Multihull Structure Thoughts

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

  1. oldmulti
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    oldmulti Senior Member

    I am often surprised of how much more accommodation is being squeezed into modern cats that can still sail well. Kurt Hughes often designs cats that have a good balance between sailing capability and space. His boats are not like EG the Lagoon 38 which have great accommodation but performance was not high on its list of priorities due to weight and hull shape. Hughes 37 (attached) is 37 x 25 foot 8430 lbs weight (actual structure weight about 5500 lbs) is lighter, has better hull shapes, has deep boards and an efficient rig to help it along. The Crowther Spindrift 37 shown previously in this thread has less accommodation, similar weight and a less efficient rig. I would suggest in a boat for boat race the Hughes cat may win but not by much. Now the real difference between the 2 boats is in the structure. The spindrift was 9 mm plywood decks, ply bridgedeck, all plywood crossbeams and hulls of 225 CSM, 600 gsm Woven rovings 225 CSM inside and outside 18 mm foam. The Hughes 37 is all foam glass construction with 1176 gsm triax 18 mm 756 gsm triax the maximum layup for most of the boat. The Hughes 37 is 5 foot wider than the Spindrift 37. The Hughes has more accommodation on the longer wingdeck etc. I would also suggest that the Hughes with flat panel foam glass construction for most of the boat would take less time to build.
     

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  2. oldmulti
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    oldmulti Senior Member

    For composite hull skins there are many choices from solid ply or glass to sandwich construction. Lets look at sandwich structures. The difference between foam cores (Airex, Corecell etc) and Western Red Cedar (WRC) often depends on $ and availability but occasionally on a "strength" issue. Lets look at WRC versus Corecell. WRC weights about 22 lbs/cubic foot, Corecell from 4 to 9 lbs/cubic foot. In many hulls the core is EG foam may be 18 mm or WRC may be 15 mm thick. The weight of a WRC core will be a least 3.66 times the foam core. In a 1000 sq ft hull that adds 1000 lbs in core weight alone. WRC is "stronger". Depends what you mean by stronger. WRC compression strength is 240 pounds/square inch (PSI), corecell foam 165 PSI. Shear strength WRC 70 PSI, corecell foam 191 PSI. In short glass skins will peel of WRC easier than it will peel from corecell. If hit hard WRC breaks, foam deforms and "bounces back". In short Corecell (or Airex, Divinycell etc) is a better material under most circumstances. BUT you say WRC cores can have lighter glass so the total skin weight will be less. For similar hulls the glass skins on a WRC hull will be about 70% of a foam core hull. So the glass skins on WRC will be about 300 lls lighter than a foam core. Translation a corecell etc will be about 700 ls lighter than a WRC hull. Remember, you can be more aggressive in specs on any hull structure to achieve an outcome. This is just a example of about a 40 foot hull. Next we will talk about the glass that goes onto the core.
     

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    Last edited: Jul 26, 2019
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  3. oldmulti
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    oldmulti Senior Member

    Glass to go onto a core comes in 4 basic formats. E glass (most common) S glass (20 to 30% stronger than E glass in a resin matrix) Kevlar incredibly tough but hard to work and requires epoxy for best results and Carbon fibre (CF) strong for its weight but only marginally stronger than S glass. Timber (WRC) only stretches about 1% before it breaks. Foam can stretch over 6% before it breaks. Why is this important? Because CF only stretches 1% before it breaks. Glass stretches 5 to 6% before it breaks. So if your going to associate materials its better to have glass and foam together or have timber and CF together. But in the real world $'s and weight come into the equation. So people try to use WRC and glass or foam and CF. Its OK as long as you design for these combinations. If you want e glass and WRC you need a lot more e glass to take up the bending load first than you would if it had a CF skin. Or if you use a foam core and CF you have to design the carbon structure to take all the loads as the foam core at best will be a separator. Also another factor is longevity. CF maintains 50% of its strength after 10 million cycles, fiberglass only maintains 22% of its strength after 10 million cycles. In simple terms imagine a hull going thru a cycle each time it goes over a wave. Some real world examples. A racing tube cat 40 x 25 x 5000 lbs x 1000 sq foot of sail. The hull was 300 gsm unidirectional glass 10 mm WRC 300 gsm unidirectional glass. The hull split after 5,000 miles of racing. Next boat a racing tube cat 43 x 27 x 7000 lbs x 1300 sq foot of sail. The hull had 200 gsm CF cloth 285 gsm unidirectional glass 10 mm WRC 380 gsm cloth on inside. The boat is 30 years old and has been raced heavily. The most extreme WRC boat I know is a racing tube cat 43 x 27 x 7000 lbs x 1400 sq foot of sail. The hull is 160 gsm kevlar 8 mm WRC 140 gsm unidirectional s glass and a 200 gsm glass cloth. This boat has survived coastal races in some rough conditions, not something I would recommend for a cruiser. Foam glass boats are common and vary from sensible to extreme. CF foam boats are mainly for racing but are becoming more common in high performance cruising boats BUT they need good design. The weight savings of CF foam are not that great unless you really optimize the structure and use high CF to resin ratio's requiring vacuum bagging or resin infusion. Sorry I am repeating myself. The word document is from Christine Demerchant A good small boat home builder. It covers a lot of information.
     

