PaddleWise by thread

From: Matt Broze <mkayaks_at_oz.net> subject: [Paddlewise] fiberglass Date: Thu, 3 Feb 2000 22:29:18 -0800 · This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:33:09 PDT

Hi Peter
Who is the builder? Does he have a company name yet? Has he built a kayak
yet? Got any dimensions of it you could send me?

I don't know about Australia if that is where you are from (is that what the
"au" means in your address?--how about the "dk" does it stand for Denmark or
something?). In North America I know of no manufacturer of kayaks that uses
chopper gun construction. A few cheap canoes are made using a chopper gun
but it is more commonly used for making shower stalls and portable toilets.
The hand laid "chopped strand mat" in mat form is a step up from a sprayed
on "chopper gun" laminate. Chopped strand mat is used in many British kayak
decks where it conforms well over and around recessed deck fitting bumps (a
nightmare for hand laid woven fabrics). This is also one of the reasons that
Brit boats are often on the heavy side. Because the chopped strand mat is
relatively brittle (due to a low glassfiber to resin ratio) it must be made
rather thick (and therefore heavier) to be strong enough. When a mat
laminate is thick enough to be strong it is also thick enough to be very
rigid. This helps for doing boat over boat rescues but it is still often
more brittle than lighter, more expensive and better lay-ups.

The next step up the fiberglass construction scale is a sandwich
construction with cloth or cloth and woven roving (very heavy weave cloth)
sandwiching glass mat between them (the glass mat is sort of a filler
material here between the woven layers that it holds apart. The woven
materials act as the tension and compression carrying parts--like an I-beam.
Gelcoat/cloth/mat/woven roving is used by many manufacturers especially in
the hull half of the kayak (which is more likely to suffer blows). A
sandwich laminate can be very tough and still be reasonably priced.

A little background:
Of the materials commonly used in fiberglass construction, for a given
thickness and area chopper gun is the cheapest followed by fiberglass
chopped strand mat (in sheets), then a big step up to woven roving, followed
by unidirectional roving and woven glass cloth. Of these the strongest
laminates for a given thickness and weight are generally unidirectional
roving (long straight strands of heavy fibers bundled in parallel groups and
set and stitched together at various angles to the next layer--biaxial,
triaxial, etc), next strongest is woven roving with its long heavy strands
of glass (which enable it to act much like a net absorbing blows by
spreading the force of the blow over a wide area down its long strands, the
next strongest is also generally the most expensive fiberglass material per
unit thickness gained (or weight)--woven cloth. Chopped strand mat and
chopper gunned follow.
Each of these materials have certain qualities that limit how they can be
used. For instance woven roving has a very open weave so a builder that
tried to make a hand laid kayak just out of it would probably find that the
resin drained out of weave on the vertical sides and pooled in the bottom of
the mold. But by putting chopped strand mat behind it in the sandwich
construction the mat serves to hold the resin from running out of the weave
on vertical surfaces as well as being an inexpensive filler and space
filling layer (more thickness can be a cheap if heavier way to increase
strength).
Next in strength for a given thickness or weight is an all cloth lay-up (at
least as long as the weave is tight enough so the resin doesn't flow out of
it on vertical surfaces).

Resin is brittle and glass fibers are relatively strong, at especially when
glued together by a sufficient amount of resin. Up to the point where the
laminate is too dry (or gets air into it) a greater percent of glass to
resin in a laminate makes it tougher. Mat with its random "pickup sticks"
orientation takes more resin to fill up the gaps between the relatively (to
woven fabrics)short fibers. Longer strands of glass in the woven or
unidirectional fabrics hold better under tension and therefore provide
better impact resistance.

