Definition
Modern yachts use an impeller or paddlewheel through-hull unit whose rotation rate is proportional to water speed, integrated over time to give distance run. Displays usually show a resettable trip distance and a cumulative total. The reading is essential for dead reckoning, since distance run divided into the elapsed time gives average speed. Fouling by weed or barnacles is the commonest fault, typically causing the log to under-read progressively over a season.
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