Casting the Log

from the National Maritime Museum

It was well that he did, for with the rising of the moon the wind increased, blowing straight into her round, foolish face and across the growing swell. By the time he came on deck Mowett had already takin in the lower studdingsail, and as the night wore on more and more canvas came off until she was under little more than close-reefed fore and main topsails, reefed courses and trysails, yet each time the reefer of the watch cast the log he reported with mounting glee, “six and a half knots, if you please, sir. — Seven knots two fathoms. –Almost eight knots. –Eight knots and three fathoms. –Nine knots. –Ten knots! Oh sir, she’d doing ten knots.”

The Reverse of the Medal, Patrick O’Brian.

Navigation was the critical function required of a ship’s captain and master during the Age of Sail. It made little difference how fast your crew could make sail or serve the great guns if you didn’t know where you were going.

We’ll cover calculating northing and easting in the future. In the last story we looked a how casting the lead allowed a ship to be sure it was in water of a safe depth. In this story we’ll answer the question “How fast am I going?”

Ships calculated their position in two ways. The most precise was the measuring of the height of the sun at the noon sighting using either a sextant or a quadrant. In the right hands this gave an accurate northing but until the advent of the chronometer the ship relied on a dead reckoning board to plot its easting between sightings.

Dead reckoning is a pretty simple process. A series of measurements of the direction of travel and speed of the ship are converted to lines on a chart.

The compass did the direction part. The speed was calculated every half hour, with the chiming of the ship’s bell, and done with the log.

The log is obviously a misnomer. It is shaped like a quarter of a circle with hole bored at each of the three corners. Cords were permanently affixed to the holes at either side of the curved side. The cord at the sharp corner was held in place with a wooden plug which had a separate line attached.

The log was tossed over the stern and a 28-second glass started running. At the end of 28-seconds, the number of knots that had run out were recorded in the ship’s log. The knots were 47 feet, 3 inches apart and equated to a nautical mile when used with the 28-second glass. To retrieve the log, which was held in place vertically by water pressure, the string attached to plug was given a sharp tug pulling the plug free. Now the log was released from the pressure of the water and could be retrieved by the use of the reel.



Filed under Age of Sail, Jack Aubrey Novels, Naval Equipment, Naval Life, Navigation and Seamanship

6 responses to “Casting the Log

  1. Randy Nini

    In the Quote above, what is meant by the refrence to fathoms in, “seven knots two fathoms? Obviously, the ship is not in two fathoms of water under the keel.

  2. billcrews

    a fathom is a unit of linear measure like a nautical mile. The speed rendered in the above quote is the speed of the ship. Seven knots two fathoms means that the speed of the ship is a fraction more than seven knots.

    • Randy Nini

      Bill thank you very much for the explanation. I had forgotten the site where I posted the question, thus the long time to say thanks,


  3. James

    In both the Jack Aubrey and Horatio Hornblower series there is mention of something like ‘giving a sharp tug to pull the peg and allow the log to be pulled back on board’… I have been trying to picture how this would work, can you elaborate?

  4. Pingback: Casting the web-log « The Pantechnicon

  5. richard spengel

    Getting an answer to the 7 knots and 2 fathoms question is not easy. Putting together info from several sites: there is a distance of roughly 8 fathoms between knots on the log line: (8 x 6′ = 48′ = approx 47’3″). The log line has small markers at every fathom in addition to the larger knots. 7 knots and 2 fathoms refers to 2 marks past the seventh knot and corresponds to 7 and 2/8 knots or 7 1/4 knots after reducing the fraction.

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