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Re: 1797 logbook notation
From: Frank Reed
Date: 2010 May 26, 20:27 -0700

This might be a good spot to point out the heritage of Bowditch's Navigator. We know that Bowditch is based on Moore, and Bowditch knew that Moore borrowed from Robertson. So Bowditch went back to the source. Compare the descriptions of "knots and fathom" and heaving the log in Robertson and Bowditch:

From Robertson's "Elements of Navigation", 1796:

"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 the reel (which is about 10 fathoms, this distance being usually allowed to carry the log out of the eddy of the ship's wake) and the first mark is going off, he cries turn! the glass-holder answers done! who watching the glass, the moment it is run out says stop! the reel being immediately stopt, the last mark run off shews the number of knots, and the distance of that mark from the reel is estimated in fathoms. Then the knots and fathoms, together, shew the distance the ship has run the preceding hour, if the wind has been constant.

In the King's ships, India ships, and some others, it is usual to heave the log every hour: but coasters, and those which make short voyages, heave the log once in two hours only."
[...]
"But because it is safer to have the reckoning rather before the ship than after it, therefore 50 feet may be taken as the proper length of each knot and each knot is now usually divided into 8 fathoms. The length of the knots in the log-line, used formerly, was only 42 feet ; and it is much to be wished that no line so divided was now in use, but custom in many things prevails over reason. "

From Bowditch's "New American Practical Navigator", 1807:

"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 rag) is going off, he cries turn! the glass holder answers done! who watching the glass, the moment it is run out says stop! the reel being immediately stopt, the last mark run off shews the number of knots; and the distance of that mark from the reel is estimated in fathoms. Then the knots and fathoms together, shew the distance the ship has run the preceding hour, if the wind has been constant. But if the gale has not been the same during the whole hour, or time between heaving the log, or if there has been more sail set or handed, there must be allowance made for it according to the discretion of the artist. 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 10, and less in proportion, if the sea be not so great; a proper allowance ought also to be made if there be a head sea."
[...]
"hence the length of a knot ought to be 51 feet: each of these knots is divided into 10 fathoms of about 5 feet each. If the glass be only 2S 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 over-run 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 a knot about 7.5 fathoms of 6 feet each, to correspond with a glass that runs 23 seconds."

And by the way, EXACTLY the same text is found in the 1880 edition of Bowditch (the last edition before the USN overhaul was published). You can see that a good portion of this is verbatim from Robertson, but Bowditch added to it. If you read the same section in Moore, he, too, borrowed from Robertson but differently and not verbatim.

Here's Moore's version from 1800:

"In heaving the log, one man holds the reel upon which the logline is wound, and another holds the half-minute glass; an officer of the watch heaves the log over on the lee-quarter, and when he observes the stray-line is run off the reel (to denote which there is fixed a red rag) he cries turn! the glass-holder answers done! who, watching the glass, the moment it is run out, cries stop! the reel being immediately stopped, the knots, or knots and fathoms run off, shew the ship's rate of sailing per hour, if the wind happens to have been constant."

and in another section, Moore has:
"But as for the most part, the ship's way is found, by experience, to be really more than that given by the log, and as it is safer to have the reckoning before the ship than after it, therefore 50 feet may be taken as the proper length of each knot, and these knots subdivided into ten fathoms, each of 5 feet, which is certainly the best adapted for practice, and will correspond with all the tables and instruments used in navigation, as they are decimally divided, and consequently, the ship's run determined with greater care and certainty. But some experienced commanders find, that the allowing 50 feet to a knot generally makes a ship a-head of the reckoning; and to avoid danger mostly divide the log-line into knots of 7 or 7.5 fathoms of 6 feet each, to correspond with a glass that runs 28 seconds. Others again divide the seconds the glass runs by 4, and take the quotient for the distance in fathoms between the knots; which of these methods are best, I leave to every captain's own experience to determine; but certain it is, that whatever length the knots are, the most convenient way is to divide them into tenths."

-FER
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