If this was an enticing way to build a boat it would be in common practice, wouldn't it? The idea seems so simple.
There is a book "Building Your Kevlar Canoe", which is really about building fiberglass canoes on a male plug (which is what you are considering)with a bit of Kevlar, that goes thru' the entire process. For one thing they go thru' the process of building a mold which is five times harder to do than laminating the canoe hull. If they could have used plastic wrap they would have but they covered the mold with a couple of pounds of melted parafin and car wax over that. They must have had a reason.
When they laminated the hull they did it in one shot. Three experienced people working as long as it takes to apply about 4 layers of glass. One mixes the epoxy the other two pouring and squeegying the glass. You can imagine what it's like to do this for 10-15 hours. Or more if it is required. This technique is used to insure there is a rock-solid bond between all the laminates.
The downside to all this is every step has to be about perfect. If the hull comes out bumpy, lumpy or wrinkled you cant' take a grinder to it. The glass is the strength of the hull. If you cut into it to make it pretty you are causing irrepairable damage.
If you go way back into the archives here you'll find there was a spate of posts about wrapping glass around a styrofoam plug like the surfboarders do and then digging the styrofoam. I'm sure some built boats this way but it didn't seem to catch on.
Your idea is not new and I'm certain it has been done innumerable times. The hook is, can you produce a boat you would be proud to own or would it be a cobbled up mess. The difference is in the skill with which it is done. If the hull doesn't come off the mold nearly perfect there is little you can do to make it look right. Grinding and filling on a glass hull aren't condusive to a good result.

: Hi,

: I was just thinking about how we use wood as the filler between two layers of
: epoxy/glass, whereas racing kayaks tend to just be thicker glass (or
: kevlar or carbon fibre)covered in gelcoat, and so are really just a 'glass
: shell made in or over a mould. Stiffness coming from heavier glass rather
: than composite like strip/S&G.

: Then I wondered about making a kayak off another one. If exact dimensions
: weren't important (except the new one would be a small percentage bigger),
: would it be possible to cover an existing kayak with plastic wrap and then
: drape 'glass cloth over the hull and epoxy? Then do the same with the
: deck, cut out cockpit, etc and then join with 'glass tape? Then paint with
: gelcoat and viola, a close duplicate.

: Kind of like using the kayak as a male mould rather than building one in a
: female mould. Obviously a heavier glass would be needed to have stiffness,
: maybe with bulkheads and some other details.

: I'm not interested in copyright discussions about copying the hull - there's
: enough of those already! :-) - but just curious if anyone's done this or
: if it would work?

: Call it late-night kayak-builder's curiosity :-)

: Thanks,
: Darren
: Melbourne, Australia