Multihull Structure Thoughts

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

  1. oldmulti
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    oldmulti Senior Member

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  2. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Sailhawaii (who is thinking about a new 42 foot cat). The following is an example of a 46.5 x 23.5 bridgedeck catamaran that displaces 12000 lbs and has 110,000 foot lbs righting moment. It carries 1256 sq foot of sail. The cat is a high performance cruiser that has done many open sea miles and has had several copies built. I will not name the design. The cat has a rear beam, main beam and a forward wing deck beam to form a box to handle the twisting loads. The main beam is 1 meter high with a 25 mm airex foam core. Under high compression area's eg mast a solid timber insert replaces the foam core. The core is doubled at the edges where flanges are attached. The web faces are 4 layers of 17 oz (550 gsm) unidirectionals laid 45/45 degrees on each face for a thickness of about 4 to 5 mm. The top flange is 80 x 50 mm epoxy unidirectional solid glass, the bottom flange is 40 x 50 mm epoxy unidirectional solid glass. The forward beam and rear beam are the same basic structure, that is 25 mm core with the same web faces. The top flange is 43 x 25 mm epoxy unidirectional solid glass, the bottom flange is 43 x 25 mm epoxy unidirectional solid glass. The attachments is an example of the cross beam layout.
     

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  3. sailhawaii
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    sailhawaii Junior Member

    That's exactly what I need to do some weight estimates. Which I will be starting soon. What would you estimate for the skins based on what you have seen from other boats? From what I have seen I'm guessing 13mm foam (M80 or equiv) with 2-3 layers of 400 Bias on each side. My estimates put this about 1.75lbs sq foot for the skin. Do you think that is a good number for an estimate?

    I'm sure I'll get it engineered before I built a full-size boat, but I'm getting to the point where I need to get some weight estimates that are better than 12 to 20K lbs total displacement.

    As a side note now that I'm back from a business trip, I'm starting the second model with a new hull shape and some other things I learned from sailing the first one.
     
  4. oldmulti
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    oldmulti Senior Member

    Sailhawaii. This is a choice of money versus lightness versus ease of build. A will go through a series of options for cats around 12000 lbs weight (displacements add 3000 to 4000 lbs) the foam is an Airex, divinycell etc or Western red cedar (WRC). Glass is E glass woven rovings (WR), biaxial (biax), triaxal (triax) . Kelsall 40 x 20 x 12500 lbs. Skin 2 x 600 gsm WR 12 mm foam 2 x 600 gsm WR. Cowther SP 40 40 x 21 x 14,000 lbs. Skin 1170 gsm triax 25 mm foam 1170 gsm triax. Woods Meander 40 x 23 x 11200 lbs. Skin 2 x 600 gsm biax, 150 gram CSM 12 mm foam 600 gsm wr 300 gram CSM. C106 40 x 23 x 12000 lbs. Skin 1176 triax 20 mm foam 766 triax. C 226 42 x 23 x 13200 lbs. Skin 1150 gsm triax 20 mm foam 750 gsm triax. WRC versions of these boats have 15 to 19 mm wrc with about 640 to 750 gsm triax on both faces. In each case (foam or wrc) the external skin is doubled below the waterline. Thinner foam cores generally mean heavier skin weights. I have found a thicker foam with a single skin is faster to build and occasionally cheaper. All of the above foam boats are polyester or vinylester resin. WRC is epoxy. If you do a skin of 1176 gsm triax 20 mm foam 766 gsm triax, it will weight about 1 lbs/square foot plus filler and paint etc. Probably a total average including double bottom and keel insert plus additional glass strips under bulkheads etc will weigh about 1.2 lbs/square foot. These are home builder layups for a boat around 12 to 14000 lbs weight, not the lightest possible. The exotic layups use S glass, kevlar, carbon fibre etc and can go down to 0.7 lbs/ square foot but the cost can double and its almost mandatory to use vacuum bagging/ resin infusion. Hope this helps.
     
