NavList:

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

 

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 <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, <Anabasis75@aol.com> 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 <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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