Sail Simulator

Pick a boat, set the true wind and trim the sail. Blue is suction pulling the sail; red is pressure pushing it. As the boat speeds up, its own motion bends the wind forward, and the wake behind it grows.

Attached flow

Attached flow

Boat speed–
VMG–
AWA–
AWS–
Heel–
Main AoA–
Genoa AoA–
CL main–
CD main–
L/D–
Peak suction–
Lift–
Drag–
Drive–
Heeling force–
Best trim–
3D view: pick a camera at the bottom left, or drag to turn it (sideways on phones), scroll or pinch to zoom, double-click to reset.

Lift and drag vs angle of attack

The full curve for this sail shape. Lift climbs, peaks, then drops at the stall while drag keeps rising. The dots show where you are now.

  • Lift coefficient (CL)
  • Drag coefficient (CD)
  • Luffing zone
  • Stall zone

Suction along the sail

How hard the air pulls (leeward side) and pushes (windward side) from the mast to the back edge. In a stall the suction goes flat where the flow breaks away.

  • Leeward side, suction (−Cp)
  • Windward side, pressure

Drive vs sail trim

For the apparent wind right now: forward push at each boom angle. Where drive is above the hull resistance line, the boat speeds up; below it, the boat slows down.

  • Drive force
  • Heeling force
  • Hull resistance at current speed

Polar diagram

Top boat speed at every true wind angle for this wind speed, with the sail trimmed perfectly. The shaded wedge is the no-go zone. Marked points give the best VMG (fastest progress straight upwind or downwind).

  • Boat speed
  • No-go zone
  • You now

Live forces, last 20 seconds

Drive against hull resistance is the tug of war that sets boat speed. When drive is above resistance the boat is speeding up; below it, slowing down.

  • Drive
  • Hull resistance
  • Heeling
  • Lift
  • Drag

Live speeds, last 20 seconds

Boat speed follows the wind within about 2 seconds. Sailing upwind, apparent wind rises above the true wind as you speed up; downwind it drops.

  • Boat speed
  • Apparent wind
  • True wind

What is slowing the boat, last 20 seconds

Hull resistance split into its parts, stacked. The boat settles where the drive line meets the top of the stack. In strong wind, heel, the deck edge dragging through the water and keel side-force drag grow fast, so an overpowered boat gets slower.

  • Skin friction
  • Wave-making
  • Heel & rudder resistance (monohull)
  • Deck-edge immersion (monohull)
  • Induced resistance (keel side-force drag)
  • Total resistance
  • Drive

Learn the physics behind it

Sail Simulator is a free, browser-based sail aerodynamics simulator. It shows how air flows around a sail, why a sail stalls or luffs, how the boat's own speed changes the wind it feels, and how heel and drag set the boat's top speed. These guides explain each idea step by step, with buttons that open the simulator already set up to show it.

How a sail works: lift, suction and stallWhy a curved sail pulls a boat forward, and what happens when the air can no longer follow it. Apparent wind explainedWhy the wind you feel on a moving boat is not the wind that is blowing. Reading telltales and trimming the sailThe little yarns on a sail tell you exactly what the air is doing. Here is how to read them. When to reef: heel, drag and speedWhy too much sail makes a boat slower, not faster, once the wind builds.

Reference tables (flow states, monohull vs catamaran) and all sailing terms are in the Learn section: Quick reference and Sailing terms.

How it is calculated: the flow around both sails (main and genoa) comes from a 2D vortex-lattice model (a potential-flow method) that solves them together, so the slot effect between them is included. A flat 2D section overstates that effect, so the simulator uses 40% of it. Simplified rules cover stall, luffing and 3D tip losses. Hull resistance uses the ITTC-57 friction line, a simplified wave-making term that rises steeply near hull speed, heel and rudder resistance plus deck-edge immersion drag for the monohull (with a simple broach model past about 35°), and keel induced drag from the side force. The 30° and 35° thresholds are rule-of-thumb estimates for a 40 ft cruiser in flat water. 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 areas of main and genoa) follows published specs for the two reference boats. Leeway, waves and current are ignored. It shows the right trends for learning, not a performance prediction for a real boat.