Parts of The Sail
The lower edge of a triangular sail is called the "foot" of the sail, while the upper point is known as the "head". The lower two points of the sail, on either end of the foot, are called the "tack" (forward) and "clew" (aft). The forward edge of the sail is called the "luff" (from which derives the term "luffing", a rippling of the sail when the angle of the wind fails to maintain a good aerodynamic shape near the luff). The aft edge of a sail is called the "leech". The curved sail area beyond a straight line from the head to the clew is known as the "roach". Typically this is greater in a racing sail and may be absent in a cruising sail.The roach is held in shape by sail battens which maybe full length or short.
Modern sails are designed such that the warp and the weft of the sailcloth are oriented parallel to the luff and foot of the sail. This places the most stretchable axis of the cloth along the diagonal axis (parallel to the leech), and makes it possible for sailors to reduce the draft of the sail by tensioning the sail, mast and boom in various ways.
Often tell-tales, small pieces of yarn, are attached to the sail. They are used as a guide when trimming the sail as they indicate the wind flow across the sail.
An alternative approach to sail design is that used in junks, originally an oriental design. It uses horizontal sail curving to produce an efficient and easily controlled sail-plan.
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