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byronink@netzero.com wrote:
> 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.
> ...
> 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. 

What would be interesting in this respect is the heading accuracy 
specified by the gyro's manufacturer used on the vessel(s) in question.

Modern state of the art FOG's (Fiber Optic Gyro's) are accurate to about 
0.03xSEC(LAT) and these are only around quite recently. The next, lesser 
order, FOG's perform to about 0.2xSEC(LAT) degrees and both are known to 
be quite a bit better than most conventional (spinning) gyro's, but 
still far too young for a vessel like the USS INTREPID. Now I am not too 
well informed about naval gyro's from those days, so there might have 
been models around I am not aware of that could have paralleled these 
FOG figures, but in general I would say 0.5 degree accuracy seems to be 
quite something for those conventional ones.

One of the gyro's widely used on navy vessels I know of was the Sperry 
MK29 (which was, I believe, even quite modern compared to the USS 
INTREPID). Although I do not know it's specs, I do know that it was 
replaced by (among others) the Kearfott KN-5054 Seanav Gyrocompass 
(accuracy: 0.5xSEC(LAT)) and the SITEP Gyrocompass Unit GB-7371(V) 
(accuracy: 0.2xSEC(LAT)), so obviously the MK29 was not (much) better 
than that Kearfott's 0.5xSEC(LAT). At Narragansett Bay (app. 41.5N) the 
MK29 would thus not be better than about 0.7 degrees.
As far as I know these figures are only 1SD, so 68% probability, which 
means that 98% of the readings (3SD) would be within 1.5xSEC(LAT) 
degrees (or about 2 degrees for Narragansett Bay). Therefore occasional 
deviations of 1.5 degrees for a conventional gyro like the MK29 or 
similar old type gyro - no matter how well calibrated - do not surprise 
me too much, seem to be quite normal even.

Apart from this I find it surprising that a navy vessel (or rather any 
vessel) would be navigated through thick fog by compass bearing only. 
One would expect those waters had a well marked navigable channel with 
radar reflector equipped buoys and that - under dense fog conditions - 
one would pay attention to the radar (and a keel clearance meter), not 
only relying on a steered course. At first sight it seems to me that 
other factors may have contributed to her running aground.

Nicol�s


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