Wood Carbon Fiber

I couldn't find a topic that covered this extensively.
I've read over most of the Gougeon Book and it only really touches on it briefly.
http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf

My logic goes as follows:
It is somewhat excepted that anyone that is going to build a one off boat should be building a cold molded laminated hull. There is an argument that foam core boats would be better than wood. My opinion is that I don't know if enough fatigue testing has been done on these types of laminates to prove that delamination doesn't occur. I believe balsa core can withstand these cyclic loads, but not foam. As for not building with balsa... I don't have a great argument for that, but lets save that argument for another time. That being said West system epoxy is recommended as it has the best protection against water absorption on wood boats. At this point there are several options. Covering with any of the following materials; fiberglass, polyester, kevlar, carbon fiber. I would like to look at fiberglass and carbon fiber. My argument would be that carbon fiber is worth the extra investment for the following reasons.

1) Part of the deterring cost of producing carbon fiber laminates is that it most be done with epoxy resin as opposed to polyester. But since it has already been stated that West System Epoxy will be used, some of the deterrent is removed.
2) Carbon fiber is more expensive than fiberglass, but it is much stronger. 16% stronger. 41% lighter. or a strength to weight of 64% better. Additionally according to the Goudeon Paper, after 10^6 or roughly 800 hours at sea, Fiberglass's strength reduces to 19% of its original strength where as Carbon Fiber only reduces to 60% At this point a carbon fiber laminate is an additional 3 times stronger or a total of almost 5 times stronger by weight. By this a 5.8oz weave of carbon fiber would be stronger than 2 10oz layers of glass. 5.8oz carbon ~ $38/yd 10oz glass ~ $11/yd 2x10oz glass ~$22. Also more cloth means more resin, more cost.
3) Since so much less Carbon fiber is needed to equate to the same about of strength, there wound be less labor involved in fairing the hull. If the wood is faired after lofting adding the thickness of one layer of carbon fiber should not make for much more fairing. Time is money.

So my question is why not use the carbon fiber? Don't the extra strength, and reduced labor offset the cost?

 

fiber glass is more flexible and will not suffer cyclic loading failure as most other materals. I do not have a lot of experiance with carbon but it seems to me it is a somewhat brittle material, and impacts are more likely to shatter the fabric. Perhaps others with more experiance with carbon composites can confirm this observation.

 

Well, this is one hell of an assumption. I can think of a few dozen building methods with most of them being capable of one off consideration.

Yep, lots of testing, but more importantly also lots of different foams, so the broad comment about fatigue isn't applicable without specifics. As to one being better than the other, it depends of the SOR's goals, budget, etc.

This flies in the face of thousands of foam cored builds, that haven't delaminated. I guess we should just wait and they will? My point is what bases are these overly broad conclusions accessed?

More assumptions? What makes you think West System is the best at resisting water absorption?

Okay, now you're in over your head. There are several ways to look at strength and this doesn't even touch the other physical attributes, such as stiffness.

Carbon has it's place in many laminates, but a good grasp of it's physical properties is necessary to understand where it can be employed, why and where not and why.

The cost of carbon generally doesn't offset the reduction in other materials, such as other fabrics and resin. This is mostly because it's considerably more costly than those material savings, typically being 4 times the cost of conventional fabrics.

Maybe it would be easier if you got down to specifics, about what you're attempting to do, as high modulus fabrics need to be well engineered or you'll regret their use, especially in the wallet.

 

I think the only way to test your theory would be to have the same boat designed using wood/fiberglass, and wood/carbon. It would then be engineered to properly take advantage of the properties of each material. It is never wise to substitute one material for another in a design without consulting with the designer.

Then take the plans to a builder and get a build cost for each. The builder would have to take into account the cost of materials and the cost of labor for each design. If your carbon boat is cheaper then your theory is essentially correct.

 

Normally when you speak "carbon fiber" you are referring to a strong lightweight structure. Lightweight structure implies sophisticated engineering and a sophisticated build technique. Both these details are expensive.

 

Trying to read between the lines it sounds to me like you are talking about sheathing a cold molded hull with one layer of 5.8 oz carbon woven cloth in west epoxy in lieu of 2 x 10 oz glass cloth, is this correct? If so this would be an unusually heavy layup of glass, more typical would be one layer of 4 to 10 oz. One layer of 6 oz glass cloth is going to consume about the same amount of resin as the 5.8 oz carbon and weigh the same for a fraction of the cost. The sheathing on a cold molded hull is not there to contribute to the strength, it doesnt need it, so any percieved extra "strength" from carbon is just wasted money imho. I would also question that carbon in a woven cloth format is going to give higher physicals anyway, especially with a low elongation gp epoxy like west.

Steve.

 

I believe the OP's understanding of "coldmolded" is a bit too broad and includes also strip planked, endgrain and foam cores.

 

I dont think so Teddy, he refers to "a cold molded,laminated hull" and the title of the thread is "wood carbon fiber". Well have to wait for the OP to respond i guess.

