Anemometer

An anemometer is an instrument that measures wind speed, and in marine applications it is typically installed at the masthead or on a sensor mast to minimise the effect of the ship's superstructure on airflow. The most common shipboard type is the cup anemometer, in which three or four hemispherical cups mounted on radial arms rotate at a speed proportional to wind speed; a companion wind vane provides direction. Sonic anemometers, which measure the time of flight of ultrasonic pulses between transducers, are increasingly used on survey vessels and weather ships because they have no moving parts and can measure both speed and direction with high frequency. The critical distinction for the watchkeeping officer is that a masthead anemometer measures relative wind — the wind as experienced by the moving ship — which combines the true wind with the vessel's own speed and heading. Converting relative wind to true wind requires knowledge of the ship's speed and course made good, either by mental vector arithmetic or, increasingly, through automatic computation in the ship's integrated navigation system. An error in the vessel's speed input or compass heading will propagate directly into the true wind calculation. Under SOLAS, wind speed information is an input to stability assessment in heavy weather, to decision-making on deck cargo lashing, and to voyage planning. Comparison of the observed wind speed with the Beaufort scale estimate from sea state provides a useful cross-check on anemometer calibration. Officers should verify the anemometer reading against a visual Beaufort assessment at the start of each watch to detect any sensor fault or ice accretion in cold regions.