Historical information
A Ship's Log looked very similar to a torpedo but was used to measure the speed of a ship. When dragged behind the vessel, movement of water past the propeller caused it to rotate, turning the small needle dials to record the distance and speed travelled.
https://museumsvictoria.com.au/immigrationmuseum/resources/journeys-to-australia/
Physical description
Ship's Log Display Head
Inscriptions & markings
Walkers Cherub 111 ships log