Minimum CPA

[Steve]

Inspired by Size4s recent posts, what is the minimum CPA set in your ship's (past and/or present) Master's standing orders?

[Clanky]

Not sure what it is on here, but I was on the bridge of a cargo ship once when the second mate called up by the Oriana (we were the give way vessel), can't remember the axact distances, but the convesation went something like...

ORIANA - You're crossing 11 cables in front of me, please alter course

US - Negative 11 cables is sufficient

ORIANA - My standing orders say that I must maintain a 2 mile CPA

US - Mine don't

Silence

[alistairuk]

For prudence sake we all maintain 1nm, if possible - but have to notify master if not possible to maintain at least 5 cables...

As Clanky has expressed so well, its common... "BLAH: Your passing 1 mile from me, what are your intentions" .... "My intentions are to proceed to my destination... what are yours?"

[HolyNougat]

Haha - yes, CPA does rather depend on the size of the ship you are sailing on.. When I worked on very large Pax ships one Captain insisted on three miles! - these days on a much smaller ship I am happy with around a mile, but of course, in the middle of the atlantic 3 miles seems ok, where as in the Singapore Staits 3 cables seems like a luxury regardless of ship size.

[size4riggerboots]

I get nervous about a 1 mile CPA, mainly because we had it hammered into us that there could be up to a 7 cable error in the ARPA calcs, and when you're in a bouncy sea (like we are today) CPA's tend to bounce about quite a lot. Apparently I've been let off the leash regarding CPA's (the main rule is "Don't hit anything"!!) but the Captain hasn't directly said as much to me, and as long as I have enough sea room, I see no reason to get closer than 1.5.

[size4riggerboots]

I'm a little amused by the fact that I have non-plussed all of the bridge team now by setting my radars up in various ways: N up, long relative trails (that I reset when things have cleared) on one for long range tracking, and if I have several bits of traffic, or am heading south, I like to go course up as it's easier to look out of the window and confirm what I'm looking at.

[Steve]

ARPA must always be treated with appropriate caution, but I must admit that I haven't heard of "up to 7 cable" errors. Presumably this was taught as part of OOW/NRAS? Were specific circumstances referred to?

It is worth bearing in mind that most of the OOWs on the ships around you will be working on a 1' CPA (or less, if they wear spurs and stetsons), so it'll usually be up to you to make up the difference when you are the stand-on vessel. If you gave me an extra 5 cables well in advance I might just turn towards you to use it up!

Radar setup: north up, relative vectors, true trails. Tells me everything at a glance. Possibly set to different ranges depending on traffic density.

[size4riggerboots]

Yeah, that's what we were told in NARAS O. No specific circumstances refered to though.

We have a new Captain on for two weeks, and my CPA limits are now if they're giving me less than 1', I'm to call them up. Our standing orders don't actually specify a CPA at all. When I called the captain this evening (see the what I did today thread for why) I had one radar on C up and one on N up, he didn't like that as he found it confusing (I had just dragged him out of bed after a long Panama Transit day). So I'll remember to change it to N up before I call him next time!

[GuinnessMan]

7 cables error? Thats a bit big isn't it?

[gadget123]

7 cables is the maximum it can be its almost always a lot less.

The 7 cables is based on the IMO performance standards and most radars should exceed the standards in most conditions.

[Steve]

The 7 cables is based on the IMO performance standards and most radars should exceed the standards in most conditions.

Does one of the lecturers at Fleetwood have a particular bee in his bonnet about this? The circumstances in which ARPA is unreliable should be apparent from trail/vector correlation and steadiness or otherwise of calculated outputs.

[gadget123]

Not sure but its not just fleetwood.

[size4riggerboots]

I think where it comes from is the minimum standards that are set out for the manufacturers, obviously if they were actually that dodgy then no-one would buy them so they do tend to be more accurate. Saying that, I've been in lumpy seas and watched the ARPA vectors jump around like fleas, so as Steve very rightly says, correlation with trails is a good way to verify what your ARPA is telling you. I'm a big fan of permanent relative trails, which I reset manually when the screen becomes cluttered.

[Clanky]

Sorry to butt in on what is obviously a fairly navigational thread, but I would suggest that if you are relying on ARPA accuracy to determine minimum safe CPA's then you might be doing it wrong?

Surely there is a difference between the CPA reading on your radar at 10 miles range and what you can observe as the actual CPA in a close quarters situation? So to say that you would always aim for a CPA showing on the radar at a some distance away of 1nm may be perfectly correct, but to say that you would only pass 1 nm away from other traffic because that is the limit of the capabilities of ARPA surely isn't?

[alistairuk]

I think what Size4 is referring to is the old IMO minimum standards for ARPA systems to be approved - last I heard the accuracy requirement (as of a few years ago) had changed to 0.3 cables within either 3 or 6 minutes (can't remember what - but I might have this confused with something else). If in doubt, RTFM it tells you at the back what the accuracy of your system is certified to be.

As for accuracy - the CPA and other information provided by ARPA will be reliable within a few minutes and pretty much accurate within 6 minutes - providing no one alters course or speed. The longer the speed and course remains constant the more accurate the information - range makes no difference (other than the target not returning a particularly good signal). Obviously if you slow down / alter or the other ship slows down or alters then you need to wait for the data to recalculate from 0 again.

As you should be aware, there is normally a delay of around a minute between acquiring and ARPA providing initial information and vectors - and over the remaining few minutes the information will continue to change as it gets more accurate.

In fact the closer a smaller target is, the more chance of its data being wrong, since it starts to be obliterated by your sea clutter settings / otherwise return weird signals.

Really, in open sea there is no need to pass any closer than 1nm (I tend to keep at least 2 miles if open sea / schedule allows due to the number of them that do stupid things) from another ship - doing so is just plain inconsiderate.

As for confined waters - then obviously you have a minimum CPA specified in the standing orders - if unobtainable, master should / must be notified - but if your in a TSS thats only 6 cables wide, you have no chance of maintaining a CPA over 3 cables - hell I've had to pass through narrow sections only 1 cable from an on coming ship because rather than reduce and let me clear the narrows first (I had already entered and was committed before they approached it - unless I wanted to run aground) - they insisted in coming through in opposite direction - certainly wakes you up at 5am and always a good wake up call for captain!

Finally, as Clanky says - minimum acceptable CPA is specified by captain (or company policy via. Captain), and Captains word is law!