log

log.
1. The shortened name for a log book.

2. The name given to any device for measuring the speed of a vessel through the water or the distance it has sailed in a given time. All the early types of devices for doing this were based on the same principle.

Common Log.

This was first described in print in A Regiment for the Sea (1574) by William Bourne. In its earliest form it comprised a wooden board attached to a log-line and hove from the stern of the vessel. The log-line was allowed to run out for a specified period of time. On the assumption that the board remained stationary in the water, the amount of line run out in a specified time indicated the distance sailed by the vessel through the water in that time. From this its speed could be easily calculated.

An early development of the first common log, introduced in about 1600, was the chip log, in which the original board was replaced by a wooden quadrant weighted with lead on the circular rim to make it float upright. This was designed to give it more resistance in the water to the drag of the log-line as it ran out, thus providing a more accurate reading.

Later, when the nautical mile was introduced, the log-line was marked by knots, the derivation of the knot as a measurement of speed at sea.

Dutchman's Log.

On much the same principle as the common log, the Dutchman's log was the method of estimating speed at sea favoured by Dutch mariners during the 17th and 18th centuries. The means of calculating the speed using this log involved measuring the time during which a chip of wood, dropped into the sea level with the bow, travelled between two marks cut on the vessel's gunwale. Knowing the distance between the marks, it was a simple arithmetical matter to find the rate of sailing.

Patent or Self-recording Log.

With the growth of the seaborne trade in the 17th and 18th centuries the need for a more accurate measurement of a ship's speed became widespread, and many inventors turned to a rotator towed by a ship as a means of measuring speed. The British engineer John Smeaton (1724–92), who improved Halley's diving bell, was one of the earliest to develop a patent log, producing a lightweight rotator in 1754, and at about the same time similar inventions came from Britain, France, and Germany. But they all suffered from an unacceptable degree of friction which falsified the readings. A partial solution came in 1792 when Richard Gower fitted his rotator in a wooden cylinder which also contained the registering dials. Although this largely eliminated the friction, it entailed hauling in the log every time the ship changed course as well as when the watch changed. In 1802 Edward Massey produced a log which began to resemble a modern patent log. A streamlined rotator was attached to a case containing the dials by four lengths of cane jointed together, the whole being towed at the end of a log-line. It still involved hauling in the log to take a reading, but the results proved impressively accurate and it was the Massey log which was used extensively at sea throughout the 19th century.

Massey's nephew was Thomas Walker, a name widely associated with the modern development of the patent log. His ‘Harpoon’ log, similar to Massey's but with the dials incorporated in the outer casing of the rotator, was patented in 1861, and his famous ‘Cherub’ log was introduced in 1884. By this time, engineering development had reached the stage where revolutions of the rotator astern could be transmitted accurately to a register inboard without distortion by friction.

Bottom Logs.

Other types of patent log are known as bottom logs, in that they are not towed astern but protrude from the ship's bottom. There are three basic types, known as the pitometer, the Chernikeeff, and the electromagnetic.

The pitometer log was based on an invention by Henry Pitot who, in 1730, used an open L-shaped glass tube to measure current flow. By placing the foot of the L facing forward beneath a ship, water was forced up the tube as the ship proceeded, its speed being measured by how far up the tube the water was forced. A number of experiments were made with this device, and improvements introduced, but it was not until the 20th century that reliable logs on the Pitot principle were evolved.

The Chernikeeff principle relied on a small rotator in a retractable tub carried a metre or so below the hull of the ship. It was conceived by Captain B. Chernikeeff of the Russian Navy in 1917 and was later developed and widely used.

At sea, speed may be measured in relation to either the seabed or the water flowing past the hull. Modern logs measure both. The electromagnetic log, which is widely used, has two electrodes beneath the ship's hull. These measure the potential difference generated by the ship's movement relative to a magnetic field, which is produced by an electromagnet. Smaller vessels such as yachts use an electronic log where an impeller fixed to the vessel's bottom is connected to the main instrument inboard which gives the readings of speed and distance run. Doppler sonar systems, as well as measuring the depth below a ship's bottom, measure the ship's speed and distance run in relation to the ocean floor.