We've compiled this glossary of sailing terms to help you learn just what that funny word means!

Apparent Wind Angle

The direction the wind appears to be blowing from the perspective of a moving observer. It is different from the true wind angle because of the effect of the boat's speed and direction. Super sail designers take apparent wind angles into account when designing sails to perform at optimal efficiency.

Apparent Wind Speed

The wind speed as measured by the moving observer. Like apparent wind angle, it is different from the true wind speed because of the effect of the boat's speed and direction. Heading into the wind creates apparent winds stronger than the true wind, and heading away from it decreases the apparent wind. Super sail designers take apparent wind speeds into account, but sailors should too when making decisions about sail selection, and reef planning.


A stiffener, usually fiberglass, used to project roach in the leech of a sail. Battens may be partial, extending only a few feet into the sail at an angle perpendicular to the leech, or full, stretching from leech to luff parallel to the boom. Consult Super Sailmakers to determine which is appropriate for your sail.

Boom Vang

A device, usually a system of line, hydraulic bar, or levers, connected between the base of the mast and the boom that holds or pulls the boom down to counteract the lifting force of the sail. It is an important sail shape control that also controls the vertical twist in the leech of the mainsail.


The bottom, aft corner of a sail, where the leech meets the foot.


A line passed through a reinforced ring above the tack to allow easy tightening of the luff of a sail. Tightening a sail in this manner flattens the sail, making it more efficient for upwind sailing. Conversely, when off the wind, easing the cunningham replaces the curve, adding more power. While tightening or loosening the halyard can accomplish the same thing, cunningham adjustment is typically easier.


The distance from the back of the mast to the aftermost point a mainsail could reach on the boom or its black band. More simply (and a little less accurately), the mainsail foot length. See also Rig Dimensions.


The distance from the back of the mizzen mast to the aftermost point a mainsail could reach on the mizzen boom. More simply (and a little less accurately), the mizzen foot length. See also Rig Dimensions.


The bottom edge of a sail.


The intersection of the boom to the mast, and any fitting that enables it.


A clip, hook, or other fitting that attaches a sail, usually a jib or genoa, to its stay.


The top corner of a sail. In the case of a gaff or sprit-rigged sail, the head is both the corner of the sail at the top of the luff and the edge that connects this corner to the peak.


A plate, usually of aluminum or plastic, at the head of a mainsail or other sail that serves as a strong attachment point for the halyard and the highest luff slider. Headboards also allow greater roach to be placed in the top of a sail, like in a racing or multihull mainsail.


The vertical height of a boat's foretriangle on a manufacturer's rig drawing. It is the distance from the point where the forestay intersects the mast at their top straight down to a point level with the sheer line abeam of the mast. Most commonly used in context with the boat’s J dimension. See also Rig Dimensions.


Not an indication of agreement, but a rig dimension similar to the I dimension, measuring the vertical height of a boat’s inner foretriangle up to its inner or staysail stay. See also Rig Dimensions.


The intersection of the boom to the mast, and any fitting that enables it.


The horizontal distance from the front face of the mast forward to where an inner stay for a staysail intersects the deck. See also Rig Dimensions.


The trailing edge of a sail.

Lecch Line

A small line internal to the leech of a sail designed to prevent the leech of a sail from fluttering as it stretches or ages, usually adjusted with a cleat or other securing method at its end.

Luff Foam

Shaped strip of aquaphobic closed-cell foam commonly used on roller-furling headsails on boats up to 50-60 feet. Sewn along the luff of the sail, the foam increases the circumference of a roller-reefed sail along its central areas, flattening the middle more to compensate for the shape distortion caused by the reef.

Luff Perpendicular (LP)

the sail that intersects the clew. In geometric terms, it would be the “height” of the sail triangle with the luff serving as the “base.” It can be used to determine the area of a sail, or, when compared to the boat’s J dimension, its percentage overlap. The “percentage” is equal to the ratio of the LP to J, or LP/J. See also J.

Luff Slides or Slugs

Small plastic or metal fittings on the luff or foot of sail that attach it to a mast or boom and allow it slide along an internal or external groove or track. Most flat slides are attached by hand with rugged tubular nylon webbing, and most round slugs are shackled on.


The primary control line used to pull in the boom or leech of a mainsail.


The line that pulls the clew of a sail out along the boom, making it flatter. On in-mast furling sails, the outhaul also serves to pull the sail out of the mast, and must be eased carefully when re-furling.

Patch: Head, Tack, Clew, or Reef

Reinforcing layers of sailcloth added to the corners and reef areas of the sail to make it strong enough to handle the concentrated loads placed upon these areas when sailing. Super Sailmakers’ patches exceed the recommendations of corner hardware manufacturers to provide the strongest possible support.

