: Paul,

: The word "strength" presents a problem to our conversations because
: there are several different kinds. For example there is tensile strength,
: modulus (i.e. stiffness), modulus of resilience, hardness, and shear
: strength. The three fabrics in my sample very widely among these strength
: variables. CF is highly superior in modulus, but inferior to s-glass in
: tensile strength and highly inferior in resilience. As stated in the
: 11/2002 issue of Sail Magazine, the sum of these properties determines
: what each laminate is best suited for. Not being an engineer, let alone a
: laminates engineer, I have to struggle to figure out what will give me the
: lightest and strongest boat. Often I rely on industry practice as long as
: I know why certain things are used, but I also pay attention to certain
: innovative individuals. The problem with industry practice is it often is
: trying to minimize cost, while that is not necessarily my focus.

: Unfortunately my blathering on about the different strength types does not
: help kayak builders very much. So let me stick my neck out. With regard to
: kayak building, I have concluded that CF is great for small things like
: fittings, hatch lips, and localized stiffeners. Since common glass
: schedules seem to yield a hull of adequate stiffness, I don't see any
: weight or overall strength advantage in using the commonly available 6 oz.
: CF for hull lay-ups. Since CF is so much less resilient, I would be
: worried that a nasty collision with a rock would result is much worse
: damage than if the hull were laminated with s-glass. Read the article in
: Sail Magazine (11/2002). It was an eye opener for me.

: Ted
Ted,

I am a Mechanical Engineer by training, HVAC engineer by trade and a hobby machinist, so what training I do have in strengths of materials deals almost exclusively with metals. I have not really had any need or interest in CF composites and, therefore, I have not done any research into the subject, as you have done. What little I do know about CF composites comes from blurbs I've read in magazines and I based my original reply and my opinions about why CF is the preferred material for many industries. Therefore, the following paragraphs are my own hypothesis, which is admittedly based on ignorance of CF properties and my own limited experience with building 2 FG skinned SOF kayaks. If I am wrong, please correct me, I don't want to steer anyone in the wrong direction.

By "strength", I agree, I could have been more clear. I was referring to ultimate strength (i.e. tensile), such that CF would have a higher ultimate strength per pound vs. FG, at least that was my opinion from what little I know about CF. Since it was my opinion that CF had a higher tensile strength than FG, I figured that the reason for using CF was to provide the same strength at a lower weight penalty. Your reply says that I was wrong and that is not the case. I can't argue, I have not done the research you have.

What you call "modulus", is technically called Yield strength. I do remember seeing something about CF being stiffer and your reply agrees with that. Perhaps it is this strength that has attracted the attention of different manufacturers to use it in their various products (like high performance bicycles and stuff).

Hardness is completely different and is really not considered as a "strength" except when referring to a material's resistance to wear, which is important on machine tools, I do not believe that is really an important factor in boatbuilding, since you are more concerned with running a log or rock through the hull vs. wearing away the hull dragging it through sand and over gravel. To put it another way, unless you plan on dragging your boat behind your vehicle without a trailer, you really do not need a wear resistant (i.e. hard) material, you need a material that is reasonably hard to allow you to drag over sand and small rocks with minimal damage. When it comes to brittleness, although in practice it goes hand-in-hand with hardness (i.e. harder materials tend to be brittle), brittleness is actually another reference to yield strength. Therfore, you want a material that is reasonably ductile so that when you run into a log or rock, the material bends, preferably bounces back too, and you keep right on floating. This is why epoxy-FG is used a lot, the epoxy resin is reasonably hard so that it wears at a reasonable rate, yet with the FG backing it can be fairly flexible. Perhaps its a bad example because I am not sure if they use epoxy resin or polyester resin, however, most modern bows are made from fiberglass. They have a very hard surface, yet can yield quite far when pulled back to shoot.

Modulous of resiliance (sp?) I am not familiar with that one, are you sure that is not a restatement of yield strength?

Shear strength should only be a concern with SOF 'yaks when the material is used either as the skin or a framing member. With strip and S&G built kayaks, where you have something to back up the epoxy-material "skin" shear strength of the epoxy-material skin should not be a concern.

I do not mean to mislead anyone, if I am wrong about my hypothesis please correct me. I am always willing to learn new things.

Paul