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Anemometer This consists of cups rotatable about a vertical shaft or a propeller rotating about a horizontal shaft. When driven by the wind at a speed proportional to wind force, the rotating shaft drives an electrical generator whose output is itself proportional to the speed of rotation. A voltmeter may thus be calibrated as wind speed. Anemometers are not normally used aboard merchant ships because of the difficulty of finding a suitable site and also because of expense. The UK Ocean Weather Ship carries anemometers on the yardarms at a suitable height above the water (23m), which seems to be the site furthest from eddying effects. But even here estimates of wind force and direction from the appearance of the sea are regularly made as a check on the instruments. (See Figure 17).

Figure 17. Anemometer and wind vane
Hand anemometer

Whilst estimates of wind force and direction made on merchant ships by observations of sea state are to be considered the norm, a hand-held anemometer can be used for verification or in sheltered waters where the appearance of the sea may be modified by topographic features. The hand anemometer is held with its axis vertical, at arm's length, with the arm at right angles to the wind direction to avoid disturbance of the airflow by the observer's body. At least two readings of mean wind speed need to be taken within the overall period of observation, each reading being taken over at least 15 seconds. The wind speed is shown by a graduated scale across which a pointer moves. Hand anemometers are calibrated in knots over a range of 0 to 60.

Digital hand anemometer

This is similar to the hand anemometer except that the speed sensing mechanism does not make use of switched contacts. After sampling for 15 seconds the instrument displays the average wind speed digitally for a further period of 10 seconds. The body and switch of the anemometer are waterproof and support legs fixed to the body protect the cups against damage when the instrument is laid down. The power is supplied by battery with an indication when battery voltage is low.

Wind vane

The wind direction may be directly observed by the position of the wind vane or remotely read at deck level by an electrical direction transmitter known as a Magslip for mains power or a Desynn in the case of a battery system operated through gear wheels and a countershaft. Both types may be joined to an anemometer unit so that the two instruments form a single transmitting head.