Re: [Paddlewise] Slamming

Original question:
>>Is Kevlar "stronger" than fiberglass or is it just lighter?
>>
Harold wrote:
>Kevlar fiber and cloth has superior tensile and tear strength to glass fiber
>and cloth.  I think the real question is:  " Are Kevlar composite boats
>"stronger" than fiberglass composite boats?"

Hank?
Agreed, but it's a slippery question because of different kinds of
strengths.  Strong isn't necessarily the same as strong.  Materials break
in many different ways.  Tensile strength (pulling apart), compressive
strength (pushing together), shear strength (cutting as with scissors),
interlaminar shear (will the plys separate easily - the boat "come
unglued"), plus a lot of slow acting factors like durability (slow wearing
away), abrasion resistance (related to the previous), weatherability
(environment, usually water, affecting the material), there are many
more...   Keep it simple --  Kevlar just doesn't reinforce exactly like
glass.  

Harold:
>I believe the (long) answer is that, for the same weight boat, the Kevlar
>composite boat (usually a combination of kevlar and glass-fiber materials in
>polyester or vinylester resins) will be stronger and far less likely to
suffer
>catastrophic failures (like the whole bow breaking off, versus just cracking)
>than the same model made solely with fiberglass.

Hank:
Agree, usually, but a combination of the two will be even better.  By the
way polyester doesn't stick very well to Kevlar.  Vinylester is better, but
epoxy is required to do a reasonable job in my opinion.  Not many boat
manufacturers must agree as few use epoxy.  In low stress situations,
vinylester and Kevlar together are probably fine.  I consider most sea
kayak applications as low stress, excepting those few instances of "abuse"
and "slamming" I keep seeing in the subject header above here (cringe,
cringe..).  I don't consider vinylester and Kevlar as good for whitewater
boats, but it probably is used in some.  

Harold:
>However, most manufacturers utilize the superior tensile strength of the
>Kevlar to make boats which are lighter but of approximately the same strength
>as their glass models, thus making them far more desirable to the customer
who
>tends to worry a lot about weight.

Hank:
And they can usually get away with it because most sea kayakers are
relatively easy on their boats.  

Harold:
>If I were ordering my "perfect" boat, I would request a heavier "expedition"
>Kevlar lay-up.  

Hank:
I've done exactly that for the last couple composite boats I bought.
Despite all this composite construction advice I'm spewing, I haven't built
a boat for me for several years now.  Plan on doing it, just haven't gotten
around to it yet.  These people keep ordering paddles....

Harold:
>some no-mere-mortal who ran the Grand Canyon (yes, Class 5 sections and all)

Hank:
The Grand Canyon is very easy on boats.  If you hit a rock while in the
Grand Canyon it's because you tried to (or don't belong there in the first
place?).  People ran Klepper folders down that stretch back in the 60s
without any equipment problems.  

Harold:
>The reason people worry about spider cracking of the gel coat on Kevlar boats
>is because the lighter Kevlar boat is usually more flexible (while not
>necessarily weaker) than the standard-weight glass version.  

Hank:
Yup.  Exactly right.  

Hank Hays
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