Centreboards give increased performance to windward over fixed skegs and provide greater 'beachability' due to reduced draft with the boards up. The two types of centreboards are daggerboards (boards which slide down in a slot like a dagger) and pivoting boards which pivot back into a long slot. Pivoting boards are not as popular as daggerboards due to the increased turbulence of the exposed long slot and the fact that the centre of lateral resistance moves aft when the pivoting board is partially raised.
To maximise the sailing advantages use them in the following ways:
When sailing to windward in light wind to moderate winds have the centreboard(s) fully down. In very light winds have only one centreboard down on a catamaran to reduce the wetted surface area (and thus friction).
To windward in heavy wind partially retract the board(s) to reduce the pressures on the case and board(s).
On a catamaran the boards should be adjusted to balance the helm. When broad reaching this usually means the windward board half down and the leeward centreboard should be nearly fully up. When running have both centreboards nearly fully up. A small amount of board down will improve downwind steering. At high boat speed the turbulence from the centreboard may affect the rudders so retract them fully.
On trimarans the centreboard is usually pulled up completely to reduce drag and turbulence and to enable the multihull to point slightly higher (and thus have a higher apparent wind) while slipping sideways to create optimum speed downwind.
When sailing in shallow waters on multihulls without kick up rudders it is a good idea to keep the centreboards down to a depth slightly beyond that of the rudders. It is usually easier to get off following a grounding when it involves the centreboard(s). Damage to a centreboard bottom is preferable to that of the rudder(s).
A sacrificial section on the base of the board(s) will save having to rebuild a complete board. To manufacture such a board make one of normal size and then cut off the lower 20-40 cm and then glue it back on.
The centreboard case should be a lot stronger than the board. If you run aground it is cheaper to build a board than repair a broken case.
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