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  4. oldmulti
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    oldmulti Senior Member

    Kurt Hughes, a multihull designer, has tried to simplify building small to large multihulls for the average person. He had a client say to him "if you need to curve plywood, curve plywood". Kurt created cylinder molding (attachment cylinder molding), where you get a simple mold shapes on a flat floor, put 2 (or 3) layers of thin ply on the mold shapes and glue them together under a vacuum bag. You now have a curved ply panel that can be tortured to hull shape more easily. This technique has been applied to multiple 12 foot to 60 foot plus cats and tris. Kurt then refined the technique to produce an air mold which minimizes the setup and mold material used. The air mold appears to have an unsupported edge but a temporary stringer is attached to keep that free floating edge consistent. The next step in this journey is many large cat builders want wanted cheap hull molds for foam glass cat hulls. As with a tortured plywood A class or Tornado ply cat you can use join the ply at the keel line then force the ply into a deck flange the shape of the gunnels to get a hull shape. The ply is now the shape of the hull but is clear on the inside of the hull. In short if the inside is waxed up it can be used as a hull mold. This is a cheap fast way to get a hull mold. Kurt Hughes site has more detail about cylinder molding. Air molding is listed in his blog. CYLINDER MOLD MULTIHULL CONSTRUCTION https://www.multihulldesigns.com/pdf/cm/CYLINDER%20MOLD%20MULTIHULL%20CONSTRUCTION.htm
     

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  5. oldmulti
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    oldmulti Senior Member

    Modern racing cruising multihulls can be complex structures. If the boat is a genuine performance design then detail can make or break the boat. Some designers design a boat to be easy to build, look OK and perform well. Others designers optimize the performance, make the boat look great and try and make the build easier but that does not mean simple. The following study book will give an idea of a very good design that has been evolved over many iterations. The design details on page 11 top left corner gives an idea of the complexity that is required to hold a high performance boat together. The door edging detail is required considering what is attached to that bulkhead. Wander around the designers site for further information.
     

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  6. oldmulti
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    oldmulti Senior Member

    The cross arms of the TR 36 above are built separately to allow the components of the boat to built independently to be joined later. Good if you want to build away from the water. But the price you pay is the assembly of the boat later which requires a lot of structural reinforcement and very careful design to distribute the cross arm loads. Another older tri the Buccaneer 36 36 x 26 x 8000 lbs with 900 sq ft of sail. You could build the hulls and cross beams independently and take them close to the water for final assembly. The advantage of this boat was the cross arms were a single unit the full width of the boat. Yes the assembly and finishing was harder but the crossarm structure was easier to build and the attachment to bulkheads is simpler. The forward cross arms on a Bucc 36 is 12 mm ply forward web, 19 mm ply aft web, a top timber flange of 8 layers of 140 mm x 8 mm clear oregan, a bottom flange of 7 layers of 140 mm x 8 mm clear oregan. The rear beam is 9 mm ply forward web, 12 mm ply aft web, a top timber flange of 8 layers of 110 mm x 6 mm clear oregan, a bottom flange of 7 layers of 110 mm x 6 mm clear oregan. There are D shaped fwd wing frames on both beams. Attached is a photo and some general arrangement plans for the Bucc 40 which was very similar to the Bucc 36.
     

