AIS: sensible values for CPA, TCPA

Pirx

Pirx

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We have just purchased an AIS radio that allows one to set values for CPA ( closest point of approach ) and TCPA ( time to CPA ) and emits a piercing shriek when a transmitting vessel is closer than CPA/TCPA.

I am wondering what the optimum values are? I feel I need lots of warning if an over-ethusiastic ferry is bearing down on me at 20 knots, but I'm not ( usually!) too bothered about an Anderson 22 attempting to overtake. Initial thought is to set CPA quite close and TCPA to, say, 15 minutes ?

Any observations for someone who has used one of these things would be very welcome.
 
Perhaps not the most sensible/helpful answer but I’ve disabled the audible alarm on my AIS since sailing in the Solent again - it’s not the moving boats that bother me, it’s ALL the moored up boats with their AIS left on.

The icon still flashes red when it’s a potential danger and we still keep a proper lookout so that works for me…
 
Can you set one value or both? If you can set both you will get two alarms (save where the values coincide).

15 minutes sounds like a sensible warning; allowing plenty of time to form and execute a plan for avoidance, without pinging madly at the proximity of vessels over the horizon.

Might want to disable the alarm in waterways such as the Solent where there’s bound to be an average of 16 AIS equipped vessels heading your way within 15 minutes range. :)
 
I'm not a fan of audible alarms unless I need to be woken up. I have a visible warning on the screen at the chart table that lights up any vessel which will be within 2 miles in the next 40 minutes. This helps plan crossings of shipping lanes but I switch it off in busy areas. Normally I only take occasional looks at the screen in good visibility but watch it much more frequently when it's foggy. I hope you can switch off your piercing shriek.
 
Even outside the Solent, we leave the AIS alarm switched off unless on a longer passage in more open waters - when there are so few boats that want warning for the exceptions. In this case typically set for 0.5 or 1 mile and I think something bizarre options wise like 12 minutes.

Our Raymarine one lacks the options to make really useful such as
- being able to flag a target as noted and silence to prevent repeat warnings for that target;
- being able to differentiate between big scary things and small boats - which merit differnt distances
Hence rarely used
 
I turn it off. Vessels more than 15/20 minutes away are no big threat & a CPA of 1/4 mile is plenty, so long as one is looking.
There are so many items of electric on my boat that I never know what one is bleeping, from charging alarm to just someone on the VHF on channel 16 pressing the transmit button for a split second, to the autopilot cutting out. All the same b..y beep, so I cannot tell which is which, until I run aground, or the nav lights go out.
I have an echomax radar unit & that bleeps for just every shipwithin 50 miles, if I am scanned. Drives me crazy, so the alarm is permanently turned off
 
I set it mostly for offshore, as in confined waters it is pretty useless. Typically set for 1.0 miles and 15 minutes, both criteria need to be met, to alarm. Properly offshore it is coupled to the general alarm which is enough to wake the whole boat (includes bilge warnings, engine exhaust water, and inverter failure.)
 
I’ll suggest that if you’re not single handed and catching the occasional few zzzs the audible alarm is pointlessly distracting. It’s pointless in crowded waters like the Solent. Offshore AIS is marvellous for giving you detailed info on a potential collision target but eyes (/regular checking of AIS and/or radar) should alert you to those if you’re keeping an appropriate look out.
 
Daydream believer said:
Vessels more than 15/20 minutes away are no big threat & a CPA of 1/4 mile is plenty, so long as one is looking.
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Agreed.
I set my CPA alarm to 1/4 mile then turn that alarm off and leave it off.
I set the TCPA alarm from 15 to 30 mins or off depending in the nature of the passage I'm making. In the Solent it's definitely off! X Channel 15 mins, longer trips where I might be snoozing, 30 mins.
 
