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    Re: Noon sun fix example
    From: Hewitt Schlereth
    Date: 2009 Oct 7, 10:36 -0400

    Thanks, Jeremy. That's all good info. I gather "cocoons" are sleeves
    for containers or maybe weather cloths for an entire stack.  -Hewitt
    
    On 10/5/09, Anabasis75@aol.com  wrote:
    >
    > You are probably correct to some extent about directional stability for
    > large ships compared to small craft as they won't be bounced around as much
    > by the waves at least.  Ships can be affected by strong winds and currents
    > dramatically, especially certain types of vessels.  My ship has "cocoons"
    > which have 50,000+ sq feet of canvas so I have had leeway of up to 10
    > degrees, so we aren't immune to nature's effects.
    >
    > You could certainly use an average slip for a given day to give a fair
    > estimation of distance run, but slip also changes due to current and wind as
    > well as factors such as hull fouling.  (for those who don't know, slip is
    > the efficiency of the propeller.  If the propeller should move the ship a
    > certain distance in one revolution, you can multiply this number, called
    > pitch, by the revs to give distance by engine.  Compare distance by engine
    > to distance made good gives slip).  Our slip during the first part of my
    > recent voyage was quite large due to substantial fouling of the hull and
    > less favorable currents.  We had slip around 15% on this first leg.  Our
    > total average slip for the trip from Diego Garcia to Cape Fear USA was an
    > exceptionally low 1-2%.  This was due to having scamped (cleaned) the hull
    > in Diego Garcia, and favorable currents for nearly the entire voyage.  We
    > tend to judge fouling by an increase in slip over time, which is now done by
    > GPS based sailings and engine speed.  The use of semi-accurate noon running
    > fixes isn't a great way to really determine slip, but this was the method
    > before electronic navigation.  Others on the list can comment better than I
    > can on those practices.
    >
    > We also have a speed log which can give a decent speed through the water.
    > Ours has not been accurate for some time however and is supposed to be fixed
    > during the current shipyard.  I will have to report on its effectiveness
    > next time.
    >
    > I have only navigated one ship by non-electronic means, and I cannot
    > remember the differences between the DR and the celestial fixes now.
    >
    > Jeremy
    >
    >
    > In a message dated 9/30/2009 10:15:17 P.M. Eastern Daylight Time,
    > hhew36@gmail.com writes:
    >
    > Thank you, Jeremy -
    >
    > I was asking because during the years I was navigating at sea the DR
    > was the only check for celestial. I'd heard ships could keep quite
    > precise DRs because of their inertia, and directional stability. I'd
    > also heard ships could keep accurate tabs on the distance run by
    > counting the number of propeller revolutions, since slip could be
    > determined accurately during acceptance trials. i even think I recall
    > reading that the degree of bottom fouling could be determined by an
    > increasing divergence of DR and celestial fixes.
    >
    > Anyway, page 200 of my 1933 edition of Bowditch has  a plot of a
    > ship's progress from dawn to dusk with 'position by
    > account'-to-celestial spans ranging from 2NM to 5NM
    > which sounds about what you experience.
    >
    > Hewitt
    >
    > On 9/30/09, Anabasis75@aol.com  wrote:
    > >
    > > That's a hard question to answer fairly.  We have constant position fixing
    > > and pretty much constant engine speed.  Usually we DR one hour ahead, and
    > > perhaps to the end of the watch (4 hours ahead).  The DR is pretty
    > > meaningless because we are constantly monitoring out position along the
    > > track line and periodically adjusting the heading to keep relatively close
    > > to the track line.
    > >
    > > To answer the question, if the current changes and the mate is too lazy to
    > > change course during the watch, you can be several miles off of the DR.
    > If
    > > your are slowed by current or weather, then you can certainly fall behind,
    > > but be on the track line.  Other times, you can be basically right on the
