fiberglass extension of a dingy

hi all... A lot of advice needed. I have a 12foot fiberglass dingy i would like to cut it in half and extend it to 15feet is it a workable idea.... What will i need to do to make sure it will be as strong as it is now..

 

It would be a great way to learn glassing, though the result will be a sort of jon boat you could buy for not much money anyway, maybe cheaper than stretching the dinghy.
To maintain strength, the glassing must carry through the gunwale and keelson (if present) sections without any weak spots. This will require feathering the glass with plenty of overlap. If your dinghy is hard-chined a epoxy-encapsulated wood chine log could be placed (bedded/bonded) into the chine corners and run long (so that the underside is curved to follow the old hull upsweep for a foot or so. I'd suggest the bottom be reinforced two ways: with two or three stringers like the chine logs mentioned, running longitudinally and running past the joining lines fore and aft, and then by using a crossing seat box (either with or without foam flotation) to brace the sides to the bottom.
As always, none of this advice means much without a photo of your particular boat. I'm just guessing the dinghy is hard-chined. No matter what type bottom, it will likely be rather flattish, so there is where you'll have oil-canning and distortion and you'll need to reinforce well.
As you must know, curvature ends pretty much where the new section begins unless you distort (by spreading) the old sides on purpose, which will allow you to increase the beam of the boat. Otherwise, the sides and bottom will be dead straight and straight sections are not very stiff. Round-sectioned boats can always be built lighter and stronger because of this.

 

It is quite possible to stretch a boat though is it economic? I have done it with two Rowing boats both 18ft long to start with. The first boat had her stern extended by 3 ft then two years later she was sawn in half & lengthened by 5ft taking her to 26ft, she was simulated clinker so was not that easy! In our case the exercise was warranted as it provided two boats for our new club at very low cost. Both boats are still in use and nobody suspects they are cut & shuts!
If you are going to do it you need a keel jig to align the two halves & some gunwale extensions. Formica or similar is useful for forming one shot inside out moulds the melamine face gives a good release with a couple of coats of wax.
As said before use a good overlap & feather the edges well.