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  5. oldmulti
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    oldmulti Senior Member

    For those who like a simple life the following 2 designs are about as basic as they get. Square hull shapes work as long as they are relatively light. Fine ends work if well designed, but a designer needs to be able to distribute the buoyancy correctly. Mr Toad is a proven design that is cable of more than you think if you want a basic camp catamaran for minimal cruising. 6 mm ply with 200 gsm cloth over will get you a pretty easy to build boat. The second boat is a Phil Bolger special. Phil was one of the most logical, original designers around. Do not assume he produced pretty or fast boats but they often did there job very well. His 8 foot Brick design is a gem. Phil also designed a 40 foot cat called Eagle which is a charter cat in Alaska. Back to Phil's 23 foot cat. Notice the integrated central foldable keel. The central rudder etc. Again 6 mm ply etc will get it built. Mr Toad will really sail well, Phil's 23 foot cat will keep you amused as you work it out.
     

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  6. oldmulti
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    oldmulti Senior Member

    Solid glass hulls are usually only done is small boats but also are done in bigger boats. EG the underwater hull bottoms of Lagoon 38's are up to 12 mm of solid glass. Also earlier Outremer cats had solid glass hull bottoms etc. There are reasons for doing a solid glass hull, but initially I will discuss how to create a simple mold. Easiest is a flat surface which is well waxed and glass laid directly over it. Next option is to actually make a hull shaped mold. You build a virtual full sized boat then a reverse glass mould is taken then you lay fiberglass into that mold for the hull. Painful. Better to build a series of mold frames and then push EG plywood into the frame molds to form the shape of the hull. This can be hard chine or round bilge. The attached photo's are of a 60 foot cat hull mold with 4 mm ply lining. Once you have several layers of glass in the mold it will strong enough to have the full hull weight in the structure. Notice the use of filler to smooth some plywood cuts in tight bends. This mold could be used for solid glass or foam glass construction. More later.
     

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  7. oldmulti
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    oldmulti Senior Member

    We will start with small boats who have solid glass or near solid glass skins. Solid glass is used in smaller size general use multihulls to provide the skin puncture resistance Yes a foam glass racing cat of eg 20 foot could have a 9 mm foam core with 200 gsm cloth either side and structurally work well. But if the boat sat on a stone when beached and you stepped on a hull the skin is likely to be broken. Racing boats are generally treated well. general purpose boats like hobie's are not. Result you need a thicker skin to resist knocks, running aground etc. At a certain point you may as well do a single fiberglass skin. Woods says about 3 oz CSM (approx 2.2 mm thick) is the minimum solid glass hull, or 1.5 oz CSM (about 1 2 mm thick) if its a core boat. Woods uses better layups than all CSM but it give the idea. So a few boats Hirondelle 23 x 12 x 2500 lbs cat 300 gsm CSM 3 layers of 720 gsm triaxle solid approx 5,5 mm thick from waterline down. Trikini trimaran 24 x 17 x 3000 lbs 3 layers of (1 oz csm and 300 gsm cloth) approx 4 mm thick. Scamper prout cat 26 x 14 x 4000 lbs 5.5 mm thick above waterline, 7 mm thick below waterline. Thomaas Firth Jones original Dandy had 9 mm ply topsides, below the waterline it used a 1708 DB Fabmat 3 mm coremat 1708 DB Fabmat for a hull thickness of about 4.5 mm. 1708 is 550 gsm biax stiched to 225 gram CSM. Coremat is a nylon fabric that basically absorbs about 60% of the resin volume as solid resin block. Its used as a cheap heavier substitute for foam. Jones says this layup needs to be rounded to maintain stiffness. Old Sailor in his former life in Canada built a bucc 24 with solid glass skins with a cloth either side of a CSM core that acted as a thickener "sandwich". So solid glass skin will work well for a multi BUT you must plan your internal furniture or provide stiffeners to strengthen the hulls in areas where the hull may flex. There will be more on this as solid glass takes more design to get light and right but its easier to build.
     

