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Subject: some navigation info. from a trip aboard the RMS St. Helena
From: Russell Sher (rsher@XXX.XXX)
Date: Mon Oct 04 1999 - 10:33:03 EDT
Hi - My wife and I recently returned after a trip to St. Helena Island
(South Atlantic Ocean). We traveled aboard the RMS St. Helena - approx. 6800
GRT. We traveled from Cape Town.
Firstly, here's a teaser for those of you who are interested, followed by
some interesting points which I learned during a bridge tour of the vessel:
Each day, various points of interest from the bridge would be read over the
vessel's public address system. e.g. the position, average speed and charted
depth, etc. These would always be given for 1200 LMT.
On one particular day the vessel's distance run (between 1200 previous day
and 1200 for the day in question), was given as 377.5 nautical miles and the
officer of the watch went on to say that this implied an average speed of
15.1 knots. Some quick calculating had me confused, since this did not seem
to work out - after a while I figured it out - who of you on our list can
give me the reason for the apparent discrepancy?
Here's some interesting info. about the navigating on the bridge:
On the second day of the trip, some of the navigational cadets were using
sextants - they were receiving some instruction on calibration and sight
taking - the sights were reduced using a simple scientific calculator. When
we toured the bridge, the officer of the watch mentioned that they had the
air navigational tables aboard (Ho 249) but that he didn't know how to use
them - he typically used a calculator. He said that he did occasional sight
taking just to remain in practice. The vessel was equipped with 3 GPS
receivers, which he claimed could give up to 3 metre accuracy (he said that
they can receive differential GPS signals from the beacons up to 1000 miles
offshore). I was also told that in fact some ferries use millimetre accuracy
in order to dock in certain ports. Every two hours at sea the position was
plotted. The vessel sailed the rhumb-line route as the difference between it
and the GC route was 2 miles, I was told.
According to the vessel's master, the depth recording equipment was accurate
to a few hundred metres, but beyond that temperature gradients caused
refraction of the ultrasonic signals. The vessel was steered with a gyro,
which had a repeater on each bridge wing. The repeaters were also fitted
with a pelorus which could be used with ease to take azimuths of bodies and
bearings of shore-based objects.
Meridional parts were used to work out final distances in order to adjust
speeds to arrive at the pre-determined ETA.
I'm sure that many of us small-boat sailors wouldn't feel too out of place
navigating on such a vessel.
Regards
Russell
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