Preliminary Scantling for Carbon Fiber Racing CAT

Discussion in 'Multihulls' started by Yacht Skipper, Oct 17, 2008.

  1. Yacht Skipper
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    Yacht Skipper Junior Member

    Dear All,

    I'm designing preliminaries for a 50ft Racer/Cruiser Catamaran.
    We're planning to build it out of Carbon Kevlar sandwich.
    Is there a way to easily estimate thickness of shell and structural members as well as boxed cross beems just for weight and cost study.
    Don't worry, I plan to consult a composite structure engineer to do final calculation, but first I want finish my preliminary design to see if it fits my expectation and budget.

    Any comments welcome

    Mat
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Generally, no, there is no easy way. You either have to start from engineering principles with a first pass at the engineering, or, more commonly at this early stage, look at other boats out there that are close to your size. Make some phone calls with people in the know on those designs, and ask for information--weights, thicknesses, etc. Otherwise, try to get published data on the designs, which unfortunately, is usually difficult to find. The point is, if you know what similar designs are like, chances are your design is not going to be too far different. You have to start with what someone else has done before.

    Weights may be easier to come by than thicknesses. This is because thicknesses can vary all over the place depending on core thickness. Lighter weight laminates have thicker cores, heavier laminates have thinner cores. This is OK to a point--if the core is too thick, the skins will be too thin, and they will buckle. So you need just the right amount of skin thickness for the right amount of core thickness, all to be strong enough and stiff enough for the task at hand.

    Other people reading this might offer some empirical data about boats they have built or are familiar with. Then you'll have at least something to go on.

    Good luck.

    Eric
     
  3. Yacht Skipper
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    Yacht Skipper Junior Member

    Thanks for your answer Eric,
    So I understand that thickness can not be estimated, but weight and cost could ?
    Is there a rule of thumb here for Carbon Kevlar Sandwich ?
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    Yes, weight and perhaps cost would be easier to come by if you don't have too much insider information on the engineering of previous similar designs. It is possible to start at the overall intended displacement of the boat and work backwards to detailed weights. You know that certain elements (hull structure, deck structure, interior fitments, exterior gear, the rig, crew, consumables, etc.) are all going to take certain percentages of the total displacement. You can probably find this from similar designs by asking around and studying other designers' work.

    So you estimate the weights of the gross sections, generally. You can then start breaking these sections down into their component parts. In a metal boat or in a stringer-frame method of wood boat, the skin weights for hull and deck are going to be about 2/3 of the total weight for their respective structures, and the internal stiffening structures will be about 1/3 of the structural weight. With composites you have less framing, so the percentage of the structural weight of the skins goes up, say to 85-90% if there is very little internal framing. You double-check this weight by estimating how much is a square foot (or square meter) of the hull or deck supposed to weigh? You know it will probably not weigh less than 1 lb/sq.ft, and it is unlikely that it will be over 3.0 lbs./sq.ft (or the metric equivalents). Looking at these two extremes, you can calculate what proposed laminates might be to achieve those weights. You know you will have to meet certain practical minimums and requirements just to get the boat built in a reasonable way. This distills very quickly down to a few set core thicknesses and then reasonable skin lay-ups. Knowing fabric thicknesses which you can get from fabric suppliers, you can make rough estimates of the skin thicknesses based on reasonable laminates. A lot of your research with other boat designs should focus on what is reasonable for a layup. As an example, if I were going to design a carbon-kevlar laminate, I would use the minimum amount of Kevlar--one layer right against the outside surface of the core for bullet-proofing against sharp object impact--and maximize the amount of carbon. How many layers of carbon are going to be practical, and are you going to use all carbon cloth, or are you going to add unidirectional here and there? These kinds of questions are what you have to answer to distill your weights down to thickness. This is why I say the job is not necessarily easy--it takes a fair bit of research, thinking, and a modicum of calculation.

