mctimlin asks:

Is it possible to use 'exterior' grade plywoods which are much cheaper than
marine plywoods?

The literal answer would be "Yes", but.....
It all depends on how long you want to keep the boat for.
Marine ply exists for a very good reason. I would recommend a google search on
a thread from a few weeks ago that rambled on forever about just this topic.

Steve
Stephen C. Baker - Yacht Designer
http://members.aol.com/SailDesign/pr...cbweb/home.htm