There is some misinformation flying around in this thread. It is possible to mould a new section into a hull, I've done it many times, but it isn't all that easy. There's more than one way of doing it, which I'll describe in a moment, but first you need a bare hull, and that means removing anything you want to re-use from the model, then stripping off the paint. I haven't tried it myself, but the car brake fluid method that was discussed here a couple of weeks ago sounds like the best method. Otherwise, it's wet & dry and a lot of patience.
First the best, but more difficult hull-lengthening method. Make a mould, preferably in GRP, though plaster of Paris also works, from the mid-section of the hull. I'm leaving out any advice on waxing and release agents here, if you don't know a fair bit about this kind of thing, then this technique may be beyond you. Then cut the hull in two using a hacksaw blade. With the two hull halves in the new mould, slide them apart to the length you want, and laminate the new section. I use double-sided tape between mould and the old hull sections to hold everything firmly in place, but you'll also need some clamps. Then you have to apply a gelcoat, a bit of extra thickness for rubbing down if the old hull isn't seating perfectly into the mould, then the usual glass mat and resin, forget epoxy, use the material that the hull was made from originally. Avoiding shipmate's 'fault lines' is easy, you have to chamfer the edges of the old hull on the inside, so that all the real strength is in the new fibreglass, but don't thin down to much less than 1mm, or the edges may start to curl, slightly. If you take your time and do it right, there won't be any weak spots, and the join should be completely undetectable under a few coats of paint. You can't fill a tight butt joint, cracks will always appear eventually, so use a Dremel to cut a shallow groove a few mm wide on the outside, and fill this with P38 before rubbing the whole thing down.
Next the slightly easier method. Prepare three lengths of straight timber about a foot long, using something like half inch ply. Ideally you should have a helper for the next bit, but tape fairly thick flexible polythene over the gap between the two hull halves, apply double-sided tape to the wood and the hull and stick everything together applying one piece of wood to the hull bottom and one to each side. If you don't get it right first time, pull it off and try again, but this should result in a pretty solid assembly eventually. Then with the hull edges chamfered as described before, apply gelcoat, glass mat & polyester resin to the inside, going at least an inch over the joint either side with the mat & resin. Once this is hard, remove the wood and peel off the polythene. Clean off any traces of tape adhesive with thinners, and fill & sand the outside until you're happy with it. There will be a lot more sanding & filling with this method, but it avoids all the hassle of making a mould that you'll probably only ever use one time.
You'll probably get some alternative advice about 'easier' methods, same as with that recent thread on the distorted Bluebird hull, but there are no easy fixes for difficult problems. I do this kind of thing for a living, these are the methods I use, and either method will work OK if done properly