    As for cost of construction, you should have a target price in mind, and again, the various components are going to command their own prices. The materials and labor for the various components are going to break down into different percentages. Depending on the laminate and how much material is going in, these percentages will vary from component to component. But an overall check will add some reality to your calculations. For example, here in the US, a good, fast production boatbuilder is going to be able to built 5-6 lbs. of boat per man hour. An average builder will build 3-4 lbs/man hour, and a slow builder or custom builder may build at the rate of 1-2 lbs/man hour. Figure out the weights that are going to be built, multiply by the going hourly rate in your area, and that will give you a target for the cost of labor on the boat, which should be a big portion of the overall cost. Again, you can probably get information on what may be common on similar designs as you ask around.

    I hope that helps.

    Eric
     
  5. rob denney
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    rob denney Senior Member

    G'day,

    Eric is pretty much correct (and would be a good choice to do the final engineering when the time comes). For your preliminary weight analysis you can work on 4 tons/tonnes all up weight if it is a serious race boat. You can also use 450 gsm carbon bidirectional either side of 19mm core as an average base hull laminate. Do not forget to add 500 gsm to a foam laminate for wetting the foam. Work on 50% fibre, 50% resin by weight in the laminate, which is low fibre if vacuumed, but allows for overlaps. Allow about 20% unidirectional carbon on top of the base weight for local strengthening, and perhaps another 25% for structural bulkheads. This will get you enough weights to design the hulls.

    I suspect that once you have enough room in the hulls to live in them, you will have far more boat than is required to support the likely weights.

    You should start a spreadsheet of all the stuff you are going to put on the boat. I can send you some examples if you like. If it is not at least 300 lines long, you have not finished it. ;-). Include weight, cost and whether it is really necessary.

    Where I would differ from Eric is the kevlar. I once saw a 12m/40' cat which had rubbed against a large steel buoy for a couple of hours. The buoy had gone through the outer 300 gsm/9 oz kevlar very easily and the foam as if it was not there. However, the inner kevlar, with nothing to support it, was still intact. As with all questions of this type, make some panels and test them yourself. Exterior kevlar is also a pain unless the boat is female moulded as it fluffs up when sanded.

    The expense of carbon in the hull may not be the best way to spend money. Not much point in saving weight in the lightweight hull if the rig is not equally light. The biggest weight saving you will make (apart from doing something completely out of the box like a harryproa ;-) will be to use an unstayed rig.

    regards,

    Rob
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    My reasons for using a minimal amount of Kevlar are two-fold. First, if Kevlar is in the outer layer and it gets abraded due to unintended impact, it is a devil to repair. The exposed and abraded edges of the torn Kevlar will not readily wet out again--they are extremely difficult to repair, refair, and repaint.

    The second reason is because Kevlar has extremely poor compression strength, especially compared to its tension strength. A sandwich laminate necessarily has a tension side and compression side in any given area. With loading on a panel from the outside of the hull, at the edges of a panel, the inside skin is typically in compression, and in the middle of a panel the outside skin is in compression. Even with a whole Kevlar laminate, the compression side will always fail much sooner than the tension side--the strengths of the two skins is severely imbalanced.

    Carbon fiber is different. The compression and tension strengths are very close to each other, so it makes sense to use balanced laminates of only carbon either side of the core. By balanced, I mean that the lay-up sequences of the skins should be mirror images of each other.

    Kevlar is about as strong as carbon in tension, but not in compression. Kevlar is also not as stiff (has a lower modulus of elasticity) than carbon. Therefore, in a hybrid laminate of Kevlar and carbon, Kevlar acts to reduce the overall stiffness of the laminate, and also reduce the overall compression strength of the skins. Panel deflection will be more for a Kevlar-carbon hybrid than for a carbon-only laminate.

