Drifting in the wind: leeway error in shipdrift data

Much of our knowledge of the large scale pattern of surface velocity in the oceans comes from historical shipdrift velocity data. One concern about these data is the possibility of a substantial systematic error due to leeway, the downwind sideslip of ships through the water normal to their steered courses. The leeway error is investigated here both theoretically and observationally and its magnitude is estimated for the tradewind region of the North Atlantic. The goal is to correct and improve the shipdrift velocity data set. was calculated from the difference between surface currents measured on two intersecting tracklines, one nearly aligned with the mean wind direction, the other nearly normal to the wind. The residual current is ascribed to leeway. The estimated magnitude of leeway was found to be 3.5±0.4 cm/sec in a region of 5.9 m/s winds. Expressed as a percentage of the mean wind velocity this leeway is 0.60±0.09%. For the general case of ship lines oriented randomly with respect to the wind, leeway would be around half of the above values. Errors of this size are usually much smaller than the mean surface currents, except in regions of slow currents and fast winds like the North Equatorial Current—Trade Wind region of the North Atlantic.