NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Noon sun fix example
From: Hewitt Schlereth
Date: 2009 Oct 7, 10:36 -0400
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.comwrote: > > 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 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com -~----------~----~----~----~------~----~------~--~---