The transformation of a stack of exotic plywood and epoxy resin into such a creature? That process chugs along. While the addition of bulkheads fore and aft help ensure that the hull will keep its shape; the next step is a little unsettling. A rather large hole is to be cut into the bottom of the boat.
As it stands, the Unnamed Boat resembles a curled autumn leaf: it would float, but it would not track upwind in a straight line. What's needed is a centerboard to give the boat some purchase against sliding sideways. This centerboard will be raised and lowered with the use of a set of pulleys (as opposed to a keel, which remains in place), allowing the Boat to venture into shallow water (or back onto its trailer) as needed.
And that means a hole. It also means that Mr. Linton has constructed a sort of case for the centerboard, so that once the centerboard is pulled up –– the nautical term is "housed" –– it will be held secure and straight. This structure hugging the centerboard in turn is called the centerboard trunk. It adds a great deal of strength to the hull.
As one can imagine, without beefy support, if the centerboard were simply dropped through the hull, the okume plywood hull would quickly splinter and crack under the strain. In which case, we might choose instead to swim the distance rather than sail it.
So, a centerboard trunk...As with Frankenscot, this trunk will also eventually support the sliding seat for rowing.