: Time for some definitions. These are mine. [...]

Wow, thanks for such a detailed response! This clears a number of things up for me.

: Yes. Basically it would be the shape of the bottom edge, and sides,
: of a bulkhead at that location. They don't need to be very thick
: to give strength and support. 1/2" high, or even 3/8"
: will help a lot. You can glue in strips of wood or lay strips of
: glass cloth over a piece of vinyl tubing after saturating the
: cloth with a liquid plastic resin. When the resin hardens you'll
: have fiberglass ribs which are cast in place.

This really doesn't sound hard at all. Probably a good first project.

: Can I persuade you to make two out of plywood? One would go behind
: the cockpit opening. The other would go in front of your feet.
: Make them slightly smaller than the interior at the location
: where you'll mount them. You don't want them
: pushing against the sides. Use a flexible adhesive to mount them.
: When the boat is in the water the waterpressure on the hull will
: try to 'oilcan' it, but those solid bulkheads will resist the
: squeezing and limit the amount of movement.
: They'll also create sealed air chambers. That is less water to bail
: out if the boat swamps.

Since looking at this boat more closely I've noticed that there are a lot of locations that I would like to repair the gelcoat. There are lots and lots of cracks, on the deck and hull, and a number of deepish scars. If these repairs go well I will certainly consider making the next project replacing the foam with plywood.

Thanks again,
Brian