The speedometers of quite a few boats I sailed, including mine, showed a puzzling behaviour.

With a moving boat, it indicates a generally correct speed from say 0.5 to 7 knots or above.

OTOH when moored to a buoy, no wind, the boat perfectly aligned with currents of >2 knots, all I get is a meagre 0.1kt at most

For example last week at springs in the Odet river there must have been around 4knots of current, the speedometer just read 0.0 or 0.1, so I reckon it is not only related to slow speeds.

(Of course, talking speed through water, transducer correctly located, not fouled, boat lying exactly bow against current, etc etc)

any explanations ?