So let’s continue to Part Two with a definition (part One of this article is here):
A foil is a solid object that has a specific shape designed to produce force when passed through a gas or liquid. For instance, when air or water move past it (or it moves through air or water as in surfing) force is produced as “lift” and “drag” (or thrust). Ask most people about foils and they will mention an airplane wing and the lift produced by the shape of the wing that allows flight when you thrust the wings through the air with a jet engine. When talking surf fins, the ‘foils’ attached as fins on the bottom of a surfboard are given a specific angle of attack relative to the stringer (center line of the board).