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    Re: Wartime (WW2) navigation
    From: George Huxtable
    Date: 2010 Feb 2, 17:52 -0000

    Thanks to Joe Schultz for providing a useful chart of the relevant part of 
    the South Pacific, with tracks.
    
    Here are a few pickings from the interesting bones he has thrown us.
    
    1. ... "Economical speed is not achievable speed.  My first tin can could do 
    36kts - economical speed was about 14kts."
    
    True, no doubt , for a warship, given massive additional power for combat 
    conditions. Not, however, for a passenger liner, powered to travel at a 
    predictable rate, for which "cruising speed" is exactly that- the speed at 
    which she cruises; stated as 16 knots.
    
    4.... "And I don't have any experience running from New Zealand to the horn, 
    so I don't know how far south they'd dare run.  There's an old saying about 
    the southern ocean: no law below 40 and no God below 50."
    
    I think Joe is being somewhat over-dramatic about such risks. Traffic from 
    Southern and Western Australia to Panama all passes South of New Zealand, at 
    about 50ºS, without undue hazard. There's plenty of navigable water, well 
    short of the extreme conditions of Cape Horn.
    
    6. "... remember the Pitcairn Archipelago!  Curvier Light --> Los Santos 
    Light great circle is not a safe route.  I believe the normal peacetime 
    routing was to the west of Pitcairn, by the way.  French islands to the west 
    of Pitcairn, so it makes sense to go east of Ducie."
    
    There was another aspect to ship routing along that track, in 1970, the 
    entirely mythical dangers of Maria Theresa Reef and Ernest Legouve Reef, 
    which, with other nearby reefs, appear as a hazard extending from about 32ºS 
    to 38ºS, at around 150ºW.. These had been reported by New Bedford whalers in 
    the 19th century, and ships had been keeping clear of them ever since. They 
    have been faithfully shown on authoritative maps, including my copy of the 
    Times Atlas of the World, revised 1988.
    
    My 1973 copy of the Adfmiralty "Ocean Passages for the World"  under "Power 
    vessel routes", says this, about the passage we need to consider first, 
    Wellington > Panama-
    "The direct great circle track between Cook Strait and the Gulf of Panama 
    crosses the meridian of  150ºW in about 38º 30'S. The area immediately N of 
    this position has in it several reported dangers, including Maria Theresa 
    and Ernest Legouve Reefs. For this reason it is considered that a better 
    route, about 45 miles
     longer, is by great circle to 41º 40'S, 160º 00'W; thence by great circle 
    crossing the meridian of 150º W in 40º 00' S, to a position on the equator 
    in 83º 00' W. From this position, steer direct to Panama ..."
    
    From Auckland, a direct great-circle track to Panama would take a vessel 
    even further into those reefs, so the advice is, from Auckland > Panama-
    "Proceed by great circle to 41º 40' S,  160º 00' W; thence by great circle 
    to a position on the equator in 83º 00' W; thence direct to Panama..."
    
    However, in 1983, I remember a copy of Admiralty Notices to Mariners 
    arriving, giving notice of a correction to Pacific charts. This was perhaps 
    the biggest chart-correction of all time, shifting Maria Theresa Reef all of 
    15 º Eastwards! The reason given was something like "   reassessment of 
    original data".
    Subsequently, I understand that it has been expunged altogether, even from 
    that new location. Indeed, it's all deep water, without any such hazards. 
    Those reefs were a "vigia", an entirely imaginary phenomenon that took over 
    100 years to disprove.
    
    Buit back in 1940, that shoal area would have been taken all-too-seriously, 
    and ships would skirt it rather than cross it.
    
    =================
    
    Joe tells us how he generated his great circle track on a Mercator chart, 
    but I wonder if he would have done it that way when at sea, with a full kit 
    of charting on hand. Would he, then, have had a gnomonic chart available, 
    for transferring coordinates from a gnomonic straight-line to a Mercator 
    chart?
    
    George.
    
