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Day Skipper - Part 6
- Position fixing
In last month's Day Skipper 5, we discussed plotting a position fix
on the chart. The problem with a fix is that it's history by the time
you have it plotted on the chart. This problem can be overcome by working
up a dead reckoned (DR), or better still an estimated position (EP).
Dead reckoning
 A
DR is simply a case of taking a past known position and adding to it your
course and speed. Let's say that we took a visual fix at 1715 and plotted
it as above. We are on a course of 110° at 15 knots. At 15 knots
we know that we will travel 1.5M in six minutes. Therefore we could mark
our planned track every 1.5M and mark it at six minute intervals.
Having established a DR, it is possible to look at your watch and have
a reasonably accurate idea of you position now rather than at the time
of the last fix.
Estimated position
 The
DR is all very well, but it does not take into account tide or wind. Therefore,
if we can fine tune it by adding the anticipated impact of wind and tide,
we could be very sure indeed of our position. As in the last example,
let's say that we took a visual fix at 1715 and plotted it as above. LetŐs
say we are on a course of 110° at 15 knots. If we calculated that
there was a southerly tidal stream of two knots and a 20-knot beam wind
which our experience told us would blow us an additional 5° to starboard,
then we could plot it like this.
Having generated an EP we may discover that the continuation of our
ground track will pass over an area of danger, or worse still we may already
have passed over an area of danger.
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