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  8. oldmulti
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    oldmulti Senior Member

    Next we will talk about 10 meter glass cats. A builder did a TIKI 30 30 x 16.4 x 4400 lbs with solid glass hulls has a 2(225 gram CSM) + 750 gsm biax + 1100 gsm qudax. The hull has bunk tops, floors etc to stiffen it. The decks were foam glass. Next is a charter cat that has been used as a high performance sport cat. The Rush 10 is a tube cat 33 x 18 x 3200 lbs x 540 square foot. The hulls is 600 gram CSM 820 gsm Woven Rovings 600 gram CSM. There are glass reinforcing strips internally and externally in the keel area. Now the fun begins. There is little accommodation or furniture in the hulls so the hulls are reinforced with vertical foam half rounds every 800 mm covered with 600 gram CSM. There is also a full length stringer half way up the hull. The deck is the same base layup with 25 mm lateral ribs every 400 mm. A 60 mm internal flange at gunnel level joins the hulls and deck together. This boat is light and fast. upload_2019-7-24_8-34-0.png
     

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  9. oldmulti
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    oldmulti Senior Member

    The weakness of design is occasionally designer group think gets them into trouble. In a recent monohull transatlantic race 8 brand new IMOCA 60 foot mono's from various racing designers competed. Each of the mono's had the latest construction technique. A less than 1 mm thick carbon fiber skin backed up with foam carbon ribs every 300 mm or so for the length of the hull. 6 of the latest design, multi million dollar 60 x 18 foot yachts broke. The hulls flexed breaking some ribs which allowed the skin to flex and fracture. Time to stop racing and hope that you and the boat can be saved. One boat that did not break won the race, and another slowed after he heard of the other boats failing. All the 8 boats were either strengthened or given new hulls. In short unless you have good sponsorship dont trust the latest fancy design until some else proves it. The photo is just a sample of an IMOCA 60 footer. A photo showing Gitana 16 carbon ribs build is on Getty Images on forums.sailinganarcy.com page 12 of New IMOCA boats. Sorry i cannot get the web address or photo to copy.
     

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    Last edited: Jul 24, 2019
  10. Doug Lord
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    Doug Lord Flight Ready

    OM, I tried to find more on these failures with no luck-do you have a url or even the name of the race and when it happened?
    All I can find is this: Royal Ocean Racing Club - Giant IMOCA 60 turn-out for 2019 Rolex Fastnet Race http://www.rorc.org/news/news-2019/giant-imoca-60-turn-out-for-2019-rolex-fastnet-race

    And this: https://voilesetvoiliers.ouest-fran...an-vol-1-1a6a7cca-7f06-11e9-b387-adda21f8bad7
    Tidbit from above:
    More and more solid
    Michel Desjoyeaux: "In the early 2000s, there was a real gap in performance, because we built increasingly light boats, thanks to a better understanding of structural calculations and progress on the sampling of materials and their implementation. artwork. Yet efforts continued to increase, Imoca going faster and faster. On PRB in 2000, we were at 33 tons / meter. On Foncia in 2008, we were at 25 tonnes / meter. Today, we go back in the curve, we made more solid, we strengthen, because the efforts are significantly larger and the impacts too. Do not forget that an Imoca today, goes faster than an Orma trimaran of the same size twenty years ago! At the time, the rule was to sleep feet forward. In other words, it's the same thing in Imoca today. " (bold emphasis dl)

     
  11. oldmulti
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    oldmulti Senior Member

    Doug. I am sorry but I cannot copy the address from forums.sailinganarchy.com The specific thread is New IMOCA Boats and at the end of page 11 and most of page 12 they discuss thin single skin carbon carbon fibre rib boat structures. If you type sailing anarchy into google it will bring up forums pages. The copy problem is if you try and copy a getty's image it "disables" your browsers copy function (actual masks the words out) to protect the image from being copied. You need to either restart your browser or machine to get the browser copy function visible again. And yes modern mono's are very fast. Ross Hobson the former owner of Mollyhawk a decade old very fast 40 foot racing tri sold it when the latest 40 foot racing mono's could beat Mollyhawk on almost all points of sail. The latest mono's have huge sail area for their size and weight. Wide beams, movable keels and water ballast have increased mono stability so they they can survive up wind and do up to 30 knots on most other points of sail. Mini 6.5 meter mono's that cross the Atlantic can average of 12 knots and peak at 25 knots. Modern open class racing monos will embarrass most multihulls.
     
