True Wind Speed

True Wind Speed (TWS) is the wind speed relative to the water or earth, with the boat’s own movement removed from the measurement. On a moving vessel it is calculated from apparent wind, boat speed and heading, and is normally displayed in knots.

For sailors, TWS is the reference used to choose sail area, reefing points and target boat speeds. It works with True Wind Angle to define whether you are sailing upwind, reaching or running, while changes in Wind Gust influence safety margins. Reliable TWS also improves Weather Routing because the forecast can be matched to your boat’s performance.

In PredictWind forecasts, TWS is the standard wind value shown in model maps, tables and routing calculations. The Offshore App can download GRIB data at sea, while DataHub can combine onboard instruments and GPS to help compare measured true wind with the forecast.