ARPA and Radar Plots

NB: All ARPA plots in Yacht Master and OOW Navigation and Radar courses follow the Criteria of MGN379. That is, it assumes the conditions of IMO minimum specifications….

TRUE MOTION = TRUE TRAIL

RELATIVE MOTION = RELATIVE TRAILS

In this example targets have trails. We have a trail, therefore the trails are true trails. In these examples a true TRAIL indicates true motion display [Relative motion trails indicate RM display] In terms of W.O.A. triangle other ships trails are W.A. and own ship trail is W.O.
In this example targets have trails. We have a trail, therefore the trails are true trails. In these examples a true TRAIL indicates true motion display [Relative motion trails indicate RM display] In terms of W.O.A. triangle other ships trails are W.A. and own ship trail is W.O.
Target A is a fixed object. It displays a trail whose A.W. indicates the direction of the tide going Away. Target B has a true trail which is slightly shorter than own vessel W.O. and so the O.A. shows that the target is dropping back. Target C shows that it has a large O.A. having cleared own ship very quickly. Target D is shown with an aspect of 90 degrees to own ship course. This vessel is shown to have a CPA well clear of own ship. Therefore under the Regulations for the Prevention of Collisions at sea no risk of collision exists. No action is required in either restricted or clear visibility.
Target A is a fixed object. It displays a trail whose A.W. indicates the direction of the tide going Away. Target B has a true trail which is slightly shorter than own vessel W.O. and so the O.A. shows that the target is dropping back. Target C shows that it has a large O.A. having cleared own ship very quickly. Target D is shown with an aspect of 90 degrees to own ship course. This vessel is shown to have a CPA well clear of own ship. Therefore under the Regulations for the Prevention of Collisions at sea no risk of collision exists. No action is required in either restricted or clear visibility.
In this radar picture Vectors are selected. Vectors can not indicate the motion of the radar. Own ship does not have a vector and so the vectors are relative. In terms of O.A.W. they are a forward projection of the O.A. line. We must run them behind 'A' in order to complete an O.A.W. triangle. Own ship course and speed may be taken as the reversed vector of the Racon on a fixed navigational mark.
In this radar picture Vectors are selected. Vectors can not indicate the motion of the radar. Own ship does not have a vector and so the vectors are relative. In terms of O.A.W. they are a forward projection of the O.A. line. We must run them behind ‘A’ in order to complete an O.A.W. triangle. Own ship course and speed may be taken as the reversed vector of the Racon on a fixed navigational mark.
In this diagram the plots for all targets are competed. RoC exists with Target C, whilst there is risk of close quarters with Target F. In restricted visibility Rule 19d(ii) applies to Target C, I would therefore avoid altering course towards a vessel abeam (on my port side). In case of Target F, rule 19d(i) applies, I would avoid altering course to port for a vessel forward of the beam where RoC or a close quarter situation is developing other than for a vessel being overtaken. As Target F is being overtaken I am not constrained and may taken any action for that target. DECISION We may therefore choose between altering course to starboard, reducing speed or stopping. MGN369 Restricted Visibility, tells us that we must apply Rule 8 to our decision to ensure that any action chosen will not create another close quarter situation. If this vessel was in clear visibility I would apply Rule 15 Crossing vessel to Target C (assuming PDV) and maintain my course and speed. In the case of Target F, I am the overtaking vessel under Rule 13 and would keep clear. TCPA will determine which vessel I take action for first.
In this diagram the plots for all targets are competed. RoC exists with Target C, whilst there is risk of close quarters with Target F.
In restricted visibility Rule 19d(ii) applies to Target C, I would therefore avoid altering course towards a vessel abeam (on my port side). In case of Target F, rule 19d(i) applies, I would avoid altering course to port for a vessel forward of the beam where RoC or a close quarter situation is developing other than for a vessel being overtaken. As Target F is being overtaken I am not constrained and may taken any action for that target.
DECISION
We may therefore choose between altering course to starboard, reducing speed or stopping. MGN369 Restricted Visibility, tells us that we must apply Rule 8 to our decision to ensure that any action chosen will not create another close quarter situation.
If this vessel was in clear visibility I would apply Rule 15 Crossing vessel to Target C (assuming PDV) and maintain my course and speed.
In the case of Target F, I am the overtaking vessel under Rule 13 and would keep clear.
TCPA will determine which vessel I take action for first.

Remember that the only time that own ship vector can indicate the mode of set up is if it is not in line with your heading marker. In this case you are in True Motion, Ground stabilised.