Abstract: A zip-fastener on each side of a working sail, for example a mainsail, makes it possible to remove from the effective area of the sail an elongated triangular area with its apex at the top and one side of it disposed close to the luff. In one embodiment the zip-fastener on one side has one tape close to and parallel to the luff and another tape extending from the head of the sail to a point on the foot spaced from the luff by up to about 40% of the length of the foot of the sail, while on the other side of the zip-fastener, which likewise extends downward from the head of the sail, has a forward tape that runs down to a location at the foot of the sail about mid-way between the fastener tapes at the first mentioned side of the sail and an aft tape which extends to near the foot of the sail at a location spaced aft of the aft tape of the first mentioned fastener, so that the aft tape of the first mentioned fastener is about mid-way between the fastener tapes of the aft fastener.