The wind as it blows over the water, independent of the boat's own motion, what a boat at rest would feel.
True wind is the wind over the water as it would be felt on a boat lying still. On a moving boat you never feel it directly: you feel the apparent wind, which mixes in the headwind from your own speed. Instruments therefore calculate the true wind from the apparent wind and the boat's speed through the water.
The values
- TWS: true wind speed.
- TWA: true wind angle, measured from the bow (0–180° to port or starboard).
- TWD: true wind direction, the compass direction the wind comes from.
Calculation
From apparent wind speed (AWS), apparent wind angle (AWA) and boat speed (BS):
TWS = √(AWS² + BS² − 2 × AWS × BS × cos AWA)
The calculation is only as good as its inputs: a badly calibrated log or a masthead unit that isn't aligned shows up as true wind that shifts every time you tack.
True wind or ground wind
Strictly, true wind is relative to the water. If there's a current, the wind relative to the seabed (sometimes called ground wind, and what weather forecasts give) differs from it. Most instruments use speed through the water, some offer both. More in the guide true vs apparent wind.
Manta does this for you
Plotter data on your iPhone over WiFi
Connect your plotter or a WiFi gateway, and Manta shows your instruments live and records wind, depth and speed through the water on every trip.
