CLIVE LOUGHLIN
SEAMANSHIP
TOP TIP 1 measurement alone we are able to travel
along a contour line on the chart. This
If we make photocopies of the form (email
sounds easy, but it is not. We will start
Clive for a copy) or for that matter any forms with a perfectly straight 5m contour line
that we regularly use, such as tidal curves, going exactly east-west and with 10m
tidal streams and tide tables, then get these
and 2m contours to the north and south
laminated with a matt finish laminate at a
respectively (Fig 4).
stationer, they can be written on with pencil
If we just ask the helmsman to follow
(and rubbed out) and so preserve the pages
the 5m contour heading east at 4kn and to
in our hardworking and valuable almanac.
head due north if the charted depth goes
Fig 3 Chart showing detail near Lymington
below 5m or due south if it goes above
the depth and record the time when
entrance (from Imray 2200.8). 5m, then the track of the boat would likely
significant events occur, such as when we be something like (a). The helmsman
cross the 10m contour, and mark these on (see Top Tip 2) and then would have a really
the paper. What we are basically doing is get ourselves to the ‘YC hard time and the boat’s
gathering information and recording it in a Starting Platform’ (Fig 3). progress along the contour
form that matches the scale of the chart. From there we could would be desperately slow.
We know our course over the ground either follow the lateral An improvement would
(from our heading and an allowance for marks in or, more be to tell the helmsman to
tidal stream) and so we can align the form sensibly, decide to hang turn to 15° either side of
and try to match our records with the around for a few minutes the basic 90° if the depth
contours on the chart. until the next Yarmouth goes above or below the
From the spacing of the contours on to Lymington ferry target depth (b). A further
our form (Fig 2) we can be reasonably comes past and follow improvement might be to
confident that we are somewhere around that in. allow the helmsman a bit
the ‘Paul Jackson’ Special Mark. But a Although in the above of tolerance by saying to
reasonable allowance for errors still leaves example we decided it head on 90° and to turn
us with quite a big area to consider. would be safer to head to 75° if the charted depth
If we are heading for Lymington, then for a more gradually goes below 4m and 105° if
one possible strategy might be to follow shelving area, this is a it goes above 6m (c).
the 2m contour on a heading of about 240° double edged sword. The
Fig 5 Following curvy
contours calls for different
(Fig 3). The main reasons for choosing very shallow gradient
strategies (Imray 2200.8).
COMPLEX CONTOURS
this contour is that a number of racing protecting us from Sometimes complex
buoys are located along it. With luck, we sudden and unexpected contours (e.g. 5m line Fig 5)
will find one and therefore be able to fix groundings also means our position do not allow simple contour following
our position with reasonable confidence becomes much less precisely defined. or comfortable positional accuracy. Then
The main channel in the approach to we are better off abandoning contour
Lymington is between 2.6m and 3m in following, bypassing the difficult area by
TOP TIP 2
depth, but the 2m contours on either side heading out into safe water and returning
of the channel are only about 100m apart to the contour further along the coast.
We should never blindly assume that a
in places. This means that if our depth Although contour following can look
buoy is where it’s marked on the chart. sounder or tidal calculations are not spot appealing, other strategies are often faster
If the chart is up to date and if the buoy on we could very easily overshoot the and more practicable. Once we have
has not dragged then the chart should
entrance without noticing it was there. established our position with reasonable
be correct, but many a ship has gone
certainty we no longer need to follow
down because it assumed a mark was
CONTOUR AND DEPTH NAVIGATION
the contour. Instead we can plan our
somewhere it was not. Buoys can drag
their sinkers and some are repositioned to
Although contour or depth following course towards our destination and
suit changing requirements. In contrast,
is not always an essential part of blind keep monitoring the depth to confirm it
however, the depths on charts are unlikely
navigation it is a very useful skill and it is matches our expectations.
to change much except in estuaries and
best practised before we have need of it.
other areas of shifting sands.
The basic idea is that by using depth
THE ESSENCE OF BLIND NAV
As with any navigation, the aim should be
10m
to use all available aids and information
and a questioning commonsense approach,
38 secs 22 secs 113 secs
to come up with the simplest plan that will
5m
do the job safely.
2m
LONGER PASSAGES
In the next example, fog descends when
Fig 4 This shows a contour following tracks with different strategies (a) red, (b) orange, (c)
we are within a mile or so of ‘A’ in
green (times are expected times between turns)
Fig 6, heading for Newtown Creek about
100 Sailing Today July 09
ST147 Seamanship
FOG_RTG.indd 80 13/5/09 13:17:25
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