Radar plotting: How to do it and its Significance in Collision Avoidance

Course & speed of targets by radar plotting As you mentioned: "Let us say your course is 045 degrees and speed is 12 knots. In 12 minutes your ship with speed 12 knots will travel 2.4 NM. SO from point “O” draw a line in direction 225 degrees and measuring 2.4 NM." Please check how did you draw 225 degrees line. I am affraid it is wrong.

Explanation well cleared and understood, thank you very much.

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First of all I want to congratulate you for your efforts to share practical knowledge in such a detailed way. After 25 years of teaching Radar Navigation, Radar plotting & use of ARPA at Constanta Maritime University I still can say that learning the use of ARPA without previously learning the basics of manual radar plotting is as been promoted from deck-hand up to OOW based only on years of on board practice without graduating any maritime school. Regarding the content of your article I just want to point something related to the practical use of target ship Aspect when you evaluate a traffic situation: • The fastest way to obtain the value of the aspect is by computing it (Asp = oppTB – TS Course) not by measuring the Aspect angle with the protractor. • The value of the Aspect is not very important. Important is the sign of the Aspect (Red or Green). Why? Because the OOW is analyzing mainly the close quarter situations (target ship(s) CPA is less than the safety distance). For these potentially dangerous targets the sign of the Aspect will show if the target ship is a Stand-On vessel or a Give Way vessel , as follows: • If the Aspect to an approaching target with a CPA smaller than the safety distance (ordered by the Master for that navigation area) is RED, than the target ship is a STAND ON VESSEL and our ship should manoeuver to avoid the dangerous ship • If the Aspect to an approaching target with a CPA smaller than the safety is GREEN, than the target ship is a GIVE WAY VESSEL and our ship should maintain course and speed (at least in the first stage of the encounter) This is the real IMPORTANCE of the Aspect. Regarding the analysis of the encounter between our ship and a dangerous target ship important is own ship true course and target ship true course, not the value of the Aspect. The OOW has to asses which are the type of encounter with the dangerous ship (crossing, head-on or overtaking). The encounter type in relation with COLREG Rules of the Road provisions are telling what the OOW will have to do if own ship is a Stand-On or Give-way vessel. That’s why the 1st thing that you have to draw on radar plotting sheet is the Own Ship Course (true). After plotting the observed positions to the target ship and determine the target ship true course it is easy to visualize on the plotting sheet how the target ship course line is interacting with the own ship course line and to define if it will be a crossing, head-on or overtaking encounter. It is the same way as you should proceed when you look at the ARPA radar screen and see the vectors of the plotted targets. You switch on TRUE VECTORS (if the ARPA radar is not already in TRUE MOTION mode) and assess how the true vectors of the plotted target are interacting (intersecting) with own ship heading line (ARPA radar working of course NUP or CUP). Hopping that I was not to boring I will rest here and wish you to continue to Keep Up the Good Work.

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Thank you sir you cleared all my doubts about radar plotting...

Sir i always find your articles intersting. I want to correct you that here you denoted aspect 60 degree approx. but the correct aspect here is approx. 90 and exact value is 92.5 R. you have taken relative brng of target from our vessel but its relative brng of own vessel from target vessel. Thankyou sir for ur articles keep writing we find these very intersting.

Thank you very much for this. I lost my radar plotting notes from college and wanted to revise it. This was perfect.