Radar Plotting

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          It's important to practice radar plotting so that you don't make mistakes when you're under pressure or stressed. It's not that hard to do, and the practice is fun and educational. We met two boats who suffered serious collisions (one with another sailboat and one with a freighter) and heard of many other such situations.

  1. Use a marked tongue depressor to estimate distance on the screen and a china marker to write on it.
  2. Keep the range the same (6 miles) while you are plotting.
  3. Do not change your course or speed while you are plotting.
  4. When you first see a target, put an "X" on the screen, and write the time beside it. If it is a big splodgy shape, it is a squall so alert the captain.
  5. After 6 minutes, put an "XX" on the screen where the target now is, again with the time beside it. 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. Alert the captain.
  6. Draw a line from X to XX and extend it in your direction to well past you. This is the DRM (direction of relative motion).
  7. Use the 6-minute rule. Measure the distance between X and XX with the tongue depressor. When the time between X and XX is exactly 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.
  8. Figure out the CPA (closest point of approach). This is the perpendicular distance from the DRM to us (center of the screen).
  9. Figure the time until the CPA by measuring how far it is from the target to the CPA, and dividing by its SRM.
  10. If the target is moving vertically down the screen at an SRM that is greater than ours, it is headed on a collision course towards us at a speed equal to the SRM minus our speed. Alert the captain. 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.
  11. If it is moving downscreen at an SRM less than ours, then we are on a collision course overtaking it, and its speed is our speed minus SRM. Alert the captain.
  12. If the target is not coming straight downscreen at us (either approaching along its bearing on a collision course, or with a DRM that doesn't bring it directly to us), figure out it's true speed and true relative course by making a vector triangle. Draw a line from X vertically down the screen, with a length equal to how far you traveled in the 6 minutes (your speed/10). Call this point Y. Then draw a line from Y to XX. It's length x 10 gives you the true speed of the target, and its direction is the target's true relative course. Figure it's true course by adding your course to it's true relative course.
  13. Record in the log the target's speed and true course. If we have to contact the vessel on the VHF, we need to know its approximate true course.
  14. Figure out who is the stand-on vessel and who is the give-way vessel.
  15. Note that if a target is moving downscreen towards you at your speed, then it is not moving. It is dead in the water, anchored, a buoy, or an oil drilling rig.
  16. If you slow down, the radar track for the target curves up the screen.


©2004 The Trouser Rollers. All rights reserved.
This page was last modified on:
August 9, 2009