Patrick O’Brian was an author obsessed with detail, and throughout our beloved series gives us great insight into the everyday running of a ship of the Royal Navy during the Napoleonic War. This period detail makes the reading experience even more vivid, but can also be somewhat confusing. There were many operations performed daily on a man-o-war that are utterly foreign to the modern reader: heaving the lead and heaving the log are two of them.
Heaving the Lead
“You are right, Mr Pullings,” said Jack. “Send a good man into the chains: we should have twenty fathom directly.” He walked to the taffrail and called over dark water, “Mr Marshall, we are standing in.” The high black bar of the land, sharp against the less solid darkness of the starry sky: it came nearer and nearer, silently eclipsing Arcturus, then the whole of Corona: eclipsing even Vega, high up in the sky. The regular splash of the lead, the steady chant of the man in the weather chains: “By the deep nine; by the deep nine; by the mark seven; and a quarter five; a quarter less five…” – Master and Commander
Since at least the sixth century B.C. the sounding lead or sounding weight was in use in the Mediterranean area for maritime navigation. It is the oldest known marine navigational instrument and it remained a primary navigational aid in coastal waters and rivers well into the twentieth century.
A sounding weight is a roughly bell-shaped mass, usually made of lead, averaging about five kilograms in weight, with a sturdy attachment lug at its apex and a tallow cup in its spreading base. A line was attached to the sounding lead (lead line) with graduated markings at fixed length intervals. The sounding lead was led out in the water hanging on the lead line and the line was veered until the lead reached the sea bed. From the marks on the lead line the depth of water could be read.
Ancient mariners used sounding-weights, not only to determine the depth of water, but also to bring up samples of the bottom (stuck in the tallow on the base of the lead), comparing the result with their knowledge of coastal geography and river sediments. With this knowledge a method of navigating from one depth to another based upon the condition of the bottom developed.
Herodotus wrote in the fourth century B.C.: “…When you get 11 fathoms and ooze on the lead, you are a day’s journey out from Alexandria”.
And a sailing directions from the 14th Century reads “Ye shall go north until ye sound in 72 fathoms in fair grey sand. Then north until ye come into soundings of ooze, and then go your course east-north-east.” (72 fathoms is about 130 meters – that’s a long line!)
From later reports we know that “heaving the lead” was a standard procedure as a ship sailed into harbor. A sailor (the “leadsman”) would throw the sounding lead as far forward as he could, and when the ship came up to where the lead had landed he would count the or markings on the lead line and call out the depth of the water to the pilot.
Typically, the leadsman stood in the main chains, though it seems that sometimes he was stationed in the fore chains, and cast a hexagonal weight attached to a length of line. As the ship passed over the weight he called off the depth. The line had markers attached so the leadsman could call the depth in darkness. The markers are located at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms.
In the British navy during the Age of Sail, the weight of the leads were 7 pounds (small boats), 14 pounds (standard), and 28 pounds (deep sea). The weights are all based on the British unit of weight the stone, or 14 pounds.
The lighter weights typically were attached to about 25 fathoms (150 feet) of line, the deep sea lead carried about 100 fathoms of line, or in Imperial nautical measurement, a cable.
The leadsman called depths according to a formula. When the depth fell on one of the markers affixed to the line, the leadsman would call “By the mark five (or seven or whatever)”. If the depth was at a depth not marked on the lead, the leads man called “By the deep six (or eight or eleven or whatever)”. When the depth fell between fathoms he would call “And a half five” for five and one half fathoms or “And a quarter less eight,” which is 7.75 fathoms.
Incidentally, the safe depth of a sidewheel riverboat on the Mississippi was two fathoms, which the leadsman would call out as “By the Mark Twain”.
Heaving the Log
“Each time the reefer of the watch cast the log he reported with mounting glee, “six and a half , if you please, sir. — Seven two fathoms. –Almost eight . –Eight and three fathoms. –Nine . –Ten ! Oh sir, she’s doing ten .” – The Reverse of the Medal
“Heaving the log” is a way to measure a ship’s progress through the water that dates back from the Age of Sail, and was well used into the 20th. century. Heaving the log was performed many times each day – typically each half hour – to record the sailed track.
Eventually heaving the log involved dropping a small wood panel – the “log” – into the water and measuring the length of a line it pulled off a reel in a given period of time. Time was measured with a special sand glass. The result is the speed of the ship through the water.
The log itself consists of a flat piece of thin board, of a quarter-circle, loaded on the circular side, with lead sufficient to make it swim upright in the water. Fastened to this log is a line of about 300 meters long, called log-line, which is wound on a reel.
The log was cast off the stern of the ship on the lee side and the log-line allowed to pull out as the log floated stationary in the water. The line had a series of marks or every 14.4 meters (48 feet). When the first knot had unreeled, the sand glass was turned which ran for 28 seconds. Then the number of in the log-line that were unreeled in this 28 seconds period was counted.
The distance between the marks on the log-line was such that if the ship was doing one mile per hour, exactly one mark unreeled after the starting mark. Generally every two-tenths of a knot was extra marked by a colored rag to do more a accurate estimation of speed.
During the course of the history of sailors terminology, the number of that unreeled while heaving the log, became a term for the speed of the vessel. On a ship doing 5 nautical miles per hour, 5 would be unreeled while heaving the log and sailors would simply say the vessel was doing “5 ” when referring to the speed of the vessel.
Using a set of “traverse tables” for each half-hour, the ships net change in latitude and longitude could be calculated from the measured speed. The calculated half-hour position changes were then summarized and entered into the ships main record book to determine the net change over the day. The ship’s main record book containing information about the ships speed, is still known as “logbook”.
The log was generally used in connection with a sand glass, and the distances between the on the log-line must be figured such that the “length of a knot” on the line has the same proportion to the nautical mile as the time of the sand glass has to one hour. For a 28 seconds sand glass:
( “length-of-a-knot” / 1nm ) = ( 28 s / 3600 s )
yielding 14.4 meters for the “length of a knot” on the log-line. The total length of the log line depends on the maximum estimated speed of the ship. For a maximum speed of 10 kn the log line will have a total length of about 200 meters.
Although commonly used, the origin of the 28 second glass, a rather unusual length of time, is unclear.
Courtesy of Erik Deman and Age of Sail.