Percent Overlap

A headsail’s percent overlap (i.e., 150 percent, 135 percent) is calculated by the ratio of the sail’s LP (Luff Perpendicular) to the J dimension of the boat it will be used on. See also LP and J.


The maximum distance from the top of the mainmast or its black band down to the top face of the boom. Used to estimate the luff length of a mainsail. See also Rig Dimensions.


The maximum distance from the top of the mizzen mast down to the top face of the boom. Used to estimate the luff length of a mizzen. See also Rig Dimensions.


In a gaff- or sprit-rigged sail, the corner at the top of the leech, connecting it to its adjacent side on top, the head.

Reef Patch

Reinforcements that serve as the new tack or clew of a mainsail or mizzen when the sail is reefed. Super Sailmakers’ sail designers carefully design reef patches, especially in laminated sails, to ensure the new load directions are properly anticipated in the reefed sail.

Reef Points

Grommets or rings in small, reinforced areas between the reef patches used to tie the bulk of the sail loosely around the boom or bottom of the sail after the tack and clew reefs have been secured.

Rig Dimensions

Standard manufacturer’s measurements for specific dimensions on a boat. While Super consultants often use these to quote replacement sails or certain types of modifications, construction of new sails almost always requires measurement of the specific boat in question. See also each dimension for an explanation: I, J, P, and E, as well as Ii, Ji, Py, and Ey.


The area of a main, mizzen, or jib that sits on or aft of a straight line between its head and clew, almost always supported by battens.


Typically used in relation to roller furling headsails, describes the process of partially furling the sail to reduce sail area in high winds. Super Sailmakers’ sail designers anticipate levels of roller-reefing and adjust sail shape and patch size appropriately to provide the best possible sail shape and endurance while roller-reefed. See also Luff Foam.

Sailmaker's Yard

An English/American unit of area for measuring the weight of sailcloth. It is 28.5 inches wide by 36 inches long. It is the corresponding unit of measure to the “weight” in ounces of sailcloth – an 8 oz. fabric weighs close to 8 ounces per sailmaker’s yard.

Slab Reefing

The most a common type of reefing a mainsail, mizzen, or jib wherein a horizontal piece of the sail (a 'slab') is removed by lowering the halyard and retrimming the sail using the reef tack and the reef clew. This is distinct from forms reefing by rolling the sail around or inside a spar or stay. See also Reef Points.

Standing Rigging

Wires which hold up a mast but which are not easily adjustable in length, i.e. they are usually adjusted with turnbuckles.


See UV Suncover.


The front lower corner of a sail; the intersection of the luff and the foot. In the case of a spinnaker, the corner connected to the spinnaker pole.

Tack Hook

A hook adjacent to the gooseneck, used for holding the reef tack of a mainsail while reefed.

Tack Horn

see Tack Hook


In sailmaking terms, three applications. 1) long thin strips of sailcloth, usually DacronTM, used for a variety of purposes, including reinforcing edges and batten pockets in sails. 2) double-sided adhesive tapes used to hold panels of sailcloth together during the manufacturing process. 3) common single-sided adhesives used for repairs.


A type of zig-zag sewing stitch in which the needle and thread pierce the fabric three times per diagonal pass. Three-step seams are usually tighter, lower profile, and longer-lasting than the more common zig-zag stitch, and can be used in place of two zig-zag rows. Larger sails utilize multiple rows of three-step seams.

Topping Lift

A control line used to hold up various spars: the main boom and the spinnaker pole are two common examples.

UV Suncover

A treatment on the leech and foot of sails stored outdoors, intended to protect the sails from UV radiation which rapidly deteriorates the sailcloth. Found typically on roller furling jibs and external roller furling mains. Common materials include SunbrellaTM and UV-resistant DacronTM. Super Sailmakers’ service division carefully inspect suncovers and the stitching attaching them as part of a regular service inspection.


An abbreviation for “Velocity Made Good,” VMG is a measurement of a boat’s geometrical progress toward a goal it cannot approach directly. A boat’s VMG upwind measures its success in pointing high enough to get closer to the goal, but sailing low and fast enough to maintain boat speed. Downwind, VMG measures the compromise between sailing lower and closer to a mark – but slower – and sailing higher and faster but at angels farther.


Common type of stitch used in sail making that results in a series of intersecting diagonal stitches. Used in many sail applications, especially in sacrificial suncovers which may need to be changed, its pattern facilitates a sailmaker’s removal of the stitching.