Degradation of paddle-wheel Aanderaa current measurements by mooring vibration in a strong tidal flow

Currents measured with modified (paddle-wheel rotor) Aanderaa current meters on subsurface moorings in a strong tidal flow are compared to those measured with a ship-mounted acoustic Doppler current profiler (ADCP), a bottom-mounted ADCP, and InterOcean S4 electromagnetic current meters. Significant discrepancies among the rate measurements at some vertical levels are found. The rates measured by the Aanderaas at interior mooring positions are up to 20% lower than the acoustic rates, particularly for speeds greater than 0.8 m/s. Field tests and a simple model suggest that this discrepancy arises from shielding of the Aanderaa rotors due to instrument misalignment (with the relative water velocity) during periods of mooring vibration, induced by vortex shedding from in-line backup buoyancy packages