    > > DR.
    > >
    > > I calculate SMG and CMG every hour by sailing so I have a very good idea
    > > where we are going, and if the current doesn't change, I can be very close
    > > to my DR, even by changing the course once in the hour, or not at all.  If
    > > you use an "instantaneous" GPS course and speed for SMG/CMG at the top of
    > > the hour for your DR, the error can increase.
    > >
    > > Despite all of this, unless you are in pilotage waters or making landfall,
    > > there isn't a lot of concern these days with exact navigation in the deep
    > > water over the course of several thousand miles.  I tend to be very close
    > as
    > > a matter of professional pride.
    > >
    > > Jeremy
    > >
    > >
    > > In a message dated 9/24/2009 12:12:32 P.M. Eastern Daylight Time,
    > > hhew36@gmail.com writes:
    > >
    > > OK, Jeremy. While we're on the  subject of ship navigation, I've often
    > > wondered how closely a ship's DR squares up with GPS.   -Hewitt
    > >
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    > >
    > > On Wed, Sep 23, 2009 at 6:40 PM,  wrote:
    > >
    > > >
    > > >
    > > > Not a problem there Hewitt.  I know that small boat sailors don't have
    > > many advantages with their shooting as I do, but I was wondering why the
    > > results of the reductions were different with the same data.  This
    > explains
    > > it.
    > > >
    > > > Jeremy
    > > >
    > > >
    > > >
    > > >
    > > >
    > > > In a message dated 9/22/2009 12:39:28 P.M. Eastern Daylight Time,
    > > hhew36@gmail.com writes:
    > > > Hi, Jeremy -
    > > >
    > > > Doing celestial from a ship is obviously a lot different from doing it
    > > > aboard a sailboat. A fair number of small-boat sailors - me for one -
    > > > would consider a 4' difference between a GPS position and a celestial
    > > > fix pretty much OK - 4' being just a little beyond the horizon at an
    > > > eye height of 9 feet.
    > > >
    > > > Anyway, the reason for the difference is that I have an aversion to
    > > > altering other people's data. So, I took your first and last sextant
    > > > shots as equal altitudes, even though they weren't quite. I did the
    > > > same thing in the 'Noon Sight Shootout' a few months ago.  Just took
    > > > George's data as presented.
    > > >
    > > > Attached is a work sheet where your last Hs is backed up to the time
    > > > when it would have equalled your first Hs.
    > > >
    > > > That done, my longitude now comes out 0.2'  West of yours.
    > > >
    > > > Thanks again for the real-world data.   -Hewitt
    > > >
    > > >
    > > > On 9/21/09, Anabasis75@aol.com  wrote:
    > > > >
    > > > > Hi Hewitt,
    > > > >
    > > > > Thanks for taking the time to do this via the table method.  I have to
    > > say
    > > > > that I am pretty disappointed with the longitude determination from
    > this
    > > > > method.  4.3' is a pretty big error in my book and would send me
    > looking
    > > for
    > > > > math or sight errors if it happened out here.  I find it curious why
    > > there
    > > > > is such a difference between this method and the ones Peter and I used
    > > to
    > > > > get much more accurate fixes.
    > > > >
    > > > > Being unfamiliar with the book that you used, I am wondering if there
    > is
    > > > > some explanation as to why the results differ so greatly.
    > > > >
    > > > > Jeremy
    > > > >
    > > > > In a message dated 9/19/2009 10:06:35 A.M. Eastern Daylight Time,
    > > > > hhew36@gmail.com writes:
    > > > > Hi Jeremy -
    > > > >
    > > > > Here's a paper-and-paper method of finding Longitude at Noon based on
    > > > > the motion correction table published in Latitude and Longitude by the
    > > > > Noon Sight.
    > > > >
    > > > > My work sheet is the .doc attachment.
    > > > > The table scan is the .jpg attachment.
    > > > >
    > > > > As you'll see, this method differs from your 1300 GPS by .4' in Lat
    > > > > and 4.3' in Lon.
    > > > >
    > > > > Thanks for providing data from actual combat conditions. :-)
    > > > >
    > > > > Hewitt
    > > > >
    > > > >
    > > > >
    > > > >  >
    > > > >
    > > > > >
    > > >
    > >
    >  >
    >
    
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