A lot of natural and/or synthetic based materials can be made to shrink a bit after applying them to a kayak frame, wither by soaking them with water (cotton,hemp, wool, linen, etc.) or with a bit of heat (dacron, and a few other polyesters). This gives the boat builder some leeway in design and construction. If the skin is made a bit loose, you just (somehow) tighten it!

To steal fron the opening lines of The Lone Ranger, "Return with us now to the thrilling days of yesteryear" Canoes were made of birchbark (which neither stretched nor shrank apreciably), and kayaks and coracles were covered with animal hides, which tended to soften and stretch when wet. Sure, you could shrink those skins -- but only when the boats were out of the water!

Somehow these boats thrived despite these limitations. I suspect one reason is that the skins had gores cut into it, which would be pulled tighter and stitched shut before being covered with a waterproofing agent. another possibility is that the frame and skin were perfectly matched during construction.

My references on building birchbrak canoes show the steps to be roughly: 1)Make the gunwales and suspend them over the birchbark. 2) Fold up the birch bark skin so that the crease rests on the ground and the folded edged come up to the gunwales. #) make your ribs and force them into the skin, inserting them at the widest point (near the center of the boat) and then sliding them fore or aft until they filled the skin tightly. 4) Repeat with progressively larger ribs until you fill the boat.

From what I have seen of kayak frames, the same building process could have occurred, but the skin would be of animal hide instead of bark. Today we commonly make the ribs to conform to some design idea, and cover them -- but the old birchbark way of filling the covering could be tried.

You'll find that the design of the boat is going to be similar to, but different from, more modern designs, as it is imposed by the shape of the skin. this can be a very limiting factor in design. Actually, tough,it does simplify one design parameter, wetted area can be figured rather easily as you know the skin can be reshaped as a flat piece -- no complicated curves to calculate.

If your sunbrella fabric is preshrunk, you may be able to still stretch it a bit to force it to go around curves better, or, you can reevaluate some of the design lines on the hull. Perhaps a different profile on the bow or stern would be a better fit for a material that had limited stretch and shrink potential. Making such major frame changes might mean building an entirely different boat, though.

PGJ