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Re: Azimuth Circle compass error.
From: Jeremy C
Date: 2009 Nov 19, 19:56 -0500
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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@netzero.com
To: NavList@fer3.com
Sent: Thu, Nov 19, 2009 12:36 pm
Subject: [NavList 10751] Azimuth Circle compass error.
-- From: byronink@netzero.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. -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList+@fer3.com