The log reel was used to measure the speed of the ship. A piece of wood called a “chip” was thrown overboard. A line was fixed to the chip and a long length of knotted rope was tied to the line.
The knots were tied at 7 fathom (42 feet) intervals and they were timed as they passed over the stern of the ship using a 30 second sandglass.
At the time of the Mary Rose a nautical mile was reckoned to be about 5,000 feet. (Today it is set at 6,080 feet). Because speed was measured with the log reel, nautical miles travelled per hour are called knots. We say a ship travels at 9 knots, never 9 knots per hour!