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  7. oldmulti
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    oldmulti Senior Member

    Whilst we are on trimarans, Wayne Barrett is an Australian multihull builder who has designed several home builder tri's. His web is M90 Trimaran | Build Your Own Trimaran https://trimarankit.com/m-90-stb-screecher These boats combine his building and sailing experience. He has built several production cats, several tris including a 48 foot Newick Traveller and a 31 foot kevlar and carbon racing trimaran called "Trilogy". Trilogy is a Tony Grainger design that Wayne, at the owners request, had an engineer check the structural specs as he built the boat. Result there was a lot of knowledge gained about carbon fibre structures and building technique learned along the way. Part of this knowledge has been used in the M90 cross beams. The M90 is basically a ply timber boat with CF cross beams in a dutch build where the builder choose fix beams. The M90 is 29.5 x 23 foot 4500 lbs displacement with 450 sq foot of sail. The beams have a timber box frame with 16 layers of 300 gsm unidirectional carbon fibre 20 cm wide on the top flange with 14 layers of 300 gsm unidirectional carbon fibre 20 cm wide on the bottom flange. Attached is the M90 and a photo of Trilogy.
     

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  8. oldmulti
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    oldmulti Senior Member

    Rob Denney of Harryproa is an innovative thinker. He has a logical mind and has developed his proa's accordingly. The proa's work. Rob also has developed construction techniques to suit his style of boats. Proas are symmetrical end to end as they can sail in both directions and as a result do not require much rocker in the keel line. The shape of the hulls can also be simplified to relatively flat bottoms to allow very high prismatic coefficients (a lot of buoyancy in the ends relative to the mid section). High prismatic coefficients are desirable for high speed multihulls. A fast mono or cruising cat has a PC of 0.55 to 0.6, a fast multi has PC's of 0.6 to 0.65, Robs proa's have PC's of 0.65 to 0.75. The higher the PC generally more wetted surface, result you need balance of PC versus wetted surface. This allows a simple but sophisticated hull shape to be built in an extremely simple mold. The follow web site describes the mold and "intelligent infusion" http://harryproa.com/?p=1845 Intelligent infusion is resin infusion of dry fiberglass under vacuum pump. Rob also has refined the mold and infusion technique to include bulkhead positioning slots, hatch substructures etc before you infuse the glass. The hull mold can be simple MDF box . The chine corners have a large rounded fillets done. The interior is then waxed. The external glass skin is then dry laid down followed by foam than as cuts in it, then any inserts required are placed in the foam. Then if your brave the internal glass skin is placed dry onto the foam. Attach your vacuum bag, fire up your pump and drag your resin into the mold. Result half a hull made in short time. A more conventional cat hull could be done if you can develop either the hull shape or mold to suit. Interesting.
     

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  9. oldmulti
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    oldmulti Senior Member

    Most designers have there own views on how to build a multihull. Designers often work from different starting points. Example the attached diagram is the proposed hull layup for a 52 foot cat. The majority of the glass/carbon fibre layup is at 45/45 degrees with on 1 layer of glass laid fore and aft. The designer of the layup is an excellent designer who has designed the hull to meet all forces required to hold the boat together and the said boat has done many miles of ocean sailing. A second well known designer criticized the layup saying that there was insufficient fore and aft glass fibres to handle the global hull forces due to the majority of 45/45 of glass fibers in the layup. The second designer was working from the principle of a cat hull needs about 70% of its structure for and aft and about 30% of its structure laterally. As most people have worked out a lot of plywood boats have a 50/50 structure. Aggressive strip plank WRC boats with only glass unidirectionals around the hull has about 85 % fore and aft 15% laterally. Yes the second designer may be correct in his assessment of the layup not being optimal but the original layup designer produces good boats, they may not be perfect but they work well. Trust your designer if they have results to back them up. Finally, triaxial glass cloths 45/45/0 has effectively 66% fore aft strength 33% side strength, good for hulls.
     

