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AXIS Foils - Basics of Foil Design

The basics of foil design

Here we help explain the key variables in hydrofoil design. It will also help you better understand the design specifications of the AXIS range

Span: The length of the foil from wing-tip to wing-tip.

Chord: The width of the foil (90 degrees to the span). There is maximum chord and mean average chord. Maximum chord is the widest part of the chord normally at the middle of the wing, the mean average chord is the average chord over the entire wing. Span divided by mean average chord gives you the aspect ratio.

Thickness: How thick the foil is, normally expressed as a percentage of the chord.

Foil section: This refers to actual cross sectional shape and curve of the foil, i.e. is the front portion blunt or sharp? Where is the point of maximum thickness? This will have a huge bearing on the lift the foil produces and the speed range it will perform in without ‘stalling’ (when the water flow dis-engages from the foil surface causing you to crash back down).

Camber: Camber is a key element of the foil section. Unlike centre fins on surfboards, the cross section of hydrofoils is not symmetrical – the resulting pressure difference as water flows around the foil is what creates upward lift. The degree of this asymmetry is called the camber. The more camber, the more lift a foil will generate. However camber also increases drag.

Area: The total surface area of the foil. While the industry has made area the key variable with which foils are compared, for us it is no more important than the other design characteristics featured above. The performance of a hydrofoil is determined by how ALL these variables combine together. For example, it is totally possible to have a foil with less area than another to produce greater lift if the thickness and camber are greater.