Whilst one of the volunteers was ferreting about in the store room, she recognised parts of a Walker’s Ship Log. After much Brasso and polishing, the stamp ‘Walker’s Cherub III Ship Log’ showed up. It’s an instrument used to record the distance travelled by a ship and so calculates the ship’s speed.
A dial on board the ship records the number of revolutions of a spinner or rotor towed behind the ship. For this model, 900 revolutions of the rotor meant the ship had travelled one nautical mile.
A nautical mile is about 1.15 statute miles. One minute of latitude is about one nautical mile and one knot is one nautical mile per hour (it all depends on where in the world you are). Nowadays, speed is determined by hull-mounted units which use RADAR, ultrasound or GPS. Speeds measured with a GPS differ from those measured by other means in that they are Speed Over Ground (accounting for the effect of current), while the others are Speed Through the Water, which does not account for current.