Skysail Training



Radar Plotting Instructions        


Find a plotting sheet here:  radar_plotting_sheet_board.pdf

Use a marked ruler to estimate distance on the plot sheet and a chinagraph pencil to write on it. 

Keep the range the same (6 miles) while you are plotting, and do not change your course or speed.

When you first see a target, put an "X" on the plot, and write the time beside it Call this ‘O’ (Original).

After 6 minutes, put an X on the plot where the target now is, with the time beside it.  Call this A (Actual).  If the target has moved nearer, it is getting closer. 

If it proceeds along the bearing directly towards you, you are on a collision course. 

Draw a line from O to A and extend it in your direction to well past you. This is the DRM (direction of relative motion). 
Use the 6-minute rule. Measure the distance between O and A.  As the time between O and A is 6 minutes, distance = Speed/10 or Speed = 10 x Distance. This is the SRM (Speed of Relative Motion.) This can be difficult to estimate in a seaway.

Find the CPA (closest point of approach). This is where the perpendicular from you at the centre of the plot meets the DRM line 

Find the time to the CPA (TCPA) by measuring how far it is from the target to the CPA, and dividing by its SRM. 

If the target is moving vertically down the screen at an SRM that is greater than ours, it is on a collision course towards us at a speed equal to the SRM minus our speed. Do not change course until you can tell which side to go on, or you may bring yourself inadvertently closer. To gain time, slow down or stop. 

If it is moving down screen at an SRM less than ours, then we are on a collision course overtaking it, and its speed is our speed minus SRM. 

If the target is not coming straight down screen at you (either approaching along its bearing on a collision course, or with a DRM that doesn't bring it directly to you), find it's true speed and true relative course by drawing a vector triangle. Draw a line from O vertically down the plot, with a length equal to how far you travelled in the 6 minutes (your speed/10). Call this point W (Way). Then draw a line from W to A. Its length x 10 gives you the true speed of the target, and its direction is the target's true relative course. Find its True course by adding your course to its true relative course. 

Record in the log the target's speed and true course. Determine who is the stand-on vessel and who is the give-way vessel. 

If a target is moving down screen at your speed, then it is not moving. It is stopped in the water, anchored, a buoy, or an offshore installation


This sheet is for reference only. You are strongly advised to attend at least a 1 day RYA radar course.
You are responsible for all decisions made.

Posted 22nd  January 2012