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Log line measurement: was: 1797 logbook notation
From: George Huxtable
Date: 2010 May 27, 15:47 +0100
From: George Huxtable
Date: 2010 May 27, 15:47 +0100
I've altered the threadname to accord better with the direction our discussion seems to be taking. Frank has pointed to conflicts and copyings between varying textbooks over the question of log line subdivision and timing. Confusion seems to have reigned, as Maskelyne pointed out in an appendix to his "British Mariner's Guide" of 1763, "Some remarks on the proper length of the log-line". Nevertheless, the confusion seems to have continued. Incidentally, Frank's quote, from Bowditch, where it reads "...If the glass be only 2S seconds in running out..." the 2S is presumably a transcription error for 28 seconds, as many timeglasses happened to be. Ian Jackson and I touch on the use of the log in our recent paper "Journey to Work", in The Journal of Navigation, April 2010, about James Cook's seven crossings of the North Atlantic in the small brig "Grenville" between 1764 and 1767. An electronic copy is available to anyone who asks. To judge by the (very) occasional entry of 8 or 9 in the Fathoms column, those numbers were presumably regarded as representing tenths of a mile, and not eighths. But the very rarity of those entries, compared with the commoner values of less-than-eight, leads one to wonder whether he and the master's mate and the men taking the readings always agreed about that matter. In those days, before he had learned to use lunar distances, or had a chronometer, Cook was completely reliant on his dead reckoning for longitude. The log was streamed regularly each hour, which was quite a committment, calling for two men out of an overall ship's complement of only 20. We don't know about the timing glass that was used, or the knot-spacing of the line, but it'transpired that in every one of those crossings (4 Eastward, and 3 Westward) Grenville ended up "ahead of her reckoning", consistently, by something like 4% to 8%. So Grenville would always arrive at the target coast a day or so ahead of the moment that a DR plot would have predicted. No doubt Cook allowed for that error-margin: certainly, he was busy taking soundings long before it became really necessary. But it was a surprise to us that he appeared to have taken no steps to fix the error, by re-knotting the line, to be a bit more closely spaced. Or, simpler still, by adding a teaspoonful of sand, or crushed eggshell, to the timing glass. ========================= Bligh's account makes interesting reading, after he, with 18 others were abandoned in the Pacific in Bounty's 23-foot launch. It can be read in "The mutiny on board HMS Bounty 1789" by William Bligh, published in facsimile copy in 1981. After the first few days, first on an island, then at sea, he wrote, on 5th May 1789- "I have hitherto been able to keep only an imperfect account of our run, but have now got ourselves a little better equipped and a line marked, & having practised at counting seconds, every one can do it with some exactness". I wonder how closely those agreed-seconds conformed to the International Second of time? His account, from then on, records log readings every hour, in accordance with Royal Navy tradition. There was no timepiece to measure those hour intervals, which didn't need to be at all precise. Bligh had been allowed a Hadley quadrant (but not his sextant), which would do for assessing time from height of Sun, if guesswork wasn't good enough. For the first few days, speeds were measured in whole or half knots, the only entries in the F column being either 4 or blank. As the voyage proceeded, and confidence grew, entries in the F column could be 2, 4, or 6, speeds being estimated to the quarter knot. By the end of the journey, the speed was being interpolated in eighth-knots. Anyway, after a jouney of 4000 miles in six weeks, Bligh reached his intended destination of Timor, having lost only one man (to an attack by natives). And what's more, adding to the knowledge of the World in charting the Fiji Islands along the way. contact George Huxtable, at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Frank Reed"
To: Sent: Thursday, May 27, 2010 4:27 AM Subject: [NavList] Re: 1797 logbook notation 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 ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------