701 Hydrofoils

How does a hydrofoil work?

Believe it or not the hydrofoil works just like the sails on a sailboat.  Essentially hydrofoils consist of a group of underwater wings set horizontally beneath the sailboat.

A wing, like a sail, works on Bernoulli's principal.  If a particle stream (either air or water) is split and one stream is caused to travel a greater distance than the other stream before they meet up again than the pressure is lower in the faster moving stream.  This pressure difference is what your sails are sucked into to create forward motion.  This same pressure difference on a hydrofoil causes the foil to "fly" in the water thus raising the sailboat up and out of the water and eliminating hull drag (See Hull Speed).  Hull drag, if you remember from earlier lessons is what limits a displacement boats top speed.  This top speed being known as hull speed.  In the diagram below the flow of water is indicated by the blue arrows and the lifting force generated is shown by the black arrows.

Now imagine you had a triple hull sailboat with hydrofoils down below the surface of the water and the three hulls were connected by air-foils or wings.  This could possibly give you more lift and reduce the size of the underwater hydrofoils.  The point of adding an air-foil would be to minimize forward resistance.  An air-foil creates much less resistance to forward motion than a hydrofoil alone.  These air-foils would be moving through air, not water.  Consequently the hydrofoils on a sailboat could be made smaller and thus create even less drag which in turn would translate to greater speed.There would have to be walks or netting above the wings to allow crew to move about freely.  The wings could even have control surfaces on them to shape the wing and alter it's characteristics while in "flight."  An interesting thought.

Basic Air-Foil Concept

Air-Foil with Control Surfaces

Crew Walks Added

Previous Course    Back to Basics for Sailors     Return to Home Page

Go to Project Gallery to see how you can build items for your boat.