Dinghy Laminate schedules

Discussion in 'Boat Design' started by member 14235, May 17, 2006.

  1. member 14235
    Joined: May 2006
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    member 14235 New Member

    Hi,
    I'm going to be making an 14' FRP dinghy out of split moulds. I aim to use vacuum bagged epoxy with glass and some carbon cloth. The deck will be sandwich and the hull may be monolithic for robustness although I will probably glass in foam stringers or possibly really thin (3mm ish) core to stiffen it up.
    What would be an appropriate laminate schedule for deck and hull assuming, say a 10mm core (deck) bearing in mind a bare laser hull weighs in at 59 kg and is made from spray layup CSM and polyester, and for similar overall dimensions I am targeting sub 45kg as a bare hull weight..

    I'm intending to use a thinner carbon back sheet and thicker glass face sheet for any sandwich surfaces.

    Obviously I could go more exotic on the materuials/processes and thicker cores/thinner laminates but I am going for best performance whilst retaining off the beach robustness.
    Cheers
    DannyBoy
     
  2. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    If you don't know enough to be able to develop a laminate schedule, how do you know that your hull should be single-skin, or that 3mm core would be appropriate?
    I'm not saying this just to be a pain (although I realise it looks that way), but you do need to be more specific, and less specific all at the same time.
    Start with what characteristics you want, and work from there. If you want a really stiff hull for speed, then you will have to make do with less "robustness", but how much is enough? THat is the question you must answer, and the info needed to go ahead with the laminate schedule.
    Steve
     
  3. frosh
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    frosh Senior Member

    What is the dinghy going to be used for? Sailing, motor or rowing.
    If so is lightness and performance a very high priority? You should be able to achieve sub 45Kg using a structural core of say 5 to 10mm everywhere and epoxy with woven fibreglass. Use of carbon cloth on the inside will allow a slightly thinner core, and slightly lighter cloth weight, but on the whole hull maybe only around a 5-7 kg weight reduction for the extra cost.
    200 gram/m2 carbon on the inside on a 5mm PVC core with around 300 gram/m2 woven glass outside should suffice the hull. If using glass on the inside layer go up slightly on the core thickness and use 300 gram/m2 inside instead. If you want 10mm core on the deck this will very stiff with even 200 gram/m2 woven glass both sides.
    You can do the calculations by looking up the web site for the foam core to get its density, and calculating the area required and thus determining the weight of foam. Also for the fibreglass or carbon, you work out the cloth weight, allow perhaps another 120% for the epoxy resin and you will be fairly close. :)
     
  4. member 14235
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    member 14235 New Member

    Cheers Guys,
    Obviously speccing up from scratch is difficult/inappropriate so really it is a case of finding out from guys with experience like yourselves what the go is. The boat is a car top singlehanded sailing dinghy, so lighter the better from a performance point of view, but I don't want it to be too fragile. Obviously on a boat this size the limiting factors when specifying a laminate will be local loads due to crew/ impacts rather than global loads (mast step excepted). Frosh's advice is the type I was after - i.e typical laminates for similar sized dinghies. Obviously this will depend on the dinghy, with some (albeit smaller) moth dinghies dow to 15kg ish hull weight but with exotic materials/limited durability. Stuff like the RS 600 / 300 are similar sized boats made from sandwich/vacuum bagged with target weights around that which I would like, so its just a case of getting some empirical ideas. Cheers again, any more info / thoughts would be much appreciated.
    Dan
     
  5. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think 300g glass (on each side) is a practical minimum.
    That's for example used in the cockpit sole of the Yngling, 12mm core + 300g biaxial e-glass on both sides.
    If the core is 1kg/m2, the total will be approx 3.2kg/m2 or slightly more than 6mm Okume :)
     
  6. member 14235
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    member 14235 New Member

    Cheers to all for the info. Does anybody have any idea of monolithic hull laminates as uses on stuff like 470s or finns where sandwich is prohibited. I'm pretty sure I'll be going for thin sandwich hull and thick sandwich deck but any info will help.
    Cheers
     
  7. wet feet
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    wet feet Senior Member

    Since you refer to RS boats of various flavours,it may be safe to assume that Bristol is not too far from the Severn.The information given by your description would not be sufficient to determine the requirements and the fact that you are asking such questions does not convey the impression that you have a great deal of experience with light,durable dinghy construction.Have you produced plugs yet?If so,how much laminate thickness have you allowed for at the hull to deck join?
     
  8. member 14235
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    member 14235 New Member

    Wet feet - I'm not experienced in dinghy manufacture but I'm a naval architect and I've made/repaired quite a lot of stuff in composite. I won't be actually doing the build myself - I'm no boatbuilding pro so I just thought i'd trawl the net to help me to inform my eventual layup schedule. I've had access to various bits and bobs to help me. After inspection of quite a few dinghies (including RS boats) I've accounted for 2mm for both hull and deck laminates at the join where the laminate will have been tapered down to monolithic. This is a bit conservative but as I'm sure you know the actual size of the gap is not that sensitive as there will be a reasonable margin for joining compound. Yes I'm in Bristol Uk not Bristol RI so I am familiar with the severn!
     

  9. wet feet
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    wet feet Senior Member

    There is a convention to balance the laminate either side of the core material.The use of a monlithic hull laminate does not do a huge amount for the stifness of the hull unless you have a laminate of such robustness that you negate the gains possible with the use of carbon/epoxy.Local cored patches may seem like the best of both worlds but as they give the potential for core punctures that a fully cored hull would have,they also give you the worst of both worlds.If you are pursuing performance,and why else would you specify carbon/epoxy,you will need to be willing to cope with the attendant friability.Given that you have set a weight target,and since you can probably determine the surface area of the laminates,how much weight per sq.m would it take to achieve?If you apply the idea of an achievable fibre to resin ratio of 1:1 see if it looks plausible.
     
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