RADAR PLOTTING 2

CANADIAN COAST GUARD AUXILIARY - PACIFIC

RADAR PLOTTING2


2008


Overtaking Situation


Own vessel 270° at 20 knots

6 minute plot

(1) Find the course you are going, and draw a line for your course

In the case of heads up , mark 000°



6 minute plot

(2) Mark the first radar bearing and range of the target.

323º R x 1.8 nautical miles.

All bearings are R as they are relative to your vessel’s head.

Mark as "O“

Mark time of 1200

12:00 O



6 minute plot

12:00 O(3) After 6 minutes mark the second radar bearing and range of the target.

309ºR x 0.65 nautical miles

Mark as “A”

Mark time of 1206

12:06 A



6 minute plot

12:00 O

12:06 A

(4) From "O" draw a line through "A" and carry the line well past the centre line on the screen



6 minute plot

12:00 O

12:06 A

(5) Transfer your course line to "O" and pull the line down the sheet



6 minute plot

12:00 O

12:06 A

(6a) Calculate own vessel distance in plot interval.

To use calculator scale, place right point of divider on 60 (right end of scale) and left point on own vessel speed (20 knots).



6 minute plot

12:00 O

12:06 A

(6b) Move the dividers at this setting, and place right point on the plot interval (6 minutes)

The number under the left point of the dividers will indicate the distance gone in the plot interval. (2 nautical miles).

6 min @ 20 knots = 2.0nm.

W



6 minute plot

12:00 O

12:06 A

(6c) Lay down 2 nautical miles on the plot,on own course vector from “O”.

Mark bottom end “W”W



6 minute plot

12:00 O

12:06 A

W

(7a) Connect “W” with “A”. “W” to “A” represents target course and distance travelled in the plot interval.

030°R @1.18 nautical miles

To convert this into speed, place right point of dividers on plot interval (6 minutes) and the left point on distance travelled in that time (1.18 nm)



6 minute plot

12:00 O

12:06 A

W

(7b) Move dividers so that right point is on 60 (right hand end of scale).

The reading under the left point of the dividers will be the vessel’s speed in knots.

11.8 knots



6 minute plot

12:00 O

12:06 A

W

(8) Where the line through “O” and “A” passes closest to the centre of the plot is the Closest Point of Approach (CPA)

In this case 0.25 nautical miles to port



6 minute plot

12:00 O

12:06 A

W

(9) The Time to Closest point of Approach (TCPA) is at the tangent point. OA is 6 minutes.

A to TCPA works out at 2.5 minutes = 12:08:30

TCPA


Results

In this case, providing neither you nor the target vessel alter course or speed:

• the target vessel is proceeding at 270° + 030° = 300°C at 11.8 knots

• CPA is at 12:08:30 at a distance of 0.25nm to port


Results – Clear Visibility

• In clear visibility, you are the overtaking vessel and therefore the give way vessel.

• You will pass clear, but not exhibiting the best seamanship.

• If a risk of collision was existing, you should have altered course to port early, and passed under her stern


Results – Restricted Visibility

• In restricted visibility, you are still the overtaking vessel and therefore the give way vessel.

• You will pass clear, but not exhibiting the best seamanship. At this stage, a big round turn to starboard will resolve the situation.

• If a risk of collision was existing, you should have altered course to port early, (under Rule 19 d.i) and passed under her stern.



• If however at 1206 you acknowledge the other vessel is on nearly a steady bearing, you can stop and assess the situation.

• The plot will then change to:



6 minute plot

12:00 O

12:06 A

The other vessel’s plot will then move across in front of you, resolving the situation, clarifying you are overtaking.

You have made a substantial alteration in speed which will be readily visible to him, whether he is observing you visually or by radar.

You can see immediately he is doing 300°C at 11.8 knots

RADAR PLOTTING 2

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