Apparent Wind Calculator

True wind vs apparent wind

True wind is the wind you feel when the boat is stationary — the actual meteorological wind. Apparent wind is the wind you experience while moving — the combination of true wind and the headwind created by the boat’s own forward motion.

When sailing, the sails respond to apparent wind, not true wind. Understanding the difference is critical for correct sail trim, tacking angles, and racing tactics.

The effect of boat motion

When a boat moves forward, it creates a headwind equal to its speed. This headwind adds to the true wind vectorially:

• On a beam reach (90° true wind angle), a boat doing 6 knots into a 12-knot true wind produces an apparent wind that is both stronger and forward of the true wind.
• Upwind, apparent wind is stronger than true wind.
• Downwind, apparent wind is weaker than true wind.

The formula

Apparent Wind Speed (AWA) = √(Vt² + Vs² + 2 × Vt × Vs × cos(β)) Apparent Wind Angle (AWA) = arctan(Vt × sin(β) ÷ (Vs + Vt × cos(β)))

Where:

• Vt = True wind speed
• Vs = Boat speed through water
• β = True wind angle (0° = dead ahead, 180° = dead downwind)

Practical implications

Apparent wind always shifts forward of true wind when sailing upwind or on a reach. This is why close-hauled boats sail to the wind they feel (apparent), not the true wind. As a boat accelerates (e.g., surfing a wave downwind), apparent wind backs — the sails need easing. Kite-surfing, foiling, and asymmetric spinnaker sailing all rely on understanding apparent wind shifts with speed.