How a Gybing Centerboard Works

by Alexander Meller, with Larry Tuttle,  11/14/2003.

A number of people have questioned how gybing centerboards work. The diagram included here should illustrate what they are and how they work:

The drawing is of two centerboard trunks, each containing a centerboard, viewed from straight above. Imagine the two boats are parallel, sailing from the bottom to the top of the page, on port tack. The centerboard trunk on the left has a conventional non-gybing board - you can see the parallel sides of the CB head up against the sides of the centerboard trunk.

The centerboard trunk on the right has a gybing board in it. The CB head does not have two parallel sides, rather the head is a parallelogram. Since the boat is on port tack, the centerboard is loaded on the leeward - right hand - side, and rocks to weather in the trunk. That is because the axis of rotation (vertical) is behind the center of area, and more important center of pressure of the board. Gybing boards take an angle of attack relative to the centerline of the boat.

While gybing centerboards with angles of attack of over 4 degrees have been built, Waterat and Lindsay foils are in the 2.5 to 2.8 degree range.

The boat's leeway angle is determined by the net side force on the sails and the area and effeciency of the foils. The hull has a small effect. An estimate of the average leeway angle is 4 degrees, so the boats in the diagram are actually sailing slightly to the right of straight up on the page.

This means that for a given set of conditions and equipemnt, if you change from a fixed to a gybing CB, the leeway angle will not change. What will change is that the bow will rotate to leeward about 2.5 degrees.

Then things get interesting. The sailplan is rotated with the hull. So the there is less drag on the hull, and you can sheet the sails for the lower heading. This causes the boat to sail faster, which creates more lift. So you point higher. Lift increases as the square of the velocity.

The effect is the most dramatic in conditions where the boat accelerates easily, flat water.

If you merely steer the gybing board equipped boat higher, then you go slower, and the board may actually achieve a higher angle to the water flow.

The gybing board is most effective in light to medium air and flattish water. In windy conditions you do not want the centerboard to gybe. Fortunately the centerboards are designed so that by bringing them up very slightly, part of the airfoil blade goes inside the centerboard trunk, and jams at full width, stopping the centerboard from gybing.

Though Lindsay no longer makes foils, a number of North American 505s are still equipped with them. Gybing 505 centerboards are available from Waterat Sailing Equipment in Santa Cruz California, Milanes & White in the UK, and a number of other sources.

For further information on 505 centerboards:

Two Bransford Eck articles on 505 foils from old Tank Talk magazines:

Bransford Eck Article Number 1

Bransford Eck Article Number 2

The University of Palermo Italy conducted a centerboard study ending in 2008. The University had chosen the 505, because it is one of the few existing classes that has not significant size and shape restrictions on the centerboard. Many class members provided data and feedback and now finally the results have been published. The report is a theoretical study on how to improve the 505 centerboard. Only one prototype was created and used on a 505 in Palermo  prior to the WC. Unfortunately the prototype broke during a high wind test, due too poor construction, and no other prototypes were subsequently built. If you love technical readings, this publication might be of interest to you:

University of Palermo Centerboard Study