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  12. sailhawaii
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    sailhawaii Junior Member

  13. Vantage475T
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    Vantage475T Vantage475T

    oldmulti,
    This thread has made for some fantastic reading.
    I've built some small ply/glass thingys like a 21 foot kayak trimaran and am now looking to do a 16 foot tri as a basically larger Weta as I found them a bit small and slow. I want something I can sail myself or 2 up and do some solo camping/raid type stuff.
    I love speed and want it to be decent so I'm looking to use the rig off my Spitfire F16 cat (tough to single hand and wholly impractical for any longer trips really) and am interested in using a foam/glass build so I can get some nicer shapes - my wife hates my lashed up isoskiffmaran test bed and refuses to go out on it, especially since we completely smashed it in half in very heavy weather...
    Anyway, I am just wondering if you had any thoughts on the thickness of Airex T92 (as recommended by the supplier) to use in the floats / main hull and the glass weight / layups for the new project?
    I've started a thread here with some rough shapes that are evolving and will be looking to make a test float on that to see how a taller, larger volume hull compares.
    I'm perfectly happy to bin it once done as it is just a test before moving on to a final evolved shape but it will help me get to that final shape.
    I'll be going to test sail an Astus 16.5 next month in France but I can't really justify the cash (wife has just spent it on a new kitchen anyway) and enjoy the building/tinkering bodging process.
    Any thoughts you have would be most helpful and gratefully received - the point being I will test it to destruction anyway.
     
  14. oldmulti
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    oldmulti Senior Member

    Vantage. Your done a lot of research and I can only give some directional hints. I am not a boat designer. The Astus looks a good boat but has limited accommodation, so I will assume you want speed over internal space. This means a weight of 200 kg or under so a relatively light weight structure. Now the choice. How much money do you want to spend and how long do you want it to last? If it really is a test boat build it in 4 mm ply with 200 gsm glass on outside. If you want a longer term boat then thicker foam and thin skins will be lighter but less resistant to knocks. A 16 foot foam glass boat boat could be 200 gsm 10 mm foam 200 gsm BUT you will have to build it well as skins this thin can be porous unless well filled and painted correctly. A class cats have hulls of 200 gsm cloth 9 mm corecell 200 gsm cloth, the most extreme is 165 gsm kevlar 5 mm foam 195 gsm cloth inside with 8 mm foam around daggercase and main beam. A more conservative 16 foot boat is the Scarab 16 (attached drawing and material sheet) is 400 gsm double bias 10 mm foam 400 gsm double bias. This hull layup is used on Scarabs up to 22 feet. The Scarab 16 folding beam is a simple approach.The plans are also cheap at $150 australian. This boat probably has to much accommodation but the plans will give you a real lead as to how you can built a light long term boat. The hull shape (eg round the bilge chines) and cabin could be modified (eg cabin could be lowered and sides narrowed to the main hull gunnel line to reflect more of what you want. The Scarab 16 weighs less than an Astus 16.5. The Spitfire 16 rig will work. The Scarab 350 is not what you want but is a nice small building guide. Also the Scarab 16/18 build guide has been posted earlier in this thread. I hope this helps.
     

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  15. Vantage475T
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    Vantage475T Vantage475T

    Old multi, thanks those are really useful guidelines for me work against - I will lash up a test float and see how I find the process and if it seems worthwhile.

    I'll do the float in 200/10/200 so reasonably light and then beat the crap out of it and see how it stands up before I spend any real money or time on the main hull.

    I can easily do it in ply if it seems pointless/too hard/expensive and I could just then look at 400 below the waterline and deck/seating areas with higher load.

    The folding element of the scarab is good - I've already done a few lash ups in the garage to trial some folding geometry here and here which took a few minutes so that end will be no problem.

    Yes, I'm not looking for any cabin/cuddy as pointless for us - I'd rather have a quickly collapsible tent that actually has some space in it across a tramp and the main hull, will weigh only a few pounds and can be left at home when not needed.
     
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