Apparent Wind

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Sail shape

The weather and apparent wind

On day sailing catamarans, which are often over-canvassed, the lighter and faster the boat the flatter the sails need to be to allow for the increased apparent wind. In contrast offshore boats can all have efficient camber sails. Close sheeting is the answer to manage the apparent wind angle increase. The sheeting angle is the angle of the gap between the headsail and the mainsail. If the headsail is sheeted out wide then it is difficult to point high into the wind. In strong conditions this may make it impossible to sail effectively to windward.

In heavy conditions the true wind and apparent wind angles are much closer therefore fully reefed sails with shape are faster than flat feathering sails.

Storm jibs should have shape, not be flat blades. When your multihull has to beat off a lee shore in gale conditions this will be appreciated.

Cumulus clouds often mean there is vigorous mixing of the lower levels of the atmosphere and thus there is less difference in apparent wind angles between the waterline and the masthead. In these conditions less twist in the sail will maximise the use of apparent wind.

APPARENT WIND: Racing use

Racing use of apparent wind

Keep the sails centre of fullness about one third of the way back from the front, especially on the jib. The windward telltales should be just lifting. In most situations a high performance multihull will sail best downwind when the apparent wind is between 90 - 110 degrees apparent. Set your yacht up for this wind angle and bring the bows toward the wind until nearly luffing. As the yacht accelerates bear away keeping the power in the sails. If the sail collapses, let the yacht slow and start again. A modern performance multihull will carry the apparent wind direction, with sails set, to 110 -130 degrees apparent.

When racing downwind in this manner lift the centreboard until it is flush with the bottom. This reduces drag and maximises leeway.

On a catamaran which needs a centreboard for steering control then lift the leeward one flush and leave the windward one down a bit. By having only the windward centreboard slightly down when the windward hull lifts the catamaran will slip sideways and the boat will have less capsize potential.

Multihull Seamanship Rule:

Pull the centreboard(s) up when on all wind angles except sailing to windward.

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