Later, sailors worked knotted cords into the log-line. Initially, the markings were simply knots in the line. With the introduction of the nautical mile as a standard unit of measure at sea in the 15th century, they began to mark the line at equal intervals proportional to the nautical mile and to the time interval used for measurement. Initially, the log-line was not knotted and sailors measured the length directly on the line. Pay distance in feet = 1 mph × 5000 ft/mi × 30 s × 1 h 3600 s At the time, a mile was reckoned as 5,000 feet, so in 30 seconds at one mile per hour, a ship would travel about 42 feet: Bourne devised a half-minute sandglass for timing. The first known description of the device in print is in A Regiment for the Sea by William Bourne, in 1574. Mariners have used the log for a long time. This box had tables on it to convert log timing to speed. The Dutchman's log could be used with a brass tobacco box, rectangular with rounded ends. The first reference to a Dutchman's log is in 1623-later than the ship log. A sailor threw a floating object overboard and used a sandglass to measure the time it took to pass between two points on deck. 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. The first known device that measured speed is often claimed to be the Dutchman's log. The length of log-line passing (the number of knots) determined the reading. The sailor let the log-line run out for a fixed time while counting the knots that passed over. The log acted as a drogue, remaining roughly in place while the vessel moved away. 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, enabling a sailor to retrieve the log.Ī navigator who needed to know the speed of the vessel had a sailor drop the log over the ship's stern. This provides more resistance in the water, and a more accurate and repeatable reading. To ensure the log submerges and orients correctly in the water, the bottom of the log is weighted with lead. 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. The shape is a quarter circle, or quadrant with a radius of 5 inches (130 mm) or 6 inches (150 mm), and 0.5 inches (13 mm) thick. Over time, log construction standardized. The log-line is wound on a reel so the user can easily pay it out. The log-line has a number of knots at uniform intervals. The small light-coloured wooden pin and plug form a release mechanism for two lines of the bridle.Ī chip log consists of a wooden board attached to a line (the log-line). The timing sandglass is in the upper left and the chip log is in the lower left. The first knot, marking the first nautical mile is visible on the reel just below the centre.
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