Definition
Built by running a measured mile or a timed leg at several throttle settings and tabulating revolutions per minute against speed through the water, ideally in both directions so any stream cancels. A typical cruising yacht might show 1800 rpm for 5.2 kn and 2400 rpm for 6.4 kn. It supplies a dead-reckoning speed when the log is fouled or dead, but needs allowance for hull fouling, loading and adverse sea state.
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