UK Sailmakers’ Encyclopedia of Sails  

10.1 – Glossary

Afterguy: The spinnaker sheet that goes through the jaw of the spinnaker pole. Big boats usually attach a separate sheet and afterguy to both spinnaker clews. Since the afterguy takes more pressure than the sheet, the afterguy is a heavier line. Most of the time afterguys are simply called “guys,” not to be confused with the “foreguy” (see next page).

Aspect Ratio: The height of a foil like a sail, keel, or rudder divided by its width. A high aspect sail has a long luff and short foot.

Bias Elongation: See Warp.

Bi-Radial: A sail construction technique where radial panels emanate from the head and clew. (See tri-radial.)

Boomvang: A block-and-tackle or hydraulic ram that controls the angle of the boom. Lowering the boom tightens the leech of the mainsail.

Clew: The back corner of a sail. On a mainsail the outhaul is attached to the clew; on genoas, the sheets are attached to the clew.

Crosscut: A sail construction technique where all the panels are parallel to each other and perpendicular to the sail’s leech.

Cunningham: A control that adjusts the position of the draft in a sail by changing the tension on a sail’s luff. The control is named after its inventor, Briggs Cunningham.

Draft: 1. The deepest part of the curve in a sail. 2. The distance from the water line to the bottom of a boat’s keel.

E: The designation for the distance from the aft face of the mast to the outermost point on the boom to which the mainsail is pulled.

Fill Threads: See Warp.

Flattening Reef: A sail control that flattens the bottom part of the mainsail. It’s called a reef because the control line passes through a grommet on the leech of the sail about a foot above the boom. When the line is tightened, the grommet is pulled down to the boom and out as far as the sail can stretch. Also called “flattener”.

Foot: The bottom edge of a sail.

Foreguy: The line that pulls the outboard end of the spinnaker pole down — not to be confused with the “afterguy.” 

Foretriangle: The triangle formed by the forestay, forward edge of mast and foredeck.

Gooseneck: The mechanical joint that connects the boom to the mast

Halyard: A line used to hoist or lower a sail.

Head: The top corner of a sail

Headstay Sag: The deflection of the headstay to leeward and aft caused by the force of wind on the sail. Because of the physics, no matter how tight your rigging, there will always be some sag in the headstay. I: The designation for the measurement of the height of foretriangle. Each rating rule has slightly different places which to measure from. J: The designation for the measurement of the base of the foretriangle, e.g., the distance between the mast and the forestay

Jib Lead: The block or fairlead, through which the jib sheet passes, between the clew of the jib and the winch. The position of the lead has a great effect on the shape of the jib.

Laminated Sail Cloth: A fabric that has multiple (at least two) layers of fiber and film that have been permanently bonded by adhesive. The film provides the structure to control bias stretch. The fiber, in the form of woven material or unwoven yarn, provides stretch resistance in the fabric’s oriented direction. Also see “Scrim Cloth”.

Lazy Guy/Lazy Sheet: Lines used on big boats for jibing the spinnaker. Each clew of the spinnaker has a guy and sheet attached, the ones not in use are called the lazy guy and lazy sheet. 

Lee Helm or Leeward Helm: The tendency of a boat to bear off when the helm is released. Lee helm is nor-mally encountered in light air or if your mast is too far forward in the boat. See “Weather Helm.”

Leech: The back edge of a sail.

LP: The abbreviation for Luff Perpendicular, which designates the shortest distance from the clew to the luff of a genoa. The size of genoas is expressed in a 54 percentage, which is the LP divided by J. For example, if a boat’s J measurement is 12 feet, a 150% genoa will have an LP of 18 feet.

Luff: 1. The forward edge of a sail. 2. The flapping of a sail caused by the boat heading too close to the wind or because the sail is not trimmed tight enough. 3. “Luffing” is altering your course toward the wind. In racing, luffing is a defense permitting a leeward boat to protect its wind from a boat passing to windward.

Outhaul: The control line that pulls the mainsail clew to the end of the boom, tightening the foot of the sail. 

Overpowered/Underpowered: A boat is overpow-ered when it heels too much from having too much sail up. Underpowered is when a boat is slowed because it does not have enough sail up. P: The designation for the measurement from the top of the boom at the gooseneck to the highest point on the mast that the mainsail will be raised.

Rake: The mast’s inclination from vertical. The amount of rake is measured from the back of the mast at the partners to a plumb line hanging from the main halyard.

Reacher: A high-clewed genoa used when reaching in heavy winds. Also know as a “blast reacher.”

Roach: The area of a mainsail that protrudes beyond a straight line from the head to the clew. The roach is supported by battens.

Scrim Cloth: An extremely loosely woven cloth. Lami-nated to Mylar, scrims make strong, lightweight sail cloths. Scrims are distinctive since there is empty space between threads.

Shelf Foot: An option for mainsails that gives extra control to shaping the lower third of the mainsail. The name comes from the flat piece of cloth that connects the bottom of the sail to the boom.

Shroud: Wires that support the mast athwartships.

Spreader: Strut attached to the side of the mast, which amplifies the shrouds ability to support the mast.

Squaring the Pole: Tightening the afterguy, which pulls the spinnaker pole back.

Stay: Wires that support the mast fore-and-aft, e.g.,”forestay” and “backstay.” Also see “Shroud.”

Tack: 1. The lower forward corner of a sail. 2. Turning the boat so that the bow passes through the eye of the wind. 3. (Port or Starboard) You are sailing on starboard tack when the boom is on the port side and vice versa.

Tape-Drive ® : The patented sail construction system used by UK Sailmakers, which uses high strength Kevlar tapes to lock in a sail’s designed shape. The tapes radiate from the three corners of the sail along computer-mapped load lines.

Telltales: Streamers attached to the sail to indicate wind flow. Tri-Radial: A sail construction technique where radial panels emanate from all three corners of the sail. (Also see bi-radial.)

Warp, Fill and Bias: Woven cloth has threads running in two directions. Fill threads run perpendicular to the longest side of the cloth and warp threads are parallel to the longest edge of the cloth. The strength of woven cloth lies only in the direction of the threads. Stresses not parallel to the threads are called bias stresses. The farther off the thread line the stresses are, the greater the distortion of the cloth. Bias stresses at 45 degrees to the threads distort the sail the most.

Warp-Oriented Cloth: Sail cloth that has more strength in the warp direction than in the fill direction. Extra strength is cre
ated by more or stronger threads run-ning in the warp direction. Some extreme warp-oriented laminates have no fill yarns. Instead, yarns are just glued to the Mylar film in the warp direction. These extreme warp-oriented fabrics rely on the Mylar for strength in all other directions.

Weather or Windward Helm: The tendency of a boat to head up when the helm is released. Weather helm is measured in degrees of angle that the rudder must be turned to sail a straight course. See “Leeward Helm.”

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