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
    Shovell disaster. was; Old question - need summary
    From: George Huxtable
    Date: 2010 Nov 2, 00:01 -0000

    Frank wrote, under "old question-need summary"  his familiar apologia for
    Sobel, and criticism of May's paper about his study of the surviving logs
    from the Shovell disaster-
    
    "That's the status of the dead reckoning alone, but Shovell had more to go
    on than "pure" dead reckoning. He also knew that they were in soundings and
    presumably he had even seen the mud pulled up stuck to the tallow. Those
    soundings, while quite deep, pin down the longitude considerably better. On
    a modern chart, we can follow the contours of the soundings and mark the
    position of the fleet with some accuracy. If we knew where Shovell believed
    that the 80/90 fathom band was located, based on his nautical information
    back in 1707, then we could narrow down better his estimation of the
    fleet's position. But this was highly uncertain information. The surveys
    themselves suffered from the same navigational uncertainty, especially with
    respect to longitude, as did the navigators of Shovell's fleet."
    
    I wasn't aware that such sounding information existed, but in the paper
    that Frank and Douglas both referred to, which can be found at-
    http://www.hmssurprise.org/Resources/SIR_CLOUDESLEY_SHOVELL.html
    these words appear-
     "On the 21st of October the Admiral made an observation, probably the
    first he had been able to take for many days. The next day, having
    soundings at 90 fathoms, he brought to and layby about 12 o'clock 18 and
    summoned all the sailing-masters of the various ships on board the
    Association, and consulted them as to the fleet's actual position "
    
    reference 18 is to Isaac Shomberg's "Naval Chronology", vol 1, page 132,
    which is available Googled at-
    
    http://books.google.com/books?id=5rFCAAAAYAAJ&dq=inauthor%3A%22Isaac%20Schomberg%22&pg=PA132#v=onepage&q&f=true 
    ,
    and states "on the 23rd October the Admiral struck soundings at 90 fathoms,
    the wind then blowing strong from the SSW, with hazy weather, he brought
    the fleet to. At six in the evening he made sail again under his courses,
    whence, it is presumed, he believed he saw the Scilly light ; soon after he
    made the signals of danger"
    
    May described the event as follows-
    "After describing the despatch of the two frigates in the morning of the
    22nd, he says "No Sun was visible on that day and around 4 pm [by which
    time I presume that the nautical-date had changed at noon to the 23rd,
    which may explain some of the date-discrepancies] the fleet hove-to for
    about two hours to obtain soundings; then, satisfied they were in the mouth
    of the Channel and clear of all danger,the ships ran to the Eastward before
    a favourable gale. Less than two hours later the leading and more northerly
    ships found themselves among rocks and fired guns..."
    
    But the crucial piece of information that's emerged is that depth, recorded
    by Schomberg, of 90 fathoms, or about 160 metres on a modern chart. I
    wonder if that statement is backed by evidence from elsewhere. Perhaps
    Douglas can tells us if his studies provide any clues. May doesn't state
    any depth. Frank suggests- "If we knew where Shovell believed that the
    80/90 fathom band was located, based on his nautical information back in
    1707, then we could narrow down better his estimation of the fleet's
    position." But we need to ask more than that. How did he make such a
    hopelessly inaccurate sounding? Because inaccurate it must have been, as
    events showed.
    
    Attached is a corner of just such a chart as "Association" is likely to
    have carried at that date, 1707. It's from a survey, "Great Britain's
    Coasting Pilot", made by Greenville Collins "by order of the King", and
    first published in 1693. Publication, by Mount and Page, continued right
    through to 1753, from which this copy was taken (and a bit further still).
    Few updates or amendments were made over that long life; not even a
    correction of the erroneous latitude of Scilly. The fleet's flagship is
    highly likely to have carried a copy of that pilot. And comparing it with a
    modern metric chart, there is remarkably good agreement between the two, in
    terms of depths of these Western approaches to the Channel, in general if
    not in fine-detail.
    
    That chart, which has unnumbered longitudes, and my modern metric charting,
    both extend to nearly 8 degrees West, and neither shows water as deep as 90
    fathoms anywhere near striking distance of the Scilly rocks. Frank wrote-
    "On a modern chart, we can follow the contours of the soundings and mark
    the position of the fleet with some accuracy." I challenge him to do so;
    there's simply no such 90-fathom contour, between the latitudes of Ushant
    and Scilly, within the confines of that chart. Possibly, Schomberg's
    reported sounding depth may be mistaken. Or perhaps the depth observation
    itself was at fault. I can suggest one possibility. Sounding at such depths
    called for the deep-sea lead, a 56 pound weight. Although shallow-water
    soundings could be taken under way, and piano-wire soundeing machines
    allowed even deep-sea soundings in the 19th century, in Shovell's day
    taking a deep-sea sounding required a vessel to heave to, to allow the
    weight to drop down straight rather than trail at an angle. Yet, the
    accounts agree in implying that first, the sounding was taken (when
    reaching in a near-gale), and next, the fleet lay-to to discuss its
    implications. That's the wrong order, and asking for trouble, in
    overstating the true depth.
    
    After passing East of Scilly, there's still Lizard to round, before
    latitudes should ever have been allowed to approach anywhere near 50ºN, and
    passing Lizard can't be relied on until the soundings have dropped to 45
    fathoms: not 90 fathoms. And yet, the evidence is that he must have thought
    his fleet clear East of Lizard when dispatching his scout frigates to go on
    ahead, because these had been sent off in a Northeasterly direction: so
    much so, that in the event they found themselves on the North side of
    Scilly, doubtless to their great surprise. How that view could ever be
    squared with a 90-fathom sounding is a mystery.
    
    The disaster can't simply be ascribed to failure in knowledge of latitude
    or longitude, but in a failure of both, together with carelessness about
    sounding information. Dead-reckoning for latitude, from an observation
    taken at noon on the previous day, taken with proper assessment of
    soundings, and caution about blundering on in the dark, should have been
    enough to keep Shovell's fleet off the Scilly rocks. Lead, log, latitude
    and lookout, were the rules that a navigator was expected to follow. In
    1707, a navigator was expected to manage without longitude, and get his
    ship to its destination.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    

    File:


       
    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