Sailing terms

Every sailing and physics term used in the simulator and the guides, explained in plain English. The name in brackets is the standard industry term or abbreviation.

True wind (TWS speed, TWA angle): The wind you would feel standing still, for example on a dock. These are the wind sliders.
Apparent wind (AWS speed, AWA angle): The wind the moving boat feels: true wind plus the headwind made by the boat's own speed. The sail only ever feels this one.
Wind triangle (wind vector diagram): In the top-right gauge: grey arrow is true wind, green is the headwind from boat speed, blue is the apparent wind they add up to.
Boat speed (SOG, speed over ground; no current here): How fast the boat moves. Shown on the dial; tap the unit to switch between knots, km/h and mph.
VMG (velocity made good): The part of your speed that takes you straight toward (upwind) or away from (downwind) the wind.
Hull speed (industry term): About 1.34 × √(waterline length in feet) knots. Above it, the boat has to climb its own bow wave. Red zone on the dial.
Hull resistance (total resistance): All water drag on the boat: skin friction, wave-making, heel resistance and induced resistance. Wave drag explodes near hull speed.
Induced resistance (side-force / leeway drag): The keel stops the boat sliding sideways by working like an underwater wing, and that costs drag. It grows with the heeling force² and is worst at low speed.
Deck-edge immersion (burying the rail): Past about 30° of heel the side deck goes under water and drags through it. On the monohull, more wind beyond this point makes the boat slower.
Round-up (broach, rudder stall): Past about 35° of heel the rudder loses its grip and the boat spins up toward the wind. The sails flap until the boat straightens up.
Heel resistance (resistance due to heel): A heeled monohull sits lopsided in the water, and its rudder must fight the urge to turn into the wind (weather helm). About +10% drag at 20° heel, and it climbs steeply past 25° as the rudder starts to lose grip.
Wake (stern wash): The band of disturbed, foamy water left behind the boat. It grows longer, wider and whiter as the boat speeds up.
Heel (heel angle): How far the boat leans sideways. On a monohull more heel also tilts the sail away from the wind and wastes power.
Righting moment (RM): The boat's resistance to heeling: keel weight and hull shape on a monohull, the distance between the hulls on a catamaran.
Hull lift (catamaran stability margin): Heeling moment as a % of what would lift the windward hull out of the water. At 100% the hull flies and capsize becomes possible.
No-go zone (in irons): Wind angles too close to the bow for any trim to make forward drive. The boat stops.
Polar diagram (VPP polar): A chart of top boat speed at every wind angle. Sailors and designers use it to pick the fastest course.
Angle of attack (AoA): The angle between the apparent wind and the sail's chord (the straight line from mast to back edge). AoA = AWA − boom angle.
Lift and drag (C_L, C_D coefficients): Lift is the force at right angles to the apparent wind; drag is the force along it. Coefficients compare sails regardless of size or wind speed.
Drive and heeling force: Lift + drag split into the part pushing toward the bow (drive) and the part pushing sideways (heeling).
Stall / luffing: Stall: angle too big, air peels off the leeward side. Luffing: angle too small, the front of the sail flaps.
Camber / draft position / aspect ratio (AR): How deep the sail's curve is, where the deepest point sits, and how tall the sail is compared with its width.
Pressure coefficient (C_p): Local pressure compared with the wind's dynamic pressure. Negative means suction.
Telltales (sailing term): Short yarns on both sides of the sail and on its back edge. Streaming straight back means the air is attached.
Windage (parasitic air drag): Air drag on the hull, cabin and rigging. It is why the catamaran can't point as close to the wind.