    Kevlar's real benefit is its impact resistance--think bullet-proof vests. But you don't want much of it, and you don't want Kevlar on the very outside layer because it is hard to repair once damaged. You do want to protect the core from impact penetration and abrasion. So all things considered, a small amount of Kevlar right against the outside surface of the core is desirable for impact protection, it will not be damaged easily by minor abrasions because hard objects have to get through the outer layers of laminate first, and a minimal amount Kevlar will not greatly affect the overall strength and stiffness of the laminate.

    Eric
     
  7. marshmat
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    marshmat Senior Member

    Some solar cars, including one whose structure and shell I designed a few years ago, use a small amount of Kevlar in the same way Eric describes. In our case, its purpose was to prevent the car's belly from abrading through on the road if the suspension were to fail (the driver's butt in these things is separated from the road by 1/2" of carbon and Nomex, and 4" of air). No point mixing Kevlar and carbon within the laminate, or putting Kevlar on the outside layer, because of the huge difference in compressive strength and modulus. The loads on a grounded boat are remarkably similar, and so are the engineering principles.
     
  8. Yacht Skipper
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    Yacht Skipper Junior Member

    Thanks again for the comments.
    Rob, I planning to step a carbon wing mast with kevlar shouds.
    What do you think would be the weight difference between Carbon kevlar sandwich, and a Glass Foam sandwich for a surface of 126 m2 (for 2 hulls not including deck)
     
  9. rob denney
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    rob denney Senior Member

    G'day,

    Dyneema is used instead of kevlar for shrouds.

    Without details of the loads and panel sizes (distance between stringers, frames, bulkheads, structural furniture) it is not easy to say. Some careful thought and engineering will allow for small panels with correspondingly low reinforcement.

    But if it was say, 19mm H80 foam core (1.5 kgs per sq m) with average of 450 gsm carbon (1.8 kgs/sq m) each side it would weigh (1.5+1.8+0.5 (wet out the foam) = 3.8 kgs per sq m. Replace the carbon with twice as much glass, it would be 1.5+1.8+1.8+0.5= 5.6 kgs/sqm. Difference on 126 sq m is 227 kgs, which is not to be sneezed at. Glass cost is approx $4 per kg, carbon cloth approx $150/kg so the difference in cost is carbon (126*.45*2*150=$17,000) vs glass (.45*4*126*4=$900).


    regards,

    Rob
     
  10. Yacht Skipper
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    Yacht Skipper Junior Member

    Thanks for the great reply.
    So for my preliminary weight calculation, I can rely on 3.8 Kgsm for Carbon/H80 sandwich and 5.6 kgsm for Glass/H80, What is the percentage to add for stringers ?
    What about lighter Cores like Nomex ?
     
  11. rob denney
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    rob denney Senior Member

    G'day,

    Stringers are far beyond the scope of this guess. Draw your hulls based on the skin weight and your contents spreadsheet, put in your necessary furniture, bulkheads and floors, then take it to an engineer.

    Nomex is funny stuff to use as it requires glue films and other tricks, is a pain to put skin fittings in and cannot be tapered. Weight saving is about half that of foam for the core.

    regards,

    Rob
     
  12. Yacht Skipper
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    Yacht Skipper Junior Member

    Ok, so Carbon/Nomex sandwich could be reduce to 2.6 kgsm for the same steinght.
    Is that right ?
    Will it be the same shell thickness ?

    What about the deck ?

    Thanks again for your help

    Mat
     
  13. rob denney
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    rob denney Senior Member

    Shell thickness is in the same category as stringers.

    Thickness can and should vary all through the boat, if minimum weight is required. Half the foam core (1.5 kgs/sqm) is 0.75 kgs/sqm. This will be roughly the saving with nomex, depending on how the boat is built.

    Deck thickness is also fraught as walked on areas have different laminate requirements to non walked on areas. Again, use my rough figures for your first sketch, then see an engineer, if lightest weight is what you require.

    regards,

    Rob
     

  14. oldsailor7
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    oldsailor7 Senior Member

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