 ", distance = speed × time), where time was measured by a half-hour-glass, and speed readings were taken with by some form of a chip log (the archaic method, used in the 14th and 15th centuries, involved heaving a piece of wood or flotsam overboard; the crew engaged in a rhythmic chant to mark the time it took for the chip to float past the length of the ship).Plotting a course required knowing the compass direction and distance between point A and point B.Knowledge of where ports lay relative to each other was acquired by navigators by long experience at sea.