The relative plot

The relative plot deals entirely in terms of relative movement, Ihe first principles of which have now been covered; it avoids having

to plot the actual move- 0⁹ ments of either ship. The

plot can, of course, be made, as was the true plot, on any suitable sheet of paper using a fixed point to repre­sent the position of own ship; it is most conveniently constructed, however, on a plotting diagram such as the one shown in Fig. 34.

On this diagram, which can be thought of as a PPI face, own ship is represent­ed at the centre, the radial lines representing bearings and the circles, ranges. The top of the diagram (0°) can be taken to represent either north or the ship's heading. When the heading-up­ward method is used, bearings are plotted relative to the ship's head. This has the disadvantage that when own ship alters course, all plots have to be transferred to suit the new relative bearings of the targets. For this reason in the examples which follow the north-upward method is illustrated. This method does not, of course, pre­vent the bearings being reported as relative to the ship's head,

A MECHANICAL AID TO PLOTTING

Devices are available for making the relative plot already de­scribed without using drawing instruments. A typical one-which is illustrated in Fig. 35 consists essentially of a disc of transparent material about 10 inches in diameter, which is engraved in degrees from 0° to 359° at its outer edge and on which lines may be drawn. The disc is free to rotate above a sheet of squared material, graduated in inches and tenths or eighths. Concentric circles at ^г/₂ inch intervals may be provided. The vertical centre-line of the paper is emphasized as a reference line and a reference arrow is marked at its upper end. A circular slide rule may be included for calculation of speeds, times, etc.

The distance between any two points is obtained by rotating the disc until they are on the same vertical or horizontal line, and reading off the distance between them on the scale in use. The direc­tion of a course or bearing line is measured by rotating the disc until the line is vertical with the direction arrow upwards and then read­ing off the number of degrees opposite the reference arrow. Similarly, to draw a line from a point in a particular direction, this direction is set at the reference arrow and the line drawn upwards from the point, and parallel to the vertical lines. Own ship is at centre of disc (C).

Example. Own ship steering 330° 10 knots; echo bearing 040° 8 miles at 1000, and 038 V2⁰ 7 miles at 1006. With this range it will be convenient to use a scale of 2 miles to an inch. Plot to be north-upwards.

Mark an arrow at the edge of the disc at 330° to indicate own course. Rotate the disc until 040° is at the reference arrow. Mark point О over the reference line at 8 miles from centre. Then place 038 ¹/₂° at the reference arrow and mark point A over the reference line at 7 miles from the centre.

Rotate the disc until О and A are on the same vertical line with A above 0. Join"(L4 and continue the dotted line upwards to the edge of the disc. Read off the relative course at the reference arrow (226V2⁰). Note the distance between О and A and calculate the rela­tive speed from the time interval (11 knots). Rotate the disc until 0° is at reference arrow. The plot is then correctly orientated, north-upwards.

The point of closest approach (T) is where the line OA extend­ed is closest to the centre C. Rotate the disc until OA is horizontal; point T will then be over the reference line and can be plotted and the distance CT (1 mile) noted.

To construct the speed-vector diagram WO A, rotate the disc until own ship's heading is at the reference arrow and draw own ship's speed vector (WO) downwards from 0; insert the arrow pointing upwards (away from W). The other ship's speed vector (WA) can then be drawn, and it will show that she is steering 278° at 12 knots.

Reference arrow

Fig. 35

To obtain a course steer from 1012 to increase the closest ap­proach to 4 miles, plot point А_г on the line OAT, to represent other ship's relative position at 1012. Rotate disc until A_x is on the hori­zontal graduated line which is 2 'inches (4 miles) above centre C. Draw a horizontal line through A₁ and mark its intersection with the reference line 7\. This line will be the new direction of relative movement. Construct the triangle W₁0₁A₁ as in the relative plot and the direction of W₁0₁ will be found (058°).

When own ship has altered course, the position of the ship's head arrow must be changed. Subsequent observations of other ship will plot on a line parallel and close to A_xT_l9 if she has not altered course or speed.

The whole of this may be done by the heading-upwards method, in which case the ship's heading arrow is marked at 0° and all courses and bearings plotted relative to own ship's head.

THE PRACTICAL USE OF RADAR AS A COLLISION WARNING

When it becomes an habitual practice for users of radar in low visibility to note the bearing of any vessel sighted, and to continue to observe it until no risk of collision remains, the contibution of radar to the safety of navigation will have been greatly increased. The greater its regularity, the more effective will the practice prove in the relatively few cases where avoiding action is called for. Unless the habit of plotting echoes and determining the aspects or the courses steered by other vessels, their passing distances and the times of meeting or crossing is formed, mistakes may be made when they can be least afforded. Needless to say, regular examination of blind sec­tors is highly important.