The four flow states side by side. Angles update to match your current sail shape.
- 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 (friction + wave-making resistance)
- Water drag on the hull: skin friction from rubbing, plus the energy spent making waves. Wave drag explodes near hull speed.
- 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 (CL, CD 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 (Cp)
- 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.
How it is calculated: the flow around the sail comes from a 2D vortex-lattice model (a potential-flow method), with simplified rules for stall, luffing and 3D tip losses. Hull resistance uses the ITTC-57 friction line plus a simplified wave-making term that rises steeply near hull speed. Heel comes from a linear righting-moment model, and boat speed moves to its new balance point within about 2 seconds. That is quicker than a real boat, whose weight would stretch a change over 30 seconds or more, but it makes every change easy to see. Boat data (weight, waterline, sail area) follows published specs for the two reference boats; leeway, waves and current are ignored. It shows the right trends for learning; it is not a performance prediction for a real boat.