Why do we need to do radar plotting? The International Regulations for the Prevention of Collisions at Sea (1972) as amended (Colregs) requires the systematic observations of targets. Extracts from Rule 7 b. and c. state:

• Rule 7b. “Proper use shall be made of radar equipment if fitted and operational, including long range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected targets”.
• Rule 7c. “Assumptions shall not be made on the basis of scanty information especially scanty radar information”.

The relative motion (relative velocity) plot (relvel) gives the deck officer an understanding of the combined movement of vessels when under way and making way. The relative motion plot takes the following factors into account – when given any four factors, the remaining two can be calculated:

1. The true course of the observing (own) vessel.
2. The actual speed of the observing (own) vessel.
3. The true course of another ship (the target observed).
4. The actual speed of another vessel (the target observed).
5. The combined movement of both vessels.
6. The combined speed of both vessels.

Plotting procedure. The relative velocity problem is solved by the use of vectors on a plotting diagram as follows:

1. Lay off the observing (own) ship’s true course.
2. Plot the first bearing and distance of the target observed and next to it insert the time
   and the letter “O”.
3. Plot the second bearing and distance and insert the time next to it (six minutes later).
4. Plot the third bearing and distance and next to it insert the time and the letter “A” (a
   further six minutes later).
5. Join “O” and “A” and extend this line past the centre of the plot. This is the projected
   movement of the target and this extension will indicate whether a risk of collision will result. Insert an arrow in the direction of movement.
6. Insert the observing (own) ship’s course in reverse from point “O”. The length of this vector is the distance travelled by own ship at it’s present speed for 12 minutes. Mark the end of this vector with the letter “W”. Insert an arrow in the direction of own ship’s course.
7. Complete the triangle by joining “W” to “A”. Insert an arrow in the direction of “A”.
8. Determine the direction of “W-A” which is the other ship’s true course.
9. Measure the length of “W-A” which is the distance travelled by the other ship in 12 minutes. From this calculate the other ship’s true speed (distance of “W-A” multiplied by five).

A graphical representation of the procedure described above.

Summary of Vector components.

• OA Relative movement of vessels which includes relative direction and the relative movement over the period between the first and last plots.
• WO Own vessel’s true course and the distance covered over the same time period as OA.
• WA Other vessel’s true course and distance covered over the same time period as OA.

Target report. The results of the plotting procedure are used to compile a Target Report which has the following elements:

1. Target identity/name.
2. Time of first plot.
3. Bearing and range of first plot.
4. Closest Point of Approach (range and bearing).
5. Time to Closest Point of Approach.
6. Target’s true course.
7. Target’s true speed.