...every day he came up on the bridge to relieve the first mate he first went to the safe, opened it, took out a piece of paper and read it, put it back, closed the safe. Every day.
The first mate wondered about it for a decade and finally the captain retired. The mate was to become the new captain and so the retiring captain gave him the combination to the safe. He promptly opened it and with slightly trembling hand read the paper in the safe. It said, "Port-left, starboard-right".