Re: [Paddlewise] Boat Construction

Doug Lloyd wrote:

> 5. I have witnesses catastrophic failure of a vacuumed bagged hull - not a
> good thing. Never seen this with a Brit heavy - just extra chiropractor
> visits :-)
>

Is there a typical failure mode or are they all different?  Or are you talking about
one failure?

> 6. All the guides/instructors that I know in Southern BC, who work with or
> for stores/manufactures and always custom order their boats (at reduced
> costs) usually ask for extra-heavier lay-ups. What does that tell you about
> their faith in the standard lay-up vacuumed bagged kayak?

But how much time do they spend in their boats compared to the weekend
warriors?  That factors into the decision.  I've also found that guides don't
want to baby their boats and will do seal landings in conditions that I'd exit
offshore and walk.  Construction workers don't buy the same tools that
weekend woodworkers do either, so I think your observations are valid in
the appropriate context.

> 9. All the above guides/instructors I know, also usually order the extra
> outside seams, especially over the extruded plastic seam. Even then, I have
> seen outside seam failure concurrent with inside seam failure. This can be
> a bit catastrophic in the middle of a gale during a long crossing. For my
> money, an extra outside seam of two layars done with _epoxy resin_ is the
> only way to go.

But reinforcing may result in local stress concentration that increases the
likelyhood of failure.  The key is a balanced design; adding reinforcement to an
existing design may not be a good idea.  I'm a former engineer who specialized
in stress analysis techniques and I can't see a reason why inside-only seams
can't work; they don't need to be any stronger than the hull and deck they join.

> 10. Kayaks are relatively small vessels, but don't underestimate the
> stresses faced. A fully loaded kayak undergoes incredible flexing motion.
> Next time your kayak is fully loaded, have two people pick the kayak up by
> the ends. As it is slowly lifted by said hardy individuals, look closely at
> the alignment between stern and bow. Whoa, there's a ton of flexing going
> on. Now, think about adding your weight in too, throw in some ruff, choppy
> seas, and then tell me stiff isn't important. (Baidarkas, folders, and
> plastic not withstanding).

My xc skating skis are almost two meters long, weight barely more than a
kilogram (the pair) and can handle my body weight repeatedly pounding on
them for many seasons without failing.  Fiberglass, cap construction (sorta
like a kayak hull) with honeycomb core.  Their ability to deal with stress comes
in large part from their flexibility.

It's important to distinguish two different kinds of stiffness in a kayak.  Your
example is one of overall bending of the kayak, supported at the ends.  In this
case, the skin of the kayak is not flexing, but is in either tension or compression.
(If held up from the ends, tension on the bottom and compression on top.)
If the skin is subjected to a local load, like hitting a rock, the skin itself flexes
and near the point of impact will be in compression in the gel coat and tension
on the inside.  Away from the impact, the flex will reverse to tension in the gel coat
and compression inside.

The thickness of the skin will have a strong influence on the stiffness where flexure
of the skin is concerned, but will be less significant where tension/compression of
the skin is involved (since the fibers carry the load).  Hence, the reputedly thicker
Brit boats should show their stuff in oil canning but should show less advantage over
a vacuum-bagged kayak on overall bending.

When I look at the difference in weight between fiberglass and Kevlar canoes (around
30 lb), I would only buy Kevlar (say 55 lb canoe).  In a kayak, the difference is only
about 5-9 lbs - my kayak, when new, was 55 lb in fiberglass (49 in Kevlar).  If I'm going
to go to Kevlar in a kayak, I'd want it for strength, not weight savings.  So - my approach
to the "idea" constructed kayak:

Kevlar built up to the same weight as (flimsier?) fiberglass.  Vacuum bagged
construction to maximize fiber to resin ratio.  Epoxy rather than vinylester resin.
Not _too_ heavy gel coat layer  - after all, I'm fixing it every couple of years anyway,
why carry excess around all the time?  (Is there not the potential for a _better_ material
to coat than gel coat?)

To increase stiffness, I wonder if a thin core material should be considered.  Not foam
or balsa or any of the thicker materials.  I'm thinking more of a light fiber layer(s) between
the Kevlar.  Walbridge's Boatbuilder's Manual discusses this in the context of home-built
WW boats in the days before plastic.  Nylon, for example, increases the toughness (ability
to survive impact) without adding much weight (4.2 oz vs 5 oz in kevlar).  Hmmm.  Builders,
please comment!

Mike

***************************************************************************
PaddleWise Paddling Mailing List - All postings copyright the author and not
to be reproduced outside PaddleWise without author's permission
Submissions:     paddlewise_at_lists.intelenet.net
Subscriptions:   paddlewise-request_at_lists.intelenet.net
Website:         http://www.paddlewise.net/
***************************************************************************