The record of courses steered and distances made good through the water from the time the ship's position was last reliably fixed. In the days of sail this record would be kept on a log-slate on which the times of altering course, and the distances made on each course as indicated by the log, were chalked up on the slate. At the end of each watch the record was transferred to the log book and the relieving officer of the watch began his watch with a clean slate. Hence the expression ‘to start with a clean slate’.
Subjects: Maritime History.