Effect of ship's roll on the Ekman current meter

Tests were made with a pair of Ekman current meters to observe the effect of ship's roll upon their measurements and their behavior. In calm seaways where the current speed ranged from five to 45 cm/sec the speeds indicated by the two instruments were consistent within five per cent, and the indicated directions were within four degrees. In weaker currents the direction difference is twice as large. When the sea/swell conditions became greater than 1/1 the scatter in the indicated speeds became greater than ten per cent, and the results were considered unusable. Directions were measured within an accuracy of ten degrees in sea/swell as great as 2/3. One instrument was given an oscillation approximating a movement caused by a ship's roll of 10 to 15°. Its recorded speeds were 1.5 times those measured by a stationary one. This increase is due to the excess revolutions made by the propeller of the oscillating instrument during its downward swing. The increase in the scatter of the speed differences of the two instruments during sea/swell > 1/1 is attributed to this also. There was no appreciable variation in the directions during this artificial roll except when measurements were made in weak currents.