When it starts to cure a laminate expands slightly as the fibers stiffen. If
there is not enough extra resin in a hand lay-up to fill in the spaces
created by that expansion air is sucked into the laminate and weakens it. If
more glass fibers could be crammed into the laminate or the laminate could
be kept from expanding as it cures it would be stronger. Both these things
are what vacuum-bagging can do. By holding pressure on the laminate while it
cures a higher percentage of glass fibers can be used and the ideal glass to
resin ratio can be achieved (Note: I didn't say "is" but "can be" achieved).
The strongest glass to resin ratio is probably impossible to achieve with an
open mold (hand-lay-up). Vacuum-bagging also lets the builder use the
strongest glass (unidirectional and woven rovings) without worrying about
the "run down" problem on vertical mold surfaces because the air pressure is
even everywhere on the mold and the resin is held up on the vertical
surfaces by the pressure against the bottom surfaces). Vacuum-bagging can
also force woven fabrics around shapes such as recessed deck fittings that
cloth won't easily form around when hand-laid. (Note: vacuum-bagging leaves
"bag wrinkles" where the Mylar bag material wrinkled going over the
obstruction and was sucked down around it by the vacuum. These wrinkles are
often like little knife edges of resin and must be removed. One method is to
scrape them off with a paint scraper. Another involves putting a thin layer
of strong tight weave nylon that doesn't bond well to resin as the last
layer over the convoluted surfaces. After the laminate is cured the nylon
"peel-ply" is peeled off the rest of the laminate taking the sharp edged bag
wrinkles with it.

A further advantage to vacuum-bagging is that it leaves a relatively smooth
surface that is totally cured and hard (just like the outside of the kayak).
This smooth surface is easier to clean and causes less wear on ones heels
than the rough surface commonly found with hand lay-ups. Of course this can
be filled in or covered by someone who sees it as a problem. Unless hand
lay-ups have a special resin or a liquid wax that seals off the surface from
contact with air the last molecular layer in contact with air never cures.
The downsides are 1)raw resin is potentially in contact with ones bare skin
where it can cause a rash, 2)it erodes easier, 3)has the odor of styrene
coming off it for a long time, 4)the softer surface gums up sandpaper
readily as well but the upside is it doesn't need to be sanded to bond more
fiberlass to it (as a vacuum bagged surface or the surface in contact with
the mold does). A totally cured surface must be sanded to increase the
surface area for a more mechanical bond (rather than a chemical bond to the
air inhibited resin). I think that the smooth interior of vacuum-bagging may
also hold together better under tension (a blow) than the more serrated
surface of hand lay-up

 Note: other lighter, weaker, and sometimes cheaper filler materials than
chopped strand mat such as "core mat" and a urethane foam ("Divinicell",
"Airex" foam, etc.--more common in stiffening canoe floors) have been used
in place of mat between layers of fiberglass cloth in a laminate. My
experience is that although it is lighter, pound for pound it is more
brittle and also more subject to sheer stresses between layers than a more
cohesive laminate. "Core mat" is often used sparingly to make stiffening
ribs on a deck or as a keel stiffener down the middle of the hull. It adds
stiffness due to the increased thickness of the laminate (but in this
application it isn't intended to be part of the hulls impact strength but
rather helps in preventing a thinner laminate from folding too easily and
drastically--which is more likely to be the cause of damage to a glass sea
kayak than a hard knock). You can see "core mat" by looking at the inside of
a kayak. It will probably look like an opaque white material--showing
through the laminate (at least if the laminate is of a relatively clear
resin--and glassfiber--rather than pigmented resin, an opaque cover coat
inside, or Kevlar or graphite cloth--which one can't easily see through).

Hopes this helps understand fiberglass laminates for kayaks.

BTW a customer looking for a new boat was in my store today looking to find
a new plastic kayak to replace the Chinook he had had for years. Seems he
dropped the old Chinook and it shattered into many pieces. I guess better to
find out it had gotten brittle that way than landing hard after jumping a
steep wave.

Matt Broze
http://www.marinerkayaks.com

original message:
<<Date: Thu, 3 Feb 2000 11:41:55 +0100 (MET)
From: Peter Joachim Unold <pjunold_at_daimi.au.dk>
Subject: [Paddlewise] fiberglass
Hi Paddlewisers
A few days ago, I was visited by a fellow who is about to start
designing and building his own seakayak. He said that one way
in which his handcrafted kayak was going to differ from ordinary
production kayaks, was that he would be using fiberglass mats.
When asked what the difference was between that and my fiberglass
kayak, he said:
Most fiberglass kayaks in production, really consists of more plastic
than fiberglass. When building these kayaks, they cut fiberglass mats
into small pieces, and then spray a mix of fiber and plastic on the
hullshape. It's simply too expensive to have a mass production of
kayaks build using larger fiberglass mats. Typically a glassfiber
kayak contains twice as much plastic as glassfiber. Anyway this spray
technique means that they have to use more material for obtaining
a sufficient strength, compared kayaks build with mats.
My question to you - is this really true?
best regards
Peter Unold>>>>>

***************************************************************************
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/
***************************************************************************