Fiberglassing the inside of the boat
I have included this in the WebLetter after receiving a phone call asking this question. It is not the first and will not be the last time this question is asked. It is important to reiterate what we have said in many places on this site: Fiberglass adds very little strength to a plywood boat. If you want to make the boat stronger, increase plywood thickness. Fiberglass is primarily applied to protect against abrasion and reduce maintenance.
The majority of the designs in our catalog are for sheet plywood with sawn frames. This discussion deals with this type of construction.
Above is a portion of the Glen-L 17 drawings that shows the bottom of a frame, keel, bottom battens and chines. In order to fiberglass the inside, it is necessary to lay the cloth around all the logitudinal members. If interior corners are not filleted, you are almost guaranteed gaps between the cloth and these interior corners. Should water get into these "runways", for any reason, it would travel along the member and make dry rot the likely outcome.
This is a plan view that shows the athwartship frames. These would also be difficult to fiberglass around, and if filleted and fiberglassed, how would you provide limbers to allow water to drain aft?
We have built a lot of boats over the years and have never fiberglassed the interior of a framed boat. We have fiberglassed decks and cockpit soles, but not hull interiors. If you wanted to spend the time, could you do it? Yes. But is is expensive, difficult to do and has very little payoff and some potential problems. I would not fiberglass the inside of my boat.