1911 Encyclopædia Britannica/Log
LOG (a word of uncertain etymological origin, possibly onomatopoeic; the New English Dictionary rejects the derivation from Norwegian lág, a fallen tree), a large piece of generally unhewn, wood. The word is also used in various figurative senses, and more particularly for the “nautical log,” an apparatus for ascertaining the speed of ships. Its employment in this sense depends on the fact that a piece of wood attached to a line was thrown overboard to lie like a log in a fixed position, motionless, the vessel’s speed being calculated by observing what length of line ran out in a given time (“common log”); and the word has been retained for the modern “patent” or “continuous” log, though it works in an entirely different manner.
The origin of the “common log” is obscure, but the beginnings of the “continuous log” may be traced back to the 16th century. By an invention probably due to Humfray Cole and published in 1578 by William Bourne in his Inventions and Devices, it was proposed to register a ship’s speed by means of a “little small close boat,” with a wheel, or wheels, and an axle-tree to turn clockwork in the little boat, with dials and pointers indicating fathoms, leagues, scores of leagues and hundreds of leagues. About 1668 Dr R. Hooke showed some members of the Royal Society an instrument for the same purpose, depending on a vane or fly which rotated as the vessel progressed (Birch, History of the Royal Society, iv. 231), and Sir Isaac Newton in 1715 reported unfavourably on the “marine surveyor” of Henry de Saumarez, which also depended on a rotator. Conradus Mel in his Antiquarius Sacer (1719) described a “pantometron nauticum” which he claimed would show without calculation the distance sailed by the ship; and J. Smeaton in 1754 published improvements on the apparatus of Saumarez. William Foxon of Deptford in 1772, James Guerimand of Middlesex in 1776 (by his “marine perambulator”), and R. H. Gower in 1772, practically demonstrated the registration of a vessel’s speed by mechanical means. Viscount de Vaux in 1807 made use of water-pressure, as did the Rev. E. L. Berthon in 1849, and C. E. Kelway invented an electrical log in 1876.
Common Log.—To ascertain the ship’s speed by the common log four articles are necessary—a log-ship or log-chip, log-reel, log-line and log-glass. The log-ship (fig. 1) is a wooden quadrant 1 in. thick, with a radius of 5 or 6 in., the circumference of which is weighted with lead to keep it upright and retard its passage through the water. Two holes are made near its lower angles. One end of a short piece of thin line is passed through one of these holes, and knotted; the other end has spliced to it a hard bone peg which is inserted in the other hole. The holes are so placed that the log-ship will hang square from the span thus formed. The log-line is secured to this span and consists of two parts. The portion nearest the log-ship is known as the “stray line”; its length varies from 10 to 20 fathoms, but should be sufficient to ensure that the log-ship shall be outside the disturbing element of the ship’s wake. The point where it joins the other part is marked by a piece of bunting, and the line from this point towards its other end is marked at known intervals with “knots,” which consist of pieces of cord worked in between its strands. A mean degree of the meridian being assumed to be 69.09 statute miles of 5280 ft., the nautical mile (1 degree) is taken as 6080 ft., which is a sufficiently close approximation for practical purposes, and the distances between the knots are made to bear the same relation to 6080 ft. as 28 seconds to an hour (3600 seconds); that is, they are placed at intervals of 47 ft. 3 in. The end of the first interval of this length (counting from the piece of bunting) is marked by a bit of leather, the second by a cord with two knots, the third by one with three knots, and so on; the middle of each of these lengths (half-knot) is also marked by a cord with one knot. It follows that, if, say, five knots of the line run out in 28 seconds, the ship has gone 5 × 471 ft. in that time, or is moving at the rate of 5 × 6080 ft. (= five nautical miles) an hour; hence the common use of knot as equivalent to a nautical mile. In the log-glass the time is measured by running sand, which, however, is apt to be affected by the humidity of the atmosphere. Sometimes a 30-second glass is used instead of a 28-second one, and the intervals between the knots on the log-line are then made 50 ft. 7 in. instead of 47 ft. 3 in. For speeds over six knots a 14-second glass is employed, and the speed indicated by the log-line is doubled.