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    Last edited: Jul 29, 2019
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  10. oldmulti
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    oldmulti Senior Member

    A small series on 30 foot tri's. This will not be about any specific design but will be a composite of several real 30 foot tri's specs. The first will be a solid glass hull tri (based on 1 old boat) that has aluminum swing arms allowing the boat to fold for docking. The tri is 30 x 20 foot folds to 10 feet. Weighs 4500 lbs carries 450 sq foot of sail. The main and float hulls are solid glass multi chine flat panel construction. The panels are gelcoat the 3 layers of (450 gram CSM plus 300 gsm cloth) for a thickness of about 5 mm. Secondary bulkheads are 9 mm ply and interior floors and bunk tops are 9 mm ply. The main and rear crossbeam support bulkheads are 2 layers of 9 mm plywood. The transom is 2 layers of 9 mm ply. All 9 mm ply is quality 5 ply. There are foam glass ribs every meter if there is not a bulkhead. The flush decks are 9 mm ply. The cross arms are 125 x 75 mm aluminum tubes with a top tube 300 mm above a bottom tube at the inboard main hull end. The tubes meet at the inboard side of the float. The connection between the tubes and the hulls are C section aluminum of 125 mm wide and 25 mm thick. The pivot pins are 25 mm stainless steel at the main hull and 35 mm pivot bolts at the float end. I do not know the performance of this boat, but smaller solid glass tris by the same designer sailed, folded and cruised very well. If there is any interest I will find the hull lines and some diagrams.
     
  11. trip the light fandango
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    trip the light fandango Senior Member

    They're always good reading
    I suppose chines affect the resale value, as you have noted previously, of that tiny fraction of people who aren't time poor, still physically able and can actually build a boat,[the dream]. Making a cheap mould as described in your other posts and doing a lay up in fibreglass with a flare in the transom similar to a Farrier could be worth investigating, not me I don't think. However using tube aluminium cuts out a lot of work so the design of this 30footers beams, housing/connections are intriguing. Reading previous posts it looks like two lengths at 3mm is the minimum wall thickness for a 10 ft span, so 5 or 6mm for a single beam?,.. whereas something like a Hobie 18 mast is nearly 2mm. Can you shed any light on beam wall thickness Oldmulti? thanks again for sharing your knowledge, it's much appreciated. I did check up at an aluminium supplier about 6061 and losing tensile strength with welding,.. they were unaware of it.. I've only tried one so far.
     
  12. oldmulti
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    oldmulti Senior Member

    Next is a plywood trimaran. The trimaran plan is for a 28 foot tri that could be easily be extended to 30 foot. The structure of the tri would be the same for a 30 footer. So I will use the full plans for the Buccaneer 28, a high performance tri with aluminum cross beams. The plans are available at MediaFire https://www.mediafire.com/folder/aaltgc3owsb1u//Buccaneer%2028 If you want to have timber cross beams another tri of the same general size and displacement has a forward beam with fore and aft 9 mm ply webs, 240 mm high in the middle, parallel over the mid 1.5 meters and tapering at either end to the thickness of the flanges. The top and bottom flanges are 2 layers 140 x 19 mm clear oregan. The cross beams bulkheads are 30 mm thick every 700 mm with the cross beam bulkheads at gunnel edges of the main hull and floats 100 mm thick. The rear beam is the same structure but the webs are only 200 mm high.
     

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  13. oldmulti
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    oldmulti Senior Member

    TTLF I have just discovered I only have a paper version of the lines and other stuff. I will get them scanned in and loaded up in within a week. Sorry about the delay.
     
  14. trip the light fandango
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    trip the light fandango Senior Member

    Thanks oldmulti, there's plenty of info here to run with, no rush, gee those Buccaneers are a lovely shape, some owners say the 24 is the best,.. I wonder how much Ian farrier admired them..Ha,. his tri's seem a bit like a sophisticated long development time version..Then I suppose Lock Crowther took some cues from earlier tri's... just the way good design evolves.. by osmosis..ha.
     

  15. oldmulti
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    oldmulti Senior Member

    TTLF Oldsailor who built and owned Buc 24 and Buc 28's also built and sailed Buc 33 and various farrier designs, says the Bucc 28 was the best of them all. He said really high performance with enough room to cruise aboard for holidays. He also said it was as easy to build as a 24 and easier than other boats. From my experience of sailing on the 24, 28, 33 I agree. The 28 can be pushed hard and is fast. Lock developed these boats at the time farrier was optimising his folding system. Farrier only started to refine his hulls and building techniques after Lock had produced the 24 and 28. So you may be right about osmosis.
     
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