Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Log line measurement: was: 1797 logbook notation
    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  george@hux.me.uk
    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
    ----------------------------------------------------------------
    
    
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site