(Chart extract from Imray Chart 2200.1)
7M away. The first thing is to narrow
down our position. Allowing for height
of tide, the echo sounder shows a charted
depth of 18m. We now head southwest
until reaching the 5m line, cross Ryde
Middle Bank and, when we reach the
10m line on the other side, track along it
until we see the West Ryde Middle west
cardinal. With that as a departure point, Fig 6 Longer passages – Starting from approximate position A we undertake a combination of depth
setting our speed as close as we can to
navigation, contour following and normal pilotage to get ourselves safely to Yarmouth.
6kn (1 cable a minute) over the ground
to make the plotting easier, or 3kn (half
a cable a minute) if 6kn is too fast for the 5m line leads within 50m of the pier due north and log our depth we can gather
the conditions or our engine, and making and entry there in fog is shorter, so, at more clues. If the depth gently shelves
allowance for tidal stream, we can travel Salt Mead we head out again, heading and reaches 20m after four miles, then
approximately along the 20m line on a 255° for 20 minutes, in the hope of seeing we are probably around A1. If it shelves
heading of about 265° and hopefully see Hamstead Ledge. Whether we see it or to 20m after two or three miles and to
Prince Consort north cardinal after about not, we’ll then head 245° for another 10 10m after four miles then we are probably
11 minutes or Gurnard north cardinal after minutes (about 1 mile) and then turn to around B1. If the depth varies wildly
a further 8 minutes. 210° until reaching the 10m line. We turn between 20m and 60m over the four miles
If we then stay outside the 15m line, to the shore early to make sure we do not then we can be pretty sure we are around
heading 245°, we cannot go aground. overshoot Yarmouth and slow right down C1. However, if after four miles the depth
About 14 minutes will get us near to detect the 5m contour and then follow has stayed relatively constant, then we are
Gurnard Ledge starboard mark. We need this west until we see Yarmouth Pier. in area D1 and can continue for a further
to stay offshore to make sure we avoid the four miles to gather more data.
rocks at Gurnard Ledge or Salt Mead, so
CHANNEL CROSSING
Wherever we work out we are we can
we then track 240° for 18 minutes to get Now let’s imagine we have crossed the devise a new strategy (e.g. follow the 20m
near to Salt Mead starboard mark. English Channel heading north from contour to the Needles Channel) and keep
Close inspection of the charted approach Cherbourg with the GPS out of action and a close lookout for any surprises.
to Newtown Creek makes us feel that, are unsure of our position. Our estimated The underlying process in the above is
although we can find the port-hand pool of errors is over 40M east/west and to take what we know and then devise
mark, we may not get into the creek with 10M north/south. We detect the 30m line strategies that will provide the information
visibility only 100m. Yarmouth now looks and could be at A, B, C or D (Fig 7). we need to further refine our Estimated
much easier than Lymington, because If we then go for a further four miles Position (EP).
Thus, from a huge pool of errors, we
have made a good landfall and navigated
safely to a more accurate position using
commonsense, time and depth. All we
need do now is offer a little prayer for
the Hydrographer and his trusted team. ■
ABOUT THE AUTHORS
CLIVE LOUGHLIN is a chartered
engineer, recreational sailor and a
A1 B1 regular contributor to Sailing Today.
Clive would be pleased to hear
from readers about any technical
D1 aspects of sailing that are of interest to them.
A
B
C1
Please email Clive if you would like a free
copy of the form used in Fig 2.
NEW POE (based on 30m contour)
Email:
clive.loughlin@sailingtoday.co.uk.
CHARLES WYLIE is an RN
C
D
Dagger Navigator (passed the
Advanced Course). He’s made every
INITIAL RECTANGULAR POOL OF ERRORS: 10M x >40M navigation error you can imagine
except two – he’s never grounded
Fig 7 Approaches to the Isle of Wight from the south. The initial pool of errors covers over his ship, nor collided with another. He and his
40M east/west and 10M north/south, but this is immediately reduced to the region around the
wife, Jean, potter their Moody 27 Pipkin IV
30m contour line when our charted depth falls to 30m.
along the South Coast and to France.
July 09 Sailing Today 101
ST147 Seamanship
FOG_RTG.indd 81 13/5/09 13:17:34
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