Taffrail log

A chip log, also called common log, ship log, or just log, is a navigation tool mariners use to estimate the speed of a vessel through water. The word knot, to mean nautical mile per hour, derives from this measurement method.

All nautical instruments that measure the speed of a ship through water are known as logs. This nomenclature dates back to the days of sail, when sailors tossed a log attached to a rope knotted at regular intervals off the stern of a ship. Sailors counted the number of knots that passed through their hands in a given time to determine the ship's speed. Today, sailors and aircraft pilots still express speed in knots.

A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots at uniform intervals. The log-line is wound on a reel so the user can easily .

Over time, log construction standardized. The shape is a quarter circle, or quadrant, and the log-line attaches to the board with a bridle of three lines that connect to the vertex and to the two ends of the quadrant's arc. To ensure the log submerges and orients correctly in the water, the bottom of the log is weighted with lead. This provides more resistance in the water, and a more accurate and repeatable reading. The bridle attaches in such a way that a strong tug on the log-line makes one or two of the bridle's lines release, so a sailor can retrieve the log.

A navigator who needed to know the speed of the vessel had a sailor drop the log over the ship's stern. The log acted as a drogue, remaining roughly in place while the vessel moved away. The sailor let the log-line run out for a fixed time while counting the knots that passed over. The length of log-line passing (the number of knots) determined the reading.

The first known device that measured speed is often claimed to be the Dutchman's Log. This invention is attributed to the Portuguese Bartolomeu Crescêncio, who designed it in the end of the 15th century or in the beginning of the 16th century. A sailor threw a floating object overboard and used a sandglass to measure the time it took to pass between two points on deck. The first reference to a Dutchman's log is in 1623—later than the ship log. The Dutchman's log could be used with a brass tobacco box, rectangular with rounded ends. This box had tables on it to convert log timing to speed.

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