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).
As the board and the attached fins/foils glide through the water, the nature of their shape creates lift and thrust as the fins “wants” to move forward through the water due to the foil. In stand up paddle surfing, combinations of fins and fin shapes create differing amounts of lift, thrust (and residual drag) while providing control and stability (especially “roll” stability for stand up).
50/50 is a symmetrical foil where both sides are slightly convex. Yields even water flow for stability and control. Found on single fins setups and center fins (in a thruster), or usually the back three fins (on a quad).
80/20 Foils (or Variants)
Often used for side fins or front fins (in a quad), these foils yield the stability of a 50/50 or center fin with smooth transitional performance for rail-to-rail surfing.
Flat (inside) Foil
Flat inside faces combine with a convex outside face, offering an even combination of drive, pivot and hold in a variety of conditions. This is a fin often found in most “stock” fin sets as the side fins in a ‘thruster’ (three fin) setup.
Concave Inside Foils
An often more expensive option since boards usually ship with flat foil side/front fins, especially if you’ve purchase a quad setup – since the ‘front fins’ option combine with 50/50s in the back. This foil consists of a convex outside face and concave inside face designed to produce more lift and less drag for more speed without sacrificing hold. Often, the convex, inside curve is nearer the BASE of the fin.