carbon fiber

starting a shearwater sport and thinking of carbon fiber bottom hul pros and cons

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RE: carbon fiber


You talking structural or decorative?

CF's main feature is high tensile strength for low weight. The price is that it's very brittle, compared to glass or wood. It doesn't bend, it just breaks. When it does, there's sharp slivers of CF flying everywhere. It also doesn't handle abrasion well.

Covering the CF with glass will help trap the slivers in the event of failure. It will also help protect against abrasion. Using a core (wood or foam) will give you more stiffness for less weight. Using correct forms (such as cylinders, hollow square beams, etc.) will also increase the stiffness. Areas of local stress concentration must be reinforced. Layup schedule matters a lot. For maximum strength and lowest weight you need to pay attention to the direction of the threads, the type of weave (if any), etc.

My feeling is that the shearwater doesn't need CF. It could be a fun learning experience using CF, but you won't end up with a better boat and it may be slightly worse in terms of weight, not to mention expense. CF's strengths don't really play to the boat design's needs.



RE: carbon fiber

thanks for the info

RE: carbon fiber

I have built a Shearwater 16  using the CLC guidelines and th e boat came out fine at 40 lbs

I would like to build one for my daughter so she can cartop the boat . She is a small woman and I would like to achieve 30 lbs

Firstly to save weight I was planning to use 3 mm okume . Not using the usual stitching , but building on top of the present shearwater form .

Next using carbon fibre twill weave 4.7 oz  on the outside : both to save weightand  for the outstanding look. Look more important .  Sealing wood on inside without glass. Using S glass rather than e glass. I was told that weight for weight (without resin) carbon is twice as strong as Glass , and S Glass is 20 % stronger than E glass.

Careful use of resin , fillets , end pours , cockpit coaming of glass or carbon composite .

Opinion of contributors would be greatly appreciated


RE: carbon fiber

Sorry, but I am just not hearing a logical argument for carbon,  You don't really need carbons good points, high tensile strength low elongation, and you do need its lesser points, average sheer strength and in this case, its low elongation.  low elongation means the fibers don't streach much before they break, the streaching spreads the load from an impact out over a larger area, so with carbon, you end up with a small but catistrofic failure.  With higher elongation fibers you get a larger damage area with less intense damage, from the same overload.  That makes the difference between "I dropped the boat and I have cracks over about 1 square foot, but I can still paddle today and fix it next week, and "I dropped the boat and I have a hole in it, I guess I won't be paddling today".  Kevlar would be a better choice, if you like the color.

If astethetics are a driving force, you can get black glass.  Have you looked into hybrid fabrics?

I think s glass is likely your best choice.

RE: carbon fiber

How about something like 6mm foam, pre laid up in sheets with 6 oz glass inside, then cut into the lofted shapes and assembled, then covered in 6 oz carbon on deck, maybe 12 oz on the bottom? Same general method as s&g, just a little different set of materials. 


I'd need to do the arithmetic to see if this lamination beats a 4mm okoume "core", but I suspect it would...

RE: carbon fiber

Bet it doesn't :-)

1. Glassing the core (especially with 6 oz) before before assembly will make bending the panels devilishly difficult.

2. The cross-grained layers in the plywood gives a lot of stiffness once the wood is bent. The foam contributes only compression resistance. Foam is also weaker in shear than wood.

3. It's going to need glass on the outside anyway for abrasion resistance and to contain the flying spikes in the event of catastrophic failure.

4. For best strength-to-weight, it's going to need carbon fiber on theinside, too. Otherwise the panel is unbalanced. It will have lots of tensile resistance on the outside and much less on the inside. Loads from the outside will propagate inwards where they'll encounter less resistance and cause failure. A composite is only as strong as its weakest component.

Even on large expensive boats (and ships such as the Zumwalt class destroyers) the builders use a wood core when they need maximum strength for weight.



RE: carbon fiber

A one sided foam panel bends fine. If you want to bend it a lot, a heat gun can be used to soften it slightly.

Plywood is a fine building material, but it's a lousy core in composite construction. Once you have enough strength in the inner and outer skins (based on their separation distance), the core's job is primarily to provide shear transfer between skins. Structural foam does this very well, at about 1/10 the weight of wood.

Carbon doesn't fail with "flying spikes". Yes, a foam composite hull wil be much less resistant to impact damage, the price to pay if seeking reduced weight.

Composite panels are built unbalanced all the time, as designers try to address local and global  bending loads. The inner skin sees tension in a local sense, but with chine construction the determining panel size is pretty small -chine to chine. I "specced" glass here, since there would be taped joints laid on top so not much chance to utilize carbon's strengths- its difficult to work with and expensive to purchase carbon in weights less than about 6oz. Globally, as in overall hull torsion and bending, the ease of applying continuous pieces to the exterior make this an easier way to achieve those goals.

Modern performance boats are built with honeycomb, foam, end grain balsa, or plain laminate, in descending order of weight efficiency. "Plywood" falls into the plain laminate category. Balsa is a good core for impact resistance, but is heavier than it needs to be otherwise. Honeycomb requires a significantly different skill set to work with. That leaves foam...

RE: carbon fiber

Good points, don't have time to debate them right now (as much fun as that really would be), but with respect I have personally experienced the flying spikes when testing a mast segment to destruction. It snapped and pieces flew as much as 4 ft. They were very sharp. So I'm sticking with the flying spike description regardless of how the rest turns out.

Hope to meet you one day for beer and composite talk,



RE: carbon fiber

A mast is a lot different than a composite hull laminate. Thick wall versus thin, a wood (or foam as discussed here) substrate versus none, and generally big loads versus small. Most kayak hulls will use a woven cloth as their outer layer, which also greatly mitigates splinter size, versus the uni fibers used in most mast construction.



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