William R. Watt wrote:
Watch the weight. It's a challenge to build a boat light enough to lift on
and off the roof of a car yet large enough to be of much use. Canoes
and kayaks are popular because they are light. Decide before hand if you
will be doing the lifting yourself or have a helper. That will govern the
size and weight of boat you can practically carry on the roof of a car.
Some people make gin pole hoists so they can lift bigger boats solo.

I don't disagree, but thought I'd point out that when I build my Bolger
Windsprint (16' sailboat), I car-topped. I had a battleship-sized
Pontiac station wagon from the 70s at the time -- neither of my current
cars would be big enough for it. And I would have a backache for a week
if I tried to put the Windsprint on top of a car today.

BTW, there are plenty of plywood kayak designs available.
http://www.clcboats.com has some nice ones, as well as a couple pulling
boats and small sailboats.

Patrick