Since the strength of the joint between parts comes from the fiberglass seam, there is no reason to have a tight fit.
There are however very good reasons to have gap between panels and other parts of the boat:
- we want to avoid hard spots. A hard spot is a place where one part pushes hard against another. Loads will concentrate in those hard spots and that may result in failure. We want to spread the efforts on the hull and this done by distributing the loads over the full length of a seam through the fiberglass tape or fabric.
This is not a weird requirement for our plans but an important factor in composite boat building. It is mentioned in all text books about fiberglass boat building.
- another reason for avoiding hard spots is to avoid distortion in the panels and "kinky" curves in the panels instead of smooth/fair lines.

We recommend gaps of up to 3/8" between panels and between the hull skin and it's internal framing: bulkheads, stringers etc.