Re: stringer problem
If you have already opened up the floor in the area of the rot (and beyond) it won't be very difficult if you're good at repairs of this nature. The best way to repair the bad section would be to cut a long diagonal on the adjacent good section and a mating diagonal on the new piece to give as long a glue joint as feasible. You could butt the old and new together and use marine ply on both sides of the joint and glue and screw the sandwich together as an alternative. Make sure you use a very strong waterproof glue such as water resorsinol two part. Some urethane glues should work well also. Assuming the rotted piece butts up to the transom, use plenty of glue on the new piece and make some angle plates out of stainless and through bolt them to the new piece and screw them to the transom. Before you drop the new piece in heavily sand the hull at least 2 inches to the each side and underneath where the new piece will sit. Mix some chopped fiberglass (you buy it that way) with resin and put a generous amount right under where the new piece will sit. Once the new piece is in and glue and screwed and cured, put at least two layers of mat over the whole mess. Then you put the floor back together. I would guess it's a two to three day job.
