RE: [Paddlewise] Composite vs. Plastic Info

Duane, since you are recycling this old troll of yours from a few years back
I'll recycle some of what I wrote back then and add a couple of new bits.

I will admit there can be vast differences in the toughness of various
fiberglass kayak models due to materials used, construction techniques, and
even hull shapes and sizes. Most paddlers are seeking a craft they can lift
up to their car rack by themselves and will pay the extra for fiberglass or
even Kevlar (TM) or carbon fiber in the laminate to save weight (at the cost
of about $100 per pound saved). Yes many fiberglass kayak owners baby their
kayaks but not necessarily because they have to. When you have nice things
you tend to treat them better. I however revert to the "ram it full speed up
the beach to keep my feet dry approach" once I have gotten a couple of
scratches in the gelcoat of a new kayak. If the rocks are especially jagged
and sharp edged or if there is enough barnacles that I can't miss most of
them I will try to avoid landings on those spots and look for a better
landing spot nearby if the waves are too big to step out in shallow water.
Of course if I had a rotomolded plastic kayak I'd do the same thing to try
to minimize the fins and curly-Q's that fuzz up the plastic and disrupt the
smooth flow of water around the hull by sticking up into it. If I put a
scratch in fiberglass it scrapes away some material rather than plowing it
up into the boundary layer where roughness can really add a drag penalty to
a hull.

I know of kayak companies that ship their fiberglass kayaks packed into a
large shipping container like cordwood and report no problems (it would
worry me though that a hard raised spot on one kayak--like the peak of a
coaming rim--might make a little dent in a new "green" fiberglass deck of
the one it may be pressed hard against). My understanding is that the first
shipment of plastic sea kayaks sent across the country were packed in a rail
car this way. Apparently, there should have been a sign on it warning the
customer that "contents may settle in transport this container was full when
it left the factory". I heard from those who unpacked them that some of the
kayaks on the bottom were only 4 inches deep.
This brings up another major problem with plastic. It is plastic (that is,
it changes shape). Here is a quote from Eddyline's Tom Derrer being
interviewed in Paddlesports Business (Spring 1999):
"I've designed polyethylene kayaks for roto-molding for other people in the
past." he says. "I'd always design a good beautiful shape that would end up
nothing like my original design after the molding largely because of the
unstable nature of polyethylene. It's always changing shape."

Not only are rotomolded kayaks likely to be different than the designer
intended but once they come out of the mold they aren't even the same as
other kayaks made out of the exact same mold. I learned this because of my
timing turn and spin speeds in a lot of kayaks. Sometimes I would test a
kayak model I had previously tested to see what effect different water
temperatures or densities might have on my results and also to test my
consistency over time. One day I paddled a plastic kayak that I had timed
years earlier. The results were astounding. It turned twice as fast as
before. Before it tracked okay but now it was downright squirrelly. I knew I
had improved my turning technique a little in the interim but not nearly
that much. I called the designer to see if the design had been changed. He
said no but my divergent results didn't surprise him at all "They are all
different" he said. He then went on to explain how they come out of the mold
all warm, rubbery and soft and someone tries to wrestle them into a
reasonable shape before they cool and take a set. I have since done retests
on other plastic kayaks and while the differences were definitely a lot more
than retests of fiberglass kayaks of the same model none was as dramatic as
the one reported above. This means that if you are going to buy a plastic
kayak it would make sense to try the one you are actually going to buy and
then be very careful not to distort the shape too much after that. Careful
storage in a cool dark place with regular rotation of position is
recommended for plastic but far less necessary with glass.

I tried unsuccessfully to find the newsletter article I read a several years
back about the roof rack with four kayaks on it that slid off the roof of a
car. Given the reputed toughness of plastic the occupants we surprised that
even though it was one of the middle kayaks on the rack it was the plastic
kayak that shattered and the fiberglass kayaks suffered either only minor
easy to repair damage or some scuffs. If I recall correctly the plastic
kayak was way beyond repair and offered in the newsletter to anyone who
might want it for use as a planter. Some of my customers had a similar
experience. One guy looking for a new boat was 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 by landing hard after jumping a steep wave in
the surf. Rotomolded plastic has a very limited time during which it stays
flexible and therefore not brittle.

Some plastic kayaks come pre-dented from the factory. Under the seat is a
favorite place for them to dish inwards. Too tight shock cord bungies can
create major distortions in their decks and most will dent just under there
own weight (but a lot more on a hot day). Oil canning, and dents and
distortions sure can be ugly and have some negative effect on the speed and
handling of a boat but they are mild problems compared with some of the
other disadvantages of polyethylene plastic as a building material for sea
kayaks.

Contrary to the pipe dreams of some folding and skin boats folks energy is
lost to flexing and this is especially noticeable when the force is applied
off center as it is when paddling a kayak. There is a reason Olympic racing
kayaks are made as stiff as possible. Every time I test paddle a rotomolded
kayak I get the feeling with the first few strokes that somebody has tied a
bucket to the boat and that is what is hurting my acceleration. I then
realize that I'm paddling a heavy and soft plastic kayak and that explains
the slow dead feeling I get when paddling it.

In fact if you made glass kayaks the same weight as you have to make
rotomolded kayaks to have them just barely keep their approximate shape you
would probably find out that a well build fiberglass glass kayak was tougher
than a roto-molded plastic one as well (even when they are both new). Of
course by the time they are both ten to twenty years old the plastic one has
stiffened up a lot more as it has lost plasticizer to the environment. It
gets stiffer and stiffer and more and more brittle in the process.
Eventually it can be easily broken or might just dry out and crack on its
own. Because it has no fiber structure to hold it together any crack will
probably be catastrophic. When this grim fact becomes widely known, as more
and more of the earlier plastic kayak fleet ages to this point, the resale
value of used plastic kayaks will likely plummet. Fiberglass kayaks are
often sold used for more (in inflated dollars) than the original buyer paid
for the boat. In other word the value of the fiberglass kayak degraded
slower than the value of a dollar. Storing a plastic kayak in the sun, (or
ozone, or around some breakdown products of natural gas) can greatly
increase the rate of plasticizer loss. Some plastic kayaks stored outside in
the Baja sun only last a couple of  years.

A fiberglass kayak can be repaired to look like new and simple repairs are
easy to do. Those plastic kayaks that can be repaired usually look like an
ugly mess afterward. Repairing an old cracked (from plasticizer migration)
plastic kayak is hopeless, it will just soon crack somewhere else.

Matt Broze
http://www.marinerkayaks.com


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