Subject: baidarka nylon, moisture and heat
From: Paul macintyre (pfmac@mindspring.com)
Date: Sun Apr 09 2000 - 19:50:39 EDT
My understanding is that to cause permanent shrinkage in nylon you need both
moisture and heat, that heat alone will allow the nylon to loosen. I've
made 2 boats with nylon skins using moist heat with good results. I've
copied a posting out of the archives from G. Dyson that explains why this
is so. Note that there is some concern about the skin getting to tight. I
think this is a problem with the 26oz. nylon I have never heard of this
problem with 14-11oz. nylon.
Paul
Re: longevity
George Dyson (gdyson@rowlf.cc.wwu.edu)
Thu, 31 Oct 1996 09:14:03 -0800 (PST)
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Date: Thu, 31 Oct 1996 09:14:03 -0800 (PST)
From: George Dyson <gdyson@rowlf.cc.wwu.edu>
To: baidarka@lists.intelenet.net
Subject: Re: longevity
In-Reply-To: <327826EB.3171@superiway.net>
Message-Id: <Pine.ULT.3.91.961031090706.14366A-100000@rowlf.cc.wwu.edu>
>The picture of his 6.6M on the beach at Burnett Bay, Queen Charlotte
>Sound portrays a perfectly finished craft. Look at the picture with a
>magnifying glass and there are still no flaws evident. Why cannot
>perfection be achieved with each and every baidarka that is skinned?
Sorry to puncture illusions, but that particular (7.37m) kayak built in
1984, besides being the prototype of the poorest design I ever released,
was skinned way too tight--wasn't yet aware of tendency to continue to
shrink--and was hopelessly crippled within a few years.
The behavior of nylon deserves a small treatise, but, in the absence of
one (yet) below is a posting from this list, going back to prehistoric
times (16 months ago). The problem--or one of the problems--is that nylon
doesn't always behave as we expect it to. We have been indoctrinated,
since the 1950s, to think of plastics as these inert, waterproof
materials, whereas nylon is constantly inhaling and exhaling moisture
like a living, breathing (and obstinate) being. (Just as aluminum, far
from being an inert material, is part of the food chain and will
slowly--sometimes surprisingly quickly--be eaten as certain nasty
microbes who were here long before we were slosh around in the bilge.) If
you try and shrink a kayak skin with dry heat (heat gun or dry iron) it
will actually get looser--driving you nuts! If you use polyester or
polypropylene skin material you avoid this moisture sensitivity, but face
other problems that can really get confusing... Also confusing (my fault)
is that I have sold several different species of nylon skin material that
behave quite differently, and require a different touch to achieve
"perfection"--which is simply a compromise, never perfect, between too
tight (on the roof of your car on a hot day) and too loose (in the water
when cold).
-----------------
Cc: List Baidarka <baidarka@imagelan.com>
Subject: Re: Skinning a kayak
The difference is that nylon absorbs atmospheric (or oceanic) moisture
whereas polyester doen't. So if you apply dry heat to nylon, it tightens
up significantly simply from driving the moisture out which will make your
skin drum tight, but when it coools off and reabsorbs moisture from the
air it will loosen up again (in fact it may be looser, since the force
exerted in tightening may have stretched it). Moist heat however loosens
(by swelling the fibers) and shrinks (by releasing polymeric tension) at
the same time and permanent shrinkage results. For the first few cycles,
moisture alone will do this, which is why slight pre-shrinking with water
is a good idea to avoid a skin that gets too tight later on its own. The
skin you are using (26oz) has already been partially heat-set but it
still has about 3 percent shrinkage left. I regularly get asked what to
do now that the skin is too tight, rarely, rarely what to do if its too
loose. Which is unfortunate since the latter is remediable, the former
hard to correct. My advice is to take a couple good sized squares off the
corners of your fabric that won't be used, staple them over a wood frame
like an artist's canvas, and go through the process to get a feel for it
before experimenting on your boat. The skin will always be tighter when
dry and arm, looser when cool and wet, so you are aiming for middle ground.
--------------------------
In clinical practice, I see about twenty cases of skin being too tight
for every one case of skin being too loose. As Craig Kelford (posting
here last year) remarked, making an analogy I won't repeat, "too loose is
better than too tight." I agree.
George B. Dyson gdyson@cc.wwu.edu
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