The record of a ship’s activities is called the “log.” In it are recorded the hour-by-hour notations of the direction in which the ship is going and her speed, all changes of direction and speed, employment of the crew, weather and sea conditions, sightings of land or unusual or unexpected objects, and anything else the captain decides is worthy of an entry.
The captain himself does not “keep” the log, but rather it is the duty of each officer standing the “deck” watch to ensure that notes are taken, and at the end of his “watch” or duty period, he writes a smooth report based on them, to be diurnally reviewed and approved by the captain.
Recently, the notes usually are kept in a ledger-size book known as the quartermaster’s notebook. For centuries, however, they were recorded on a slate usually hung on the binnacle (compass stand) near the ship’s wheel. At the end of his watch, when he had been succeeded as “officer of the deck,” the off-going officer would take the slate below, write his smooth log, and then return the slate to its usual position, having first “wiped the slate clean.” This same activity is the source of our use of the word “scrub,” meaning “to cancel.”