Re: [Paddlewise] Boat Construction

At 06:02 PM 1/9/00 -0500, you wrote:
>Doug Lloyd wrote:
>
>> 5. I have witnessed 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?
>

Yes, this was a one time deal. A patch about 4 x 8 inches just fell away
from the hull, allowing water to rush in. A one time event does not suggest
vacuumed bagged kayaks are intrinsically no good. If you want, I can find
the picture and have it scanned for public consumption. However, for me,
one time would be too many if I was out at sea in rough water. I'm not sure
what the total occurrence of this type of failure is, or with any related
problems that ensue from vacuum bagging where _too_ much resin has been
squeezed away. The problem with dealers are they always say the same thing,
"This is the first time this has ever happened".
  
>> 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.

Not talking about seal landings here - just things like average surf, back
deck rescue maneuvers (self and assisted, etc), and blue water touring. The
people I'm thinking of in the "trade" have to deal with all the customers
coming back with complaints of premature breakage (and the usual response
is probably "This is the first time...") These guys and girls _know_ in
order to avoid similar problems, they had better go heavier duty. A perfect
example is the Gulfstream. The fellow had seen one too many with cracked
back decks, etc. He ordered a heavy duty lay-up. It came in just under what
the tougher hand-laid British version from P&H weighs. I rather think the
vacuum bagging did save some weight in this case, and from what I saw,
stiffness was addressed with judicious use of carbon fiber.

I'm not a construction worker, but I always buy good tools, as they perform
the job in a more efficient, accurate and _safer_ manner when I do do
weekend renovations, etc. And it is the "context" that precisely gets me a
bit perturbed in your note above. Advertising in North America often
suggests to customers that modern light weight kayaks are quality products
with the implication that strength for rough water is concomitant with
that. Conversely, I also disagree with advertising aimed at getting people
into the sport, suggesting light is best and that heavier duty is only
needed for the few. To me, that's herding the masses into a lightweight
category of boat, thereby restricting participants from expanding to harder
use. I get this all the time at the club: "Oh, I don't want to do that, my
boat is too flimsy", or "Sorry, rule me out of the surf workshop, my kayak
isn't built for that".

>> 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.


Yes, agreed for the most part. This is part of the reason the Brits stay
with straightforward lay-ups. However, as you know, stress concentration
can be broadened out such that failure from the "cure" is minimized, yet
the goal of strengthening a problem area is taken care of (such as adding
kevlar to the bow). BTW, the seam debate has engineers on both sides of the
fence. The problem to me, anyway, is that if you have witnessed seam
failure, it is very dramatic. The whole seam, once a portion lets go, can
unravel like a zipper. I can post a letter from Valley Canoe defending the
use of inside seams only, if you like. I disagree with VCP's letter myself,
and that is my subjective right. Admittedly, I deal with the sport at a
much more extreme level than most, as do some of the people I paddle with,
so I've seen a bit of these types of failures over the years. Perhaps
people take some of these boats beyond the manufacture's design criteria,
yet, the rhetoric you get from the representative dealers suggests their
boats should stand up to the most extreme abuse. I will never let myself be
caught in a situation again, in a kayak at sea, without extra outside seams
- done in epoxy. Everyone else is free to paddle as they wish.
  
>> 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.

Skis and kayaks are different animals. Kayaks are more three dimensional
than a flat plane of a ski. Too much flexing in a long sea kayak, to my way
of thinking, must have a deleterious effect eventually. I can't completely
defend my notion scientifically nor from any engineering orientation. I
simply am biased in favor of a stiff kayak. Stiff is good. Stiff is fun.
Stiff is wonderful. Stiff lasts, stiff survives. If its not stiffy, its
iffy! If I want non-stiff performance, I'll get a folder which utilizes to
advantage conformity in waves. But I paddle a hardshell kayak, and I want
it hard and stiff. I don't want to paddle a jellow-jealopy. I added 10 lbs
of epoxy and cloth to my hull, inside seams and new outside seams on my
Nordkapp over the last two years, and it is finally a boat I have complete
confidence in, in almost any condition now. Its a Nordkapp on Viarga, and I
love it. I did redo the inside seams, but spread it out with 1-, 2-, 3-,
and 4-inch tape. It will take hours of abuse in rough, open sea conditions,
taking green water over the bow or clearing right over large breaking
whitecaps to the point where the hull is loosing suction, and it doesn't
miss a beat. Certainly, this is all far too subjective to be of much
further use to Paddlewise. And...my next boat will be wood-strip, built by
myself, to my specifications and covered in epoxy so that the whole kayak
is a homogeneous unit. Unless you can point me toward a one piece
fiberglass kayak, that's the direction I'm taking. Sorry if I used up too
much space. As always Mike, you get us re-thinking our posts. Good on you,
mate!

Anyway, it was blowing 50 knots today, and a 4-5 meter swell was running up
island, but I had to help teach Sunday School and finish a Sea Kayaker
article - its just not fair God!

BC'in Ya
Doug Lloyd (who wishes his body wasn't so stiff as he gets older)    

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