Measuring speed - Hand log
In the ships’ books, distances were referred to in the beginning as days of the journey. Later they were calculated from the speed of the ship multiplied by the time it took. Measurement of the speed was useful also to confirm the position. The oldest method for measuring the speed was the “Dutchmen’s log”. They threw a log into the sea from the prow. This floated, and they measured the time it took for the log to reach the stern of the ship. They entered the result in a logbook.
Later the log was tied to a line, and from the length that unwound in the time it took for a sandglass to empty, the speed of the ship could be found. English sailors perfected the method in the 17th century by making the hand log, which the Galaxidian sailors used.
It consisted of the reel, the log-chip and the log-line. The log-chip was a flat piece of wood cut as a sector of a circle with lead weights on the round edge so it would stay vertical in the water. It was attached via three short pieces of line (the crow’s foot) to the log-line that had knots every 47¼ feet with a numbered white piece of cloth at each knot. The line was wound round a reel with two handles. As the log was thrown into the sea from the prow, it remained stationary, and with the movement of the ship the line unwound at a rate corresponding to the speed of the ship. As soon as the first knot appeared, the captain turned the sandglass upside down, and when all the sand had emptied, he shouted stop. The sailor who was holding the reel stopped it from unwinding more and read the number on the cloth of the last knot. The speed was estimated in nautical miles per hour (or knots) from the number of knots and the fixed unit of time determined by the sandglass.
he towed log (such as the harpoon) developed from the hand log. It registered the number of revolutions of the rotator, which turned according to the speed of the ship. Later they made mechanical logs that were permanently fixed to the bottom of the ship, the hull-fitted logs. Their revolutions were recorded on instruments at the bridge.
