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The first navigation aid to
trawlermen was the Decca land-based radio beacons which were
civil installations developed for the military by Decca. To
use this form of navigation the fishermen had to rent the
equipment from Decca and also buy special Decca charts.
The basic Decca chart was an
ordinary Admiralty chart with lines in various colours criss-crossing
the chart. ie: Red lines ran north east to south west, purple
lines ran south east to north west with green lines being
used outside the area. The first Decca navigator to be used
was a Mark 5 which consumed vast amounts of space on the boat
and used an enormous amount of electrical power to warm up
it’s valves. In the late 70s this was replaced by the
Decca Mark 12 which was more widely available and more accurate.
It used half the power and half the space of the old Mk 5.
Using said machines, if a trawl came fast or, to use layman’s
language, became stuck in a ledge, rock or wreck, the skipper
was able to read the numbers on the red and purple dials and
mark the fastening or obstruction on his Decca chart. This
was a vast improvement on the previous system although, depending
on atmospherics and weather conditions, accuracy was somewhere
in the region of 100 yards. With the recent development of
GPS these machines have now become obsolete and satellite
navigation has enable the charting of the seabed to become
much more reliable, thus saving a lot of expensive damage
to trawls and gear.
This form of navigation is now
used in all types of fishing and commercial vessels. Due to
improved technology trawlers now carry computerised charts
which are capable of showing the track a trawler has taken
while it is towing its net ,down to an accuracy of 18 inches.
This enables the trawler skipper to miss all undersea obstructions
and also to re-trace his path if he has found good fishing
in a particular area.
Other forms of electronic aids
include the echo-sounder which has evolved from submarine
warfare, it then revolutionised the fisherman’s understanding
of the sea bed and, more importantly its topography.
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