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This is a dangerous situation that risks serious damage. A sudden wind shift can cause a square-rigged vessel to be "caught aback" with all sails aback.
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The purpose may be to reduce speed (such as when a ship-of-the-line is keeping station with others), to heave to or to assist moving the ship's head through the eye of the wind when tacking. On a square-rigged ship, any of the square sails can be braced round to be aback. A aback A sail is aback when the wind fills it from the opposite side to the one normally used to move the vessel forward.
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