Distance Log

A log measures speed through the water and distance run, the input to dead reckoning. The towed patent (taffrail) log spun a rotator on a line to turn a counter; the modern electromagnetic or Doppler log reads speed through water (and Doppler logs over the ground in shallow water). Speed through water differs from speed over ground by the current, so a fix corrects the dead-reckoning position the log builds. The log distance and the engine revolutions together flag a fouled hull or adverse set.