Concept of TCPA in collision avoidance

#Officers, #Colregs, #radar, #Arpa, #CPA, #TCPA #NavigationTricks


This post is specifically targeted for young Navigating officers but even senior officers and Masters may also find it usefull if not for themselves but to atleast train their juniors.


I will share one very interesting line in Night orders of one of my very senior Master.

"Remember you are travelling 1 Nm in every 3 minutes "


Around 2005, it was quite common to do speeds of 20 to 26 knots on container vessels. My container vessel used to do sea speed of 20 knots.


When I read that statement, it really rang a bell in my head and it stuck with me forever whenever I was navigating for purpose of collision avoidance of , Navigation in congested waters, TSS etc to plan action not only in terms of distance and CPA , but also keep a check on how much time you have to take action.


First of all let say you are sailing on vessel A, doing speed 20 knots.

You are travelling 1 Nautical mile in every 3 minutes i.e. 620 mtr per minute.


Now if you spot a vessel Head On 8 at a range 8 miles, doing speed of 16 knots.


relative speed: 16 + 20 = 36 Knots

i.e. 1.6 min per Nautical Mile

and only 13 minutes TCPA.


Now imagine if you are in congested traffic area, attending various other activities like VTS reporting, monitoring other traffic, answering a phone call on bridge. And in mean time somewhere a vessel echo pops on your radar almost right ahead about 5 mile range. You just acquire the target and till the time ARPA processes the target, you get involved with other activities on bridge, and after about 3 minutes you check the target.


With relative speed calculation as above, your TCPA is only 7 minutes now and range is 4.2 miles.


You take action immediately by altering course to stbd by 60 Deg (Wide alteration to be readily apparent both visually and by radar, Provided you have sufficient space on your stbd.)


But if you get on with other activities and not monitor the target closely and return to target after 5 minutes, your range is already abt 3 NM, Dangerously close and require a bold action. If you waste any further time like trying to contact vessel on VHF, its going to be impossible to avoid collision.


So I want to emphasize here that taking your attention away from watchkeeping and navigation even for five minutes may put you in very difficult situation.


With Present days ECDIS, ARPA, I always suggest that you should keep your vector length sufficient as per your speed.


This post is specifically targeted for young Navigating officers but even senior officers and Masters may also find it useful if not for themselves but to atleast train their juniors. sense of importance timing in your actions.


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Mumbai, India