This site is an absolute treasure trove of technical material on sail and hull design, with a focus on model yachts but most of the physics scales up. It covers aerofoil theory (momentum, circulation, lifting line), sail twist, downwash, planform effects, Reynolds number, hull resistance, appendage design, VMG, and more.

Lots of spreadsheets and practical calculators too—bulb shape optimization, wetted surface area, simple VPP models. If you're into sail performance modeling or yacht design, this is one of the best resources out there.