Судовой лаг: История и принцип измерения скорости

Tool used to estimate the speed of a vessel through water
the ship's 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 pay it out. Over time, log construction standardized. The shape is a quarter circle, or quadrant with a radius of or , 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. Mariners have used the log for a long time. The first known description of the device in print is in A Regiment for the Sea by William Bourne, in 1574. Bourne devised a half minute sandglass for timing. 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:

Initially, the log line was not knotted and sailors measured the length directly on 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 markings were simply knots in the line. Later, sailors worked knotted cords into the log line. Many ships used knots spaced 8 fathoms (48 feet or 14.6 meters) apart, while other ships used the 7 fathom prescription. The time interval needs to be adjusted according to the distance between knots. Substituting 6,000 feet for 1 mile, the above formula yields 28.8 seconds for a distance of 8 fathoms. In fact, 28 second and 14 second glasses used to be common among navigation equipment.

Mechanical speed logs called patent logs Examples of taffrail logs include those made by Thomas Walker & Son. Since the second half of the twentieth century, sailors continue to use more modern mechanical and electro mechanical versions based upon a small impeller or paddle wheel attached to (or through) the bottom of the hull, especially on smaller yachts. Most commonly used however (especially on merchant ships) is the electromagnetic log, more reliable as there is no mechanism subject to breakdown. In recent years ultrasonic speed sensors have become available. These use two ultrasonic transducers—one forward, one aft—that send ultrasonic pulses through the water flowing past the hull. By calculating the time differential in pulse propagation from one sensor to the other, the device calculates the speed of the hull through the water. Doppler log: Today, a very accurate maritime speed measurement comes from Doppler measurement, either derived acoustically with Doppler Sonar —or with radio interferometrically by Doppler measurement of satellite signals, such as those from the Global Positioning System (GPS). However, most commercial GPS systems are not configured to operate in this mode.