The safe CPA bit is an "it depends" - 1/4 mile might be fine for another small boat, but you would need to be extremely brave about that for a converging Panamax tanker or box-boat - as well as very confident about where the AIS position is relative to the bow, as this could be 250metres ahead of the AIS position !! (or 350m with new-Panamax)
Hence, as alarm off anyway in busy / inshore waters, I use a bigger warning CPA if alarm on in open waters in case of big ships
 
Depends where you are sailing.
In busy inshore waters CPAs are close and you will be a lot safer with a competent visual watch rather than relying on electronic toys. Offshore I would want advanced warning of anything approaching within 1 mile as this was regarded as the minimum safe open water CPA on Merchant Navy bridge officer training courses.
 
dunedin said:
The safe CPA bit is an "it depends" - 1/4 mile might be fine for another small boat, but you would need to be extremely brave about that for a converging Panamax tanker or box-boat - as well as very confident about where the AIS position is relative to the bow, as this could be 250metres ahead of the AIS position !! (or 350m with new-Panamax)
Hence, as alarm off anyway in busy / inshore waters, I use a bigger warning CPA if alarm on in open waters in case of big ships
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+1
100 yards is more than enough for another yacht, also plenty once I know I'm going behind that Panamax, but I think I want a good mile ahead of it. I seem to remember someone getting that calculation wrong of Cowes a few years ago...

It's a bit sobering to think that if you happen to meet Ever Ace, a mile is only a bit more than four boat lengths (LOA 1309 ft). That probably isn't enough for the rudder to start to bite if she has to dodge you :eek:

I'm going through the same calculations for my new AIS, and reckon that, in the Solent, leaving the alarm off and setting detection distance to 2 miles and CPA 100m is somewhere near right, but crossing the Channel, I think alarm on, detection at 10 miles and CPA of 1 mile is probably about right. I can then decide if I need to do anything about whatever's set it off in good time, and what to do which is probably to chase it's stern until it's clear past, as I'm likely to be motorsailing.

I very much agree that the Mk 1 eyeball is far more important than any electronics, especially as there are likely to be targets out there that don't transmit, but a bit of warning can only be a good thing, especially in dubious visibility.
 
Stemar said:
It's a bit sobering to think that if you happen to meet Ever Ace, a mile is only a bit more than four boat lengths (LOA 1309 ft). That probably isn't enough for the rudder to start to bite if she has to dodge you :eek:
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On the STCW radar simulator training courses I ran up to ten years ago for MN deck officers candidates were expected to have recognised potential close quarters situations at five miles and to make course corrections no later than three miles from the target. Obviously the very large vessels could need even more lead time if a significant course change is required. Collision avoidance is normally taken by change of course as speed changes require large fractions of an hour to have any effect, assuming that the engine room is on maneuvering standby.

The message for us is that big ships need to reliably detect us (by visual, AIS, RADAR) at ranges in excess of five miles. Once they are inside four to five miles range, with the best will in the world, they are unlikely to be able to avoid us by their actions alone.
 
Which is why, in open water, I prefer to ensure that a close quarters situation doesn't arise. I can dodge them easily enough, and my AIS receiver will allow me to detect them far enough away and take suitable steps to ensure the big boys can go on their merry way without worrying about that bloody WAFI
 
The CPA can be a pain with vessels having a course with small angle with ours, the small variations in speed/course of a sailing vessel can cause the alarm to be ON/OFF every few seconds, the time to acknowledge the alarm it starts again, one eventually turns it off and waits for the ship to move away before setting the alarm on again (or forgetting to do it). There may be system allowing to acknowledge one single MMSI alarm but alas not in my case.
 
Roberto said:
The CPA can be a pain with vessels having a course with small angle with ours, the small variations in speed/course of a sailing vessel can cause the alarm to be ON/OFF every few seconds, the time to acknowledge the alarm it starts again, one eventually turns it off and waits for the ship to move away before setting the alarm on again (or forgetting to do it). There may be system allowing to acknowledge one single MMSI alarm but alas not in my case.
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Hmmm experienced a problem. With AIS and alarms last week. cancelled alarm, sat down and alarm. Restarted, turned off!
Politically incorrect but think it was a nagging female AIS.
 
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