The log-line, after being well soaked, stretched and marked with knots, is wound uniformly on the log-reel, to which its inner end is securely fastened. To “heave the log,” a man holds the log-reel over his head (at high speeds the man and portable reel are superseded by a fixed reel and a winch fitted with a brake), and the officer places the peg in the log-ship, which he then throws clear and to windward of the ship, allowing the line to run freely out. When the bunting at the end of the stray line passes his hand, he calls to his assistant to turn the glass, and allows the line to pay out freely. When all the sand has run through, the assistant calls “Stop!” when the log-line is quickly nipped, the knots counted, and the intermediate portion estimated. The strain on the log-ship when the log-line is nipped, causes the peg to be withdrawn from it, and the log-ship is readily hauled in. In normal circumstances, the log is hove every hour. In a steam vessel running at high speed on an ocean route, with engines working smoothly and uniformly, a careful officer with correct line and glass can obtain very accurate results with the common log.
Ground Log.—In the deltas of shoal rivers, with a strong tide or current and no land visible, a 5 ℔ lead is substituted for the log-ship; the lead rests on the bottom, and the speed is obtained in a manner similar to that previously described. Such a “ground-log” indicates the actual speed over the ground, and in addition, when the log-line is being hauled in, it will show the real course the ship is making over the ground.
|Fig. 3.—The A1 Harpoon Ship Log.|
Patent Log.—The screw or rotatory log of Edward Massey, invented in 1802, came into general use in 1836 and continued until 1861. The registering wheelwork was contained in a shallow rectangular box (fig. 2), with a float plate on its upper side, carrying three indicating dials, recording respectively fractions, units and tens of miles (up to a hundred). The rotator was connected to the log by a rope 6 ft. in length, actuating a universal joint on the first spindle of the register; it consisted of an air-tight thin metal tube with a coned fore-end, carrying flat metal vanes set at an angle. Alexander Bain in 1846 suggested enclosing the wheelwork in the rotator. In Thomas Walker’s harpoon or frictionless log, introduced in 1861, the wheelwork was enclosed in a cylindrical case of the same diameter as the body of the rotator or fan, and the latter was brought close up to the register, forming a compact machine and avoiding the use of the 6-ft. line. Two years later a heart-shaped float plate was attached to the case, and the log called the A1 Harpoon ship log (fig. 3). The log should be washed in fresh water when practicable, to prevent oxidization of the wheels, and be lubricated with suitable oil through a hole in the case.
These logs were towed from the ship, but with quick passages and well surveyed coasts, the need arose for a patent log which could be readily consulted from the deck, and from which the distance run under varying speeds could be quickly ascertained. To meet this requirement, Walker in 1878 introduced the Cherub log (fig. 4), a taffrail one, which, however, is not as a rule used for speeds over 18 knots.
Fig. 4.—The Cherub Log.
Owing to the increased friction produced
by a rotator making approximately 900 revolutions per mile,
towed at the end of a line varying from 40 fathoms for a 12-knot
speed to 60 fathoms for 20 knots, the pull of the line and rotator
is borne by coned rollers, having their outlines tapering to a
Fig. 5.—Neptune Pattern for securing Rotator. common point in their rotation, thus giving a broad rolling surface. Strong worms and wheels are substituted for the light clockwork. In fig. 4 the shoe H is secured to the taffrail, and the rotator in the water is hooked to the eye of the spindle M by the hook D. The case A contains the registering wheelwork and a sounding bell. The half gimbal B pivoting in the socket of the base C allows the register to receive the strain in the direct line. The bearings and rollers are lubricated with castor oil every twelve hours through holes in the sliding case E, and can be examined by unscrewing the case E and the eye M. When not in use, the register is removed from the shoe by lifting a small screw button near C. The tow line is usually plaited, and to avoid a knot close to the rotator, the latter is secured to the former by a knot inside an egg-shaped shell (fig. 5, Neptune pattern).
Fig. 6.—Walker’s Neptune Log.
