Re: [Paddlewise] Slamming

I think the issue with some paddlers doubting the " strength " of kevlar is
related to the specific property of hull stiffness.  Kevlar FRP layups have
tremendous tensile strength for their weight and are superior to other materials
for tensile strength such as polyolefins, marine plywood, E-glass FRP layups etc.
However since it is so strong per weight and volume the layups often have much
thinner hull than the other materials.  I am sure some mechanical engineer will
call me on the equation, but I recall that the stiffness of a sheet of material
(its resistance to deflection or bending) varys by the square of the thickness of
that sheet.  Even a slight increase in hull thickness pays big dividends in
stiffness and resistance to denting or " oil canning ".  My father is the plastics
expert who has designed everything from FRP car bodies to a proposed design for
FRP tank treads, and could pass along all of the proper tech-talk and equations to
anyone who really wants to immerse themselves in the technical aspects of FRP
composites.

 If you are putting a load on a sheet of material you may be much better off
stiffness wise with a a weaker material but a greater thickness.  An inch thick
hunk of styrofoam is much thicker than a thin sheet of Kelvar FRP for example.  A
thicker hull such as a ceder strip or marine plywood is going to be much more
resistant to denting or bending in most cases than a thin Kevlar layup.  Some race
designs use a PVC foam core or other filler material between Kevlar FRP skins or
other structure modifications such as ribs to give the layup thickness and much
greater stiffness.  This is unfortuantely ususally very expensive.  My experience
with a lot of thin Kevlar layups is they bend, and dent but they don't break.  You
may crack the resin, but it is damned hard to tear or break apart a kevlar layup
boat.  However if your kayak has a big dent in it or folds in half still in one
piece you might be wishing you had traded some of that tensile strength for a more
rigid hull mad from a " weaker " material.

Hank Hays wrote:

> 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
> ***************************************************************************
> PaddleWise Paddling Mailing List
> Submissions:     paddlewise_at_lists.intelenet.net
> Subscriptions:   paddlewise-request_at_lists.intelenet.net
> Website:         http://www.gasp-seakayak.net/paddlewise/
> ***************************************************************************



***************************************************************************
PaddleWise Paddling Mailing List
Submissions:     paddlewise_at_lists.intelenet.net
Subscriptions:   paddlewise-request_at_lists.intelenet.net
Website:         http://www.gasp-seakayak.net/paddlewise/
***************************************************************************