Research Note

Log line construction

Daburb, David Huff, Karel

What was the construction of the log line used to determine speed on ships circa 1800? Specifically, is there a predetermined dimension to the wood block at the end? And was there a standardized distance between the knots in the line? It seems that it would be necessary to have some standardization lest one ship's two knots be another's four.
Thanks for the help.

OF THE LOG-LINE AND HALF-MINUTE GLASS

Various methods have been proposed for measuring the rate at which a ship sails; but that most in use is by the Log and Half-Minute Glass. The Log is a flat piece of thin board, of a sectoral or quadrantal form (see Plate VI. fig. 3), loaded, on the circular side, with lead sufficient to make it swim upright in the water. To this is fastened a line, about 150 fathoms long, called the log-line, which is divided into certain spaces called knots, and is wound on a reel (see Plate VI. fig 4) which turns very easily. The Half-Minute Glass is of the same form as an Hour Glass (see Plate VI. fig 2), and contains such a quantity of sand as will run through the hole in its neck in half a minute of time.

The making of the experiment to find the velocity of the ship, is called Heaving the Log, which is thus performed: - One man holds the reel, and another the half-minute glass; an officer of the watch throws the log over the ship's stern, on the lee side, and when he observes the stray line is run off (which is about ten fathoms, this distance being usually allowed to carry the log out of the eddy of the ship's wake), and the first mark (which is generally a red flag) is gone off, he cries Turn; the glass-holder answers Done; and watching the glass, the moment it is run out, says Stop. The reel being immediately stopped, the last mark run off shows the number of knots, and the distance from that mark is estimated in fathoms. Then the knots and fathoms together show the distance the ship has run in the preceding hour, if the wind has been constant. But if the gale has not been the same during the hour, or interval of time between heaving the log, or if there has been more sail set or handed, a proper allowance must be made. Sometimes, when the ship is before the wind, and a great sea setting after her, it will bring home the log. In such cases it is customary to allow one mile in ten, and less in proportion if the sea be not so great. Allowance ought also to be made, if there be a head sea.

This practice of measuring a ship's rate of sailing, is founded upon the following principle - that the length of each knot is the same part of a sea mile as half a minute is off an hour. Therefore the length of a knot ought to be 1/120 of a sea mile; but, by various admeasurements, it has been found that the length of a sea mile is about 6120 feet; hence the length of a sea knot should be 51 feet. Each of these knots is divided into 10 fathoms, of about 5 feet each. If the glass be only 28 seconds in running out, the length of the knot ought to be 47 feet and 6 tenths. These are the lengths generally recommended in books of navigation; but it may be observed, that, in many trials, it has been found that a ship will generally overrun her reckoning with a log line thus marked; and, since it is best to err on the safe side, it has been generally recommended to shorten the above measures by 3 or 4 feet, making the length of the knot about 7 1/2 fathoms, of 6 feet each, to correspond with a glass that runs 28 seconds.

In heaving the log you must be careful to veer the line out as fast as the log will take it; for if the log be left to turn the reel itself, the log will come home and deceive you in your reckoning. You must also be careful to measure the log-line pretty often, lest it stretch and deceive you in the distance. Like regard must be had that half-minute glass be just 30 seconds; otherwise no accurate account of the ship's way can be kept. The glass is much influenced by the weather, running slower in damp weather than in dry. The half-minute glass may be examined by a watch, with a second hand, or by the following method: - Fasten a plummet on a string, and hang it on a nail, observing that the distance between the nail and the middle of the plummet be 39 1/8 inches; then swing the plummet, and notice how often it swings while the glass is running out, and that will be the number of seconds measured by the glass.

Just one hundred years after the Bowditch quotation was published, a similar description appears in Navigation by George L. Hosmer and may well also date back to wooden ship days:

"The chip log, formerly much used on sailing vessels, consists of a light piece of board weighted on one edge so as to float in an upright position when thrown into the water. The line attached to the log is divided into lengths called knots, beginning 15 or 20 fathoms from the chip, at a point marked by a piece of red bunting. Each of these distances is marked by means of pieces of fish line run through the log line, one for the first, two for the second, etc., each one representing a nautical mile or knot. Every two-tenths of a knot is marked by a piece of white rag. The log was generally used in connection with a sand glass, and the distances between the marks on the log line must be figured such that the length of a knot on the line has the same relationship to the sea mile (6080 ft) that the time the glass has to one hour, or expressed as a proportion, for a 28s glass, x:6080::28:3600, from which x = 47.29 ft.

"When the log is hove the sand glass is started just as the red bunting passes out.... If the line is now given a sudden jerk it will free a wooden peg, fastened to two of the lines, so that the chip will lie flat on the water and can be easily hauled aboard."

The common log consists of the log-chip, or logship, often exclusively called the log, and the log line, the former being commonly a thin wooden quadrant of five or six inches radius, loaded with lead on the arc to make it float with the point up. It is attached to the log line by cords from each corner. This line is divided into equal spaces, called knots, each bearing the same proportion to a mile that half a minute does to an hour. The line is wound on a reel which is so held as to let it run off freely. When the log is thrown, the log-chip is kept by the water from being drawn forward, and the speed of the ship is shown by the number of knots run out in half a minute.

A line or cord about a hundred and fifty fathoms long, fastened to the log-chip.
{David Huff}

William Bourne wrote about the log in his book A regiment for the sea published in London 1574,.The first part of the log line is marked with a piece of cloth Two sailors were involved in taking the reading, one held the hasp with the log line the other handled the hourglass and checked the speed reached when the timer was finished.

Lines were marked with a piece of cloth this was the point where the timer was turned. The line had markings a woven line with no knot in it at every 1/240 of a NM (nautical mile - 1852 Meters ) in metrics measurement a knot was placed every 7.72 meters. The hourglass was designed to be empty in 15 sec.

The rope had every mile equivalent a piece of rope with one knot- two knots etc. the interspersed ones did not have a knot in them.

The mariners held the line to stop it unwinding any further and checked the line rope marking when the hourglass was empty this indicated the approximate speed or knots per hour. From this practices comes the expression we were doing so many knots (NM). To retrieve the log line was easy because of the pressure on the log quadrant one side of the straps was tapered and under pressure of the current hitting the quadrant the tapered peg cam out and the log was now floating and had no longer any resistance.

Many other log types were developed with more or less success. Most were designed as a type of patent log based on rotation of a propeller type instrument.

Edward Massey 1802 propeller type Harpoon log developed in 1861

Carl Uden Denmark in 1850 developed a log whereby the counting mechanism was located on the searailing

The Walker cherub Log developed in 1879 by Thomas Ferdinand Walker was probably the most useful and long serving often it had the hourly and accumulated miles recorded as well. This Walker patent log was very commonly used. Later on the readings were electrically transferred to the bridge. In my sailing days I remember this walker log as pretty reliable providing you had a bit of speed; otherwise it was useless.

A different type log was developed by the Russian chief of Hydrography of the Sebastopol Russian Navy. It consisted out of a tube in the bottom of the ship in which a small propeller was inserted and via an electrical generated impulse the speed and total distance was recorded at the bridge. When not in use the propeller was withdrawn and the tube closed.

Later on the Pitot tube speed sensors were developed which are still often used on speedboats or yachts. but all the romance has disappeared with the introduction of GPS, of course.
{Karel (Modelkiwi Karel polman NZ) }

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