC or Duracore is epoxy, foam is polyester or vinylester or epoxy.

    Foam glass. (1) from outside 270 gsm 10 mm foam 200 gsm (2) 330 gsm 12 mm foam 270 gsm

    Duracore (1) 200 gsm DK 12.7 mm 200 gsm (2) 270gsm DK 15 200 gsm

    Strip plank cedar (1) 200 gsm 10 mm WRC 200 gsm (2) 330 gsm 12 mm WRC 200 gsm.

    The lightest laminate is foam glass (1) at 0.36 lbs/ square foot, the heaviest is Strip plank cedar (2) 1.18 lbs/ square foot. This is assuming that you have a relatively fair hull that did not require excess filler. As you can see the hull weight of a foam glass hull can be half of a strip plank cedar hull. In the real world it does not always work out that way due to the weight timber or glass inserts in foam glass hulls for local reinforcing etc that add weight. Also a hull is only part of the total weight of a boat. EG the mast, winches, ropes, internal gear etc will be the same weight in either hull medium.

    Strip plank cedar can be made thinner EG down to 8 mm in a 30 foot boat and 6 mm in a 23 foot boat and still have global strength but it will be harder to build well and have less knock resistance. Also all hulls are assumed to be round bilge. Flat panel construction can require heavier panels or more internal stiffening according to the design.
     

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