Definition
A bladed wheel protrudes a few millimetres through the hull, usually forward and below the turn of the bilge where flow is least disturbed, and magnets in the blades trigger a sensor once per revolution. Pulses are counted for speed and totalised for distance. It is cheap and reasonably linear over normal yacht speeds but stalls at very low speed, is sensitive to hull position, heel and aeration, and requires regular cleaning through its seacock valve.
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