My daughter and I were poking around an antique shop yesterday and found this stadimeter. Bradley Alan Fiske, an officer in the U.S. Navy, designed this optical rangefinder.
While initially designed for gunnery ranging during early field tests the stadimeter was also found incredibly useful for navigation. Using available data about the height of the masthead on various models of ship, it is possible to set that figure with a dial. The operator then aligns the image of the masthead with the ships waterline and reads the distance.
This instrument was made by Schick, more popularly known for its razor blades, who produced them under contract for the US Navy during the second world war. Here is a much nicer example.
It will probably come in handy.