A Two-Component Electromagnetic Ship's Log

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Abstract
A ship's log is described which works on the principle of electromagnetic induction. The measuring head contains a coil with a vertical axis and is arranged so that voltages are picked up proportional to the two components of water flow past it. The electronic circuits are designed to give a true and stable zero by using a low-frequency squarewave energizing current. Laboratory tests show the output to be linearly related to water velocity. Because of the complicated flow patterns round a ship, the log must be calibratedin situat sea. Calibration of the forward component meter follows the usual procedure and this meter is accurate to about ±0·1 knot or ± 1 per cent, whichever is greater. However, calibration of the athwartship component meter is difficult and time-consuming, and the sensitivity is dependent on forward speed. With care, it seems that an accuracy of ±0·2 knot or ± 10 per cent, whichever is greater, should be possible with this meter, but more experience of such calibrations is required. The paper is discussed on p. 319.

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