Use in coastal waters

In coastal waters the PPI will be showing echoes of land or off-lying dangers, light-vessels, buoys and so on, in addition to those of ships. If, in these circumstances, time is available for plotting ship targets it may be useful to be able to forecast the arrival of other vessels into narrow channels or at turning points and thus minimize the risk of close-quarter situations. Other applications will no doubt suggest themselves.

At night in clear weather at congested points such as pilot sta­tions, radar can be of great use in helping to avoid other ships and to discover the whereabouts of the pilot vessel.

Use in pilotage waters

Finally, a word must be said about collision risks in pilotage waters, buoyed channels, etc. The use of radar for navigation in such circumstances is described in Chapter 8. It should be mentioned here that over half the "preventible" collision casualties have occurred in rivers and river entrances. In such waters the radar observer will seldom have enough time to make a comprehensive plot of ships. He will, however, with practice, be able to determine by very simple plotting which vessels are moving and which are not. When a vessel is ahead, or nearly so, and is anchored, her relative movement will be exactly the reverse of own ship's course and speed over the ground (i.e. including set and drift). If this not the case, the other vessel cannot be anchored. When other ships are close to and about to pass or be overhauled, the radar observer probably can do no more than give a rapid and highly important succession of bearings and ranges.

In other chapters reference has been made to echo "trails" which result from the afterglow or persistence of the cathode-ray tube. In close-range work, when plotting may be impracticable, these trails may be of some value in giving a rough indication of the relative move­ment or of a change in it. A stationary object will, of course, have relative movement and hence its echo may show a trail. Afterglow trails should not be confused with the other ship's wake which may sometimes be seen against a sea-clutter background. The direction of the wake will, of course, indicate the course being steered.

Reporting from the plot

No doubt every master will decide in what form he wishes in­formation from the plot to be given to him. Based on the six stages of reporting described earlier in the chapter and using the situation given in the examples, the reports might take the following form:

Stage (1) 'Echo A bearing 040° 10 miles at 1000; probably a ship target.'

Stage (2) 'At 1006 echo A bearing 038° drawing slowly forward, range 8,6 miles decreasing.'

Stage (3) 'Ship A, nearest approach will be 2 miles on bear­ing 321° at 1039/

Stage (4) 'Ship A is steering 276° at 12 knots, aspect 58° to the left (red). Bearing 036V₂^o, range 8 miles at 1009.'.

Stage (5) 'Course to steer from 1012 to give a nearest approach

of 4 miles is 037° (or 281).'

Stage (6) 'Ship A still steering 276°, bearing Огб¹^⁰ rapidly

drawing aft, range 5,8 miles at 1018.'