A number of people have questioned how
gybing centerboards work. The diagram included below 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 pivot line is
behind the center of area 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.
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