Steve

 

This was the point of my initial post, conflicting and assumed assessments, with highly engineered structures. If you just use one or two physical properties as a gauge, for a material's suitability, possably suggesting it's superiority over another, you've missed the point. Carbon has some wonderful physical attributes, cost withstanding, but it also truly sucks in other areas, which has to be balanced and compensated for in the structure.

 

Fpjeepy05, Listen to PAR he is absolutely right. A boat is a floating collection of compromises. You need to be specific regarding the requirements. For example, you could design a structurally optimized carbon hull that is light and stiff. The problem is optimizing for weight leaves you with a hull that will sink the first time it strikes anything in the water.

 

Imho

Read the original referenced book. Page 13.

I apologize. I was using "cold molding" as a general term. I thought consistent with the reading, but perhaps not. Let me rephrase by saying building by means of placing material over a male mold, jig, frame or other means. Specifically Not building with a female mold.

My base for these overly broad conclusions are that many, not all, foams to not absorb water or resin, they also do not melt with resin therefore the bond between the resin and foam is based on the shaping on the interface. Also foams used can be lower density than woods which means lower strength. I have no fear of these foams failing in compression, my fear would be failure in shear. I have not seen the tests that disprove my fears. That does not mean that they do not exist. And if you can point me to them I will gladly secede. It would be more unbelievable to think that wood was the best material. That is why in my original post I suggested that this argument not be part of the discussion.

I meant to mean epoxy. I was using West System as an Eponym, Frisbee, Cellophane, Band Aid, etc. As for epoxy being less permeable than Vinyl-ester or Polyester, I don't have a reference, but I thought that was generally excepted. Most modern wood boats use epoxy.

I think I have a good grasp of carbon fiber's physical properties. Anyone that has heard of carbon fiber knows of its stiffness. Stiffness is not a measurement of strength. Ultimate tensile strength, tensile yield strength, or maybe even energy to failure (area under stress strain curve) But not stiffness. Stiffness is important in boat building and design, but not in the same way as strength.

As for the cost... the strength to weight ratio, you can look up for yourself. I came up with 1.64 times greater for carbon fiber. I'm sure you will find that it is somewhere between 1.4 and 1.8. As for the Fatigue numbers, in the Gougeon Book, from testing they did. As for cost, I used Jamestown Distributors. Do you have a reference for your numbers?

I apologize for sounding caustic, but I was talking in generalities to make the discussion easier, and felt criticized for doing so.

No doubt you would know more than me. I guess my only rebuttal would be maybe trying to track down some 3.5oz carbon fabric. Or dropping a layer of wood?

True. But 3.5oz of Carbon fiber verses 10oz of fiberglass over 1/2" of Veneer isn't going to make a boat fall apart.

 

You might want to study how glues work. It is possible to have quite high strength bonds between smooth surfaces.

Why would lower density necessarially mean lower strength? Earlier in this thread you were talking about how carbon fiber is stronger than glass but lower density.

Stiffness can be more important than strength in many situations. A simple example is a plate in axial compression. Critical load can be determined by linear buckling stiffness rather than strength.

Remove a layer of wood and the stiffness of the hull to local bending stiffness and buckling resistance will drop significantly. If you make the core thinner you would need to make the sheathing considerably thicker for the the same bending stiffness and buckling resistance, and overall weight may well go up even if carbon fiber is used in the sheathing.

For a homogeneous construction bending stiffness goes as the thickness squared. For sandwich construction with thin skins it goes as the core thickness squared if the contribution of the core to the thickness is neglected.

 

Glue or Resin? I put resin on a smooth surface [formica] to make smooth streets of material, that peel off easily. I'm not planning on using contact cement for my boat either.

The point of building with foam core sandwich is to reduce weight. Therefore lightweight foams are used which are not as strong as wood. You could use higher density foams but that would defeat the purpose. Foam sandwich construction relies on the core being able to resist compression, It doesn't contribute to strength in any other way. By the nature of their function, foams are not strong in themselves, and therefore the shear along the foam laminate interface is most important.

I understand it is important. But personally I would rather have my hull buckle rather than break. Buckling doesn't cause my boat to start sinking.

True, but the opposite is true as well. I could build a boat out of balsa or foam and make the hull 2" thick with one layer of glass on the outside. Plenty of stiffness, however the boat will have insufficient strength. It is illogical to assume that one layer of 6oz glass cloth is the only solution to the parameters.

 

The other misunderstandings of physical properties are being covered but this one is also wrong. Fiberglass has a long history of fatigue failure. Flexing of fiberglass hulls causes them to get "soft" and many, including me, have direct experience with that. Wood is far superior in that respect since trees spend a lifetime swaying and flexing in the wind and are able to take it.

One method used in industry to do non-destructive failure testing of fiberglass pressure vessels is to pressurize them to specific levels and detect the degree of internal fracturing of fibers with a sensitive ultrasonic listening device. Fiberglass is also subject to crevice fracturing which is much greater in the presence of moisture. The loss in strength due to this alone is usually rated at 50%.

 

These two sentences are mutually incompatible. The second is opposite to the first.