Radar Plotting

[smatson]

Hello

I have a little issue with a radar report that i am compiling:
it consists of, a multiple radar plot, one vessel is on starboard quarter has same course and speed, we alter to starboard 90degrees to avoid a vessel in a crossing situation ahead. I have calculated their new CPA, but struggle to calculate TCPA for vessel running at same course and speed.
Also prior to course alteration our vessel was being overtaken by another target on our starboard quarter. having altered own course by 90degrees, the overtaking vessel has a CPA of 0nm. Does the role of the overtaking vessel remain as give way, or have we entered into a new situation?

any help would be gratefully appreciated!
cheers
sam

[CharlieDelta]

Have a read of rule 13 re: overtaking. Overtaking vessel remains obliged to keep out of the way.

Are you struggling to find TCPA post-alteration? Pre-alteration your TCPA is immediate and will remain so as long as you're on the same course and speed.

[smatson]

Excellent, i did read that, so that's solved.
For TCPA I am struggling post alteration calculation, as O A line is the same point, therefore do not know where to get a time scale to measure off TCPA.

Cheers
sam

[CharlieDelta]

You should have a new WO line = WO1. Then join O1 to A and extend for new relative vector, which you should get your CPA from. TCPA is then measured along that line.

I think. My radar plotting's gone a bit rusty!!

[Rich]

I'm confused, for other plots I have always measured the TCPA from the original OA line then transferred the 6 minute intervals from OA along the new CPA line.

Is it just for this scenario that you would use that method Charlie?

The Lecturer must of enjoyed setting this one....

[CharlieDelta]

Let me go read up on it, I could be wrong.

TCPA should be measured initially along the line extended from OA. The new TCPA should be measured along the line extended from O1A. The time intervals, as far as I know, should correspond to the line being used, since O1A would represent your new relative vector. But I'll go check.

[CharlieDelta]

Yeah just done a little plot here with a similar situation. So to begin with you have no OA line as such, and WO = WA. When you alter, you get WO1 at 90 degrees to original, and then O1A. Measure O1A as (insert total plot interval here) and then run it along from A to CPA.