"Do you know, where the unit "knots" has it's origin? See here: Knot (unit) - Wikipedia, the free encyclopedia".
And another piece of nautical trivia: The lump of wood the piece of string was attached to was not unreasonably knwon as the "log'', and the results were entered in the ''log book''.
Modern boat water speed indicators are usually paddle wheel devices, but are still known as ''logs''.