A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Azimuth Circle compass error.
From: Greg Rudzinski
Date: 2009 Nov 19, 11:21 -0800
From: Greg Rudzinski
Date: 2009 Nov 19, 11:21 -0800
Byron, On the USN fleet oilers that I sailed on the gyros (2) were modern self contain sealed units about the size of a rolled up sleeping bag. One was designated as a master to drive the repeaters at the helm, bridge, bridge wings, flying bridge, engine room, after steering, and chart room. Each repeater was set/checked and marked as high or low degrees and tenths. Sometimes there were voltage variations to the gyros which seemed to cause constant errors. I don't recall errors ever exceeding 1.5 degrees though. Taking a low sun azimuth using the shaded telescopic alidade was my preferred method of checking the master gyro. For piloting in hazardous waters my preferred method was to use turn bearings and ranges that were directly grease penciled on the radar in addition to a bridge wing gyro repeater visual turn bearing. An apparent motion track of a navigation aid/mark/point of land was also grease penciled in to give an immediate awareness of cross track error. I was the only one doing the navigating so three LOP fixes every three minutes were not done. A pair of radar ranges was my best fix. Greg On Nov 19, 9:36�am,
wrote: > � � � � �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 unsubscribe, email NavListfirstname.lastname@example.org