: about being cheap - I think the skin for a sof kayak is something
: one should NOT (!) be cheap about.

: about that elast-o-seal that stretches 1000% & 400%,

: I don't think that's a good idea.
: I think we need a kayak to be as stiff as possible, so the energy
: goes to pushing the boat forward, & not to stretching/bending the hull.

Erez,

I think you may be confusing flexibility of the skin with flexibility of the boat.

There are two competing points of view on hull stiffness: rigid hull, or very flexible hull. Those who build a very rigid hull (like aluminum baidarkas with welded bracing) make the same claim as you, that the stiffness contributes to speed and steering. Those who build flexible hulls (like the wood baidarkas with ribs lashed to the stringers with string) believe they gain speed by not fighting the waves, but by bending with them a bit, instead.

You can take whatever side of the argument you want here. However, you have to understand that the stiffness that is most important comes from the sheer chines, or what would be gunwales on a canoe. Think of a canoe for a moment. The gunwales point forward, and hanging under them are the ribs. As the waves flow past, each rib is put under pressure and it pushes up on the gunwale, flexing the hull of the boat up and down a bit -- but the boat is still pointing straight forward. It is kinda like the wheels on your car being attached with springs and shock absorbers. They can still steer straight, and transmit power from the engine to the road, but they can move freely up and down when there are bumps.

Flexible skins are the norm on skin-on-frame boats. For skins that are to be removed, as in collapsible frame kayaks, flexibility is the main requirement. A skin that is not very flexible would crack, and leak.

Some believe that flexible skinns contribute to the speed. One reason for this is that when you hit a wave you deflect a light piece of fabric, which changes shape. Rather than fighting the externals forces by lifting the boat, and making you drive more weight forward with your paddle, this movement either absorbs some of the external energy, or deflects it without causing deflection of the hull's frame. Supposedly the waves hit you, but they don't move you off course.

Hypalon probably stretches as much or more than the 400% to 1000% of the Sno- cote. They are all synthetic rubber compounds. The Sno-cote is a water based product that is about as safe to work with as latex paint. Hypalon is smelly and has some rather nasty solvents in it. Commercial companies can apply the hypalon to sheets of fabric in controlled conditions, and have proper ventilation and safety. For a home builder there are a few more concerns. You only need a few yards of this material to skin a boat. You may decide to buy it already made rather than devote the time and cost to fabricate it yourself.

Cost is ALWAYS a factor.

I have not been able to find a good supplier of hypalon-coated polyester or nylon fabric at a price I want to pay, although I know that it is available, and that it is used in the construction of inflatable boats.

On the other hand, the heavy-duty vinyl-coated polyester fabrics that are used for commercial tents and awnings are available and at reasonable prices.

Skins on SOF kayaks are very durable as they flex and can move away from obstructions, rather than get torn by them; but they do get ripped, torn, damaged, and abused. They are usually replaced at regular intervals. A 5 to 10 year life is good.

Figure your costs for a "cheap" skin (canvas or dacron) Remember that with a collapsible kayak you are taking the skin off each time you disassemble the boat, so changing to a new skin is actually the same as your usual assembly time. You can get launched with a cheap skin, and then later build a replacement skin from more expensive materials. Put that on when you feel like it, and your first skin will serve as a spare -- or something to use when you expect it will get abused -- landing on rocky beaches, for instance.

Hope this helps.

PGJ