Walker’s Neptune log (fig. 6) is used for vessels of high speed. Case A contains the wheelwork, and case E the spindle and steel ball bearings; in each case are openings, closed by sliding tubes, for examination and lubrication. In fig. 6 the cases A and E are shown open. Fig. 7 shows the dial plate. In fig. 8 the ball bearings are shown unscrewed from the body of the log, with eye, cap and spindle. They consist of two rows of balls rolling in two pairs of V races or grooves. The outer pair receive the strain of the rotator, and the inner are for adjustment and to prevent lateral movement. The balls and races are enclosed in a skeleton cage (fig. 9) unscrewing from the cap F (fig. 6) for cleaning or renewal; the adjustment of the bearings is made by screwing up the cage cap b, locked by a special washer and the two screws a, a (figs. 8, 9). If the outer races become worn, the complete cage and bearings are reversed; the strain of the line is then transferred to what had previously been the inner with practically unworn balls and races. It is for this purpose that the skeleton cage is screwed internally at both ends, fitting a screwed ring inside the cap F (fig. 6). To enable the indications of the log register on the taffrail to be recorded in the chart room or any other part of the vessel as desired, a chart room electric register has been introduced. By means of an electric installation between the log register aft and the electric register in the chart room, every tenth of a mile indicated by the former is recorded by the latter.
|Fig. 7.—Dial-plate of Neptune Log.||Fig. 8.—Ball Bearings of Neptune Log.|
Walker’s Rocket log (fig. 10) is a taffrail one, with bearings of hardened steel, and is intended to be slung or secured to the taffrail by a line; the gimbal pattern has a fitting for the deck. In taffrail logs, the movement of the line owing to its length becomes spasmodic and jerky, increasing the vibration and friction; to obviate this a governor or fly-wheel is introduced, the hook of the tow line K (fig. 11) and the eye of the register M being attached to the governor. Fig. 11 represents the arrangement fitted to the Neptune log; with the Cherub log, a small piece of line is introduced between the governor and the eye of the register. The two principal American taffrail logs are the Negus and Bliss (Messrs Norie and Wilson). The former bears a general resemblance to the Cherub log, but the dial plate is horizontal and the faces turn upwards. The main shaft bearings are in two sets and composed of steel balls running in steel cones and cups; the governor is an iron rod about 16 in. long, with 1 in. balls at the extremities. The Bliss resembles the Rocket log in shape, and is secured to the taffrail by a rope or. A governor is not employed. The blades of the rotator are adjustable, being fitted into its tube or body by slits and holes and then soldered. The outer ends of the blades are slit (fig. 12) to form two tongues, and with the wrench (fig. 12) the angle of the pitch can be altered.
All patent logs have errors, the amounts of which should be ascertained by shore observations when passing a well surveyed coast in tideless waters on a calm day. Constant use, increased friction (more especially at high speeds), and damage to the rotator will alter an ascertained log error; head or following seas, strong winds, currents and tidal streams also affect the correctness.
|Fig. 9.—Ball Bearings of Neptune Log in Skeleton Case.||Fig. 10.—Rocket Log.|
Fig. 11.—Neptune Log fitted with Governor.
A Log Book is a marine or sea
journal, containing, in the British navy, the speed,
course, leeway, direction and force of the wind, state of
the weather, and barometric and thermometric observations.
Under the heading “Remarks” are noted (for
vessels with sail power) making, shortening
and trimming sails; and (for all
ships) employment of crew, times of
passing prominent landmarks, altering
of course, and any subject of interest and
importance. The deck log book, kept by the officers of the
watch, is copied into the ship’s log book by the navigating
Fig. 12.—Bliss Log.officer, and the latter is an official journal. In steam vessels a rough and fair engine room register are kept, giving information with regard to the engines and boilers. In the British mercantile marine all ships (except those employed exclusively in trading between ports on the coasts of Scotland) are compelled to keep an official log book in a form approved by the Board of Trade. A mate’s log book and engine room register are not compulsory, but are usually kept. (J. W. D.)