FOG
BOX B POOL OF ERRORS
The PoE concept was devised decades ago
to help British submariners submerged on
DR/EP for days without electronic aids or
echo sounder (a clandestine Captain might
allow one ping per day if in sufficient doubt
about his position). An unknown compass
error of 1º could introduce a one mile error
in a 60 mile passage and an unknown
current of half a knot could move the
submarine 12 miles from its EP in a day.
Box A When checking the calibration of our depth sounder it is important to lower the lead
line in the same fore/aft position as the sounder itself and to check on both sides of the boat,
We must always remember that this Pool
in case we are over a sloping sea bed. (We have arranged for a calibrated lead line to be
available from www.jimmygreen.co.uk.)
of Errors is not static, but moves with both
the boat through the water and with the
tidal stream over the ground and continually
changes as the passage of time adds
new sources of error and makes previous
estimates outdated.
Getting into shallow water (2m) is
useful, but it will be handy if in the
process we can have established more
accurately where we are.
CROSSING CONTOURS
During blind nav exercises it is very easy
for the navigator to become fixated on
the depth sounder and to lose all track of
Fig 1 Solent area around Newtown Creek, Lymington, Yarmouth (derived from Imray 2200.8).
time. If we concentrate on depth alone
we will be throwing away
half the information that
think again. Substantial vessels of 150m our instruments are telling
in length and many thousands of tons will us. If we can maintain
typically draw 3m or less. a simple log at regular
intervals as our journey
AT SAFE SPEED
progresses, then with
If we assume a visibility of 100m, then luck we should be able to
a boat travelling at 5kn will cover the narrow down our location.
distance in just under 40 seconds, so two Fig 2 shows a simple
boats travelling in opposite directions may
Fig 2 The special form, marked up with chart behind (chart
form that we have
collide in less than 20 seconds. We take
extract from Imray 2200.8).
developed for this purpose,
this to imply that you should travel more but any record of time/
slowly, but there is a strong argument that If we were to head for
Speed Time to go mins/
distance against depth will
speed can get us out of trouble as well as the Isle of Wight on a
(kn) 1M (mins) cable
give us the depth track we
into it. As always, the skipper needs to heading of 140°, the worst
1 60 6
are looking for (Top Tip 1).
assess the situation and make a balanced case shelving is under the 2 30 3 The first thing we need
judgement. Bouldnor Cliff and goes
3 20 2
to do when the fog comes
from 5m charted depth to
4 15 1.5
down is hold the form
WHICH WAY TO GO
dry land in about 50m. If we against the latitude scale
For our first example we are somewhere were travelling at 5kn we
5 12 1.2
and mark lines every
between Newtown Creek and Yarmouth would cover this distance in
6 10 1
nautical mile (which is
when the fog comes down (Fig 1). We less than 20 seconds and so
Table A Speed/Distance
the same as one minute
Calculation
don’t know where exactly, but we can a considerably more cautious of latitude). Then, after
be reasonably confident that we’re speed would be advised. deciding the speed at
somewhere within a 2M diameter circle. On the other hand, if instead we which we will travel, and using the 0.1M
This defines our initial ‘Pool of Errors’ headed for the mainland on 320°, then marks on the latitude scale as a reference,
(Box B). the depth shelves much less steeply draw in the minute markers on the time
The depth sounder shows 14m, so and we would have over a mile (12 line (Table A). These simple, but essential
we can immediately reduce our pool of minutes at 5kn) to slow down from steps will instantly calibrate our piece of
errors to somewhere in the middle of the when the charted depth fell below 10m, paper to the scale of the chart and to the
channel, but which way should we go to before we were in any danger of touching speed of the boat.
reach safety? the bottom. As the journey progresses we monitor >>
July 09 Sailing Today 99
ST147 Seamanship FOG_RTG.indd 79 13/5/09 13:17:19
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148