    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: "Joe Schultz" 
    To: 
    Sent: Tuesday, February 02, 2010 10:20 AM
    Subject: [NavList] Re: Wartime (WW2) navigation
    
    
    A few bones to chew on while you're all thinking:
    
    1. At that point in the war the merchie losses weren't yet large enough for 
    them to "get in line" with naval instructions.  Time plus distance equals 
    fuel.  Profits matter.  Monsarrat (sic) in his novel "The Cruel Sea" shows 
    the naval point of view, albeit for the Atlantic.  Economical speed is not 
    achievable speed.  My first tin can could do 36kts - economical speed was 
    about 14kts.
    
    2. A significant portion of a merchie captain's pay is a commission on the 
    ship's profits.  Good captains pay bonuses to their crews from this 
    commission - builds loyalty.  Everybody is financially motivated to make the 
    runs profitable, and loyalty means you're not going to publicly bad-mouth 
    the man that feeds you.
    
    3. The sinking location is almost "too good" to the rhumb track.  Perhaps 
    done to save the good captain's reputation?
    
    4. When I plotted the complete rhumb line tracks I thought "looks OK if 
    you're going east of Ducie and not west of Pitcairn, but WP1 is too far 
    south" - with experience you learn to visualize the great circle tracks 
    before you plot them.  Then I thought that perhaps Curvier Light --> WP1 
    rhumb was a joint track, meaning traffic for the canal shares the track with 
    traffic for the horn, and reduces response time for naval forces.  Perhaps 
    they hadn't thought that far ahead at that point in the war.  And I don't 
    have any experience running from New Zealand to the horn, so I don't know 
    how far south they'd dare run.  There's an old saying about the southern 
    ocean: no law below 40 and no God below 50.  I believe Kermit has been down 
    there (yes, a nickname.  Hard earned, flying jets off a carrier.  I'll use 
    it until he tells me to stop.).
    
    5. Location of WP2 makes sense if the "joint track" to WP1 is a valid 
    thought, and if you wanted to keep traffic "out of sight" of Ducie, which 
    the Pitcairn'rs used as a campsite when fishing.
    
    6. Remember the Alamo!  I mean remember the Pitcairn Archipelago!  Curvier 
    Light --> Los Santos Light great circle is not a safe route.  I believe the 
    normal peacetime routing was to the west of Pitcairn, by the way.  French 
    islands to the west of Pitcairn, so it makes sense to go east of Ducie.
    
    Finding the great circle is easy in "the real world."  Lay out the rhumb 
    track, then compute (or use the conversion tables) the initial and final 
    great circle courses.  Plot as short straight lines.  Then compute the 
    vertex LAT and LON.  Then use a big French curve or a flexible stick (two 
    people) to "fair" the great circle.  If you had time then you might compute 
    two intermediate points - confirmation that you faired the track properly, 
    and you get two points in one computation if you use the old method.  It's a 
    ten minute drill by logs, but no big deal with today's electronic number 
    crunchers.
    
    The attached plots took me about 20min (I did succumb to electronic means 
    for the great circle points) - I'm getting slow!  I used my Meridional Parts 
    cheat sheet and a six-sided engineer's scale to quickly build the chart 
    axes.  Wasn't sure which ellipsoid to use.
    
    My opinion, as to a possible compromise between profit and "get in line"? 
    The good captain had no intention of going south of the vertex for the 
    Curvier Light --> WP2 great circle.  Rhumb (in the direction of WP1) until 
    you cross the Curvier Light --> WP2 great circle.  Then "cut the corner," 
    going east until you hit the WP1 --> WP2 rhumb which gets you within 30mi of 
    WP1 as you pass by.  WP1 --> WP2 and WP2 --> Los Santos Light rhumb and 
    great circle should be nearly equal distance, by the look of it.  And all a 
    moot point as he didn't make it that far.
    
    All this assumes, of course, that the given coordinates are valid, meaning 
    that they came from (then) classified sources.  Unclassified numbers 
    published in wartime are very suspect.
    
    Joe
    
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