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    Re: Azimuth Circle compass error.
    From: Jeremy C
    Date: 2009 Nov 19, 19:56 -0500
    Byron,
     
    I have a few suggestions for this.  When berthed at the shipyard, the ship should be either square in the dry dock, or the amount of error determined in the static state.  At this point the true heading of the ship can be determined with good accuracy from the charts.  This can also be done at the wet-berth by warping the lines to place the ship parallel to the pier.
     
    Second, I am not familiar with Naval Alidades, but commerical units offer shades and mirrors so that azimuths can be taken at reasonable angles.  This is my standard tool for star, planet, and moon azimuths.  I have also used it with the sun.  These shots are far more percise than the azimuth circles. These could probably be used to at least double check the rather inaccurate "sun bar" that the azimuth circle provides.  Even if the naval alidades can't be elevated for sights, if a ship can get a shade of some sort, an amplitude of the sun can be used and a second check of the azimuth circle can be made.
     
    Lastly, I am quite suprised that the ship isn't using a range departing from the harbor or entering a harbor to check compass error.  I have had numerous pilots tell me my gryocompass error by being on range and checking the difference between the course being steered and the true course of the channel.
    Jeremy

    -----Original Message-----
    From: byronink---.com
    To: NavList@fer3.com
    Sent: Thu, Nov 19, 2009 12:36 pm
    Subject: [NavList 10751] Azimuth Circle compass error.

             Azimuth of the sun and gyro/compass error                                                                             
    While I was crew on NAVY Ships undergoing overhaul in the Shipyard, the yard 
    worker align the gyro and related systems.  The azimuth of the sun is the 
    preferred reference to setup the system and my pet peeve.
    The Azimuth Circle used to align the gyro is a delicate instrument that is with 
    mirrors and a prism that throws a beam of light on the compass for a reading of 
    the compass/ gyro. The Azimuth is timed and the true bearing of the sun is 
    worked out using reduction tables for a tabulated Azimuth. The light beam on to 
    the compass card has a thick beam of about .3 degrees, the Circle leveling 
    bubbles must be level to get a good reading.   There exists a reasonable error 
    of 0.5 degrees of error when the sun is high. Even so, the delicate, mirrors and 
    prism may be unknowingly out of alignment (small knock can do this) for 
    additional error.  After the ship is out of the yards and underway, the Azimuth 
    circle is use to further check on the compass/gyro error.  At sea everything is 
    fine, the ship may often correct course to close the Dead Reckoning (DR), 
    because of set and drift found by underway fixes. There is no problem. After the 
    ship enters  to the hazard waters and the ships is to transit for a distance to 
    the birth or anchorage, another set of rules are in place, large NAVY ships are 
    to take on a pilot to advise the conn of harbor up dates , recommend courses and 
    place tugs.  The navigation team will use visual, radar, and electricronic fixes 
    ever three minutes, every third fix must be of different equipment (according to 
    naval instructions.) The compass, helmsman and the visual bearing s are a big 
    part of the navigation aids used in these more dangerous, waters. The bearing 
    takers for visual bearing are stationed with telescopic Alidades not the Azimuth 
    circles. The bearing taker with Alidade can call in a round (3 or more) of 
    bearing in a few seconds, with the accuracy of 0.1 or .2. (Looking back, a good 
    azimuth accuracy may set the gyro to 0.5 + or -.)  The Azimuth Circle used in 
    the yard to set up the gyro is known to be inaccurate. Bowditch "it is 
    reasonable to round calculations to the nearest half or perhaps whole degree for 
    most purposes."   We have the Alidade that can take bearings to 0.1 or 0.2 and 
    helmsman that can steers less than 0.5 and we introduce large error because of 
    past azimuth practice. I have seen the results many times in my past, when I 
    evaluated piloting problem aboard both submarines and surface ships. My best 
    example was the USS INTREPID.She left the Philideliy Yards after overhaul. 
    Sailed to her home port of Norfolk VA. Sailed to her new home in RI. And ran 
    aground in Narragansett Bay RI. Than to her birth at the pier. She ran aground 
    in heavy fog, I am sure that is the main reason. After I went aboard and she got 
    underway, I saw two small triangles on the chart, her second fix. What I saw was 
    an east error of approximately 1.5 degrees. I requested to the Navigator that he 
    add 1.5 degrees to the next round of bearings. With that correction the next fix 
    was a three point intersection, that 1.5E correction was used for fixes until 
    months later when the gyro error was manually corrected. The AZIMUTH CIRCLE use 
    at the overhaul was the original problem. The grounding in the fog was due to 
    the visibility, although the bottom INTREPID hit was to the right of the center 
    channel. With a 1.5 East error the ship will track you to the right of your 
    ordered course (17 YARD FOR EVERY 1000 YDS TRAVELED.)  INTREPID traveled the 
    ordered course for about 6000yrds and tracked 150 yds to the right of the 
    desired track. The Pilot didn't know this compass error nor did the crew.  I 
    found it after the grounding. Reconditions, don't use the Azimuth to set the 
    compass. Use the Alidade and bearing to chart NAVAIDS to a three point fix. (Use 
    the two Alidades (use in transiting hazardous water,) to ensure they are the 
    same. See Franklin piloting technique. I may rewrite this for the naval 
    proceedings (a naval Magazine.)I would like to get Merchant / Navy /every one, 
    ideas and experience on this subject.
    
    
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