TPCA and CPA Settings

[SimonFa]

Done a search and its a while (2011 by my search) since this was discussed so maybe people have leaned a lot more.

So, what are good settings for CPA and TCPA, especially when crossing the channel? Setting the CPA too high gives lots of false alarms for bats that are going behind, but too near and it can be quite frightening. Similar, setting TCPA too high and you're forever cancelling alarms and in danger of missing something, too low and no to time avoid.

Obviously the Mk 1 eyeball and compass are essential as well.

 

[johnalison]

I'm afraid I'm not a good person to answer this because I am one of those who never uses alarms (neither AIS nor depth). In good visibility, I find it adequate to view the screen when a vessel comes into sight, and keep an eye on it if necessary. In poor visibility, either I or my wife will be viewing the screen regularly anyway. I set an alarm zone around my boat at 1 mile, which appears on my screen as a red ring, which makes quick assessment easier.

 

[prv]

I tend to treat the CPA beeper as a reminder that there's a ship I need to be aware of, rather than an "imminent crash" alarm. So maybe some of my alerts would count as false alarms to you but I don't see them that way.

Unfortunately I can't remember off the top of my head what my CPA threshold is set to for Channel crossings (my display has four profiles that can be set independently and switched between) but I would guess maybe two miles? I am fairly sure that the TCPA is set at 20 minutes in "cross Channel" mode.

When I cancel an alarm it's disabled permanently for that ship (I don't know if all displays work that way) - I see this as taking over that ship from the AIS and saying "ok, thanks, I'll keep an eye that one now". Of course the AIS is still giving me the CPA plot, but it's not alarming any more.

Pete

 

[RobbieW]

I use 30 mins and 1 mile on OCPN

 

[mainsail1]

CPA 1 mile.
I never use TCPA

 

[prv]

I never use TCPA

Doesn't that mean you get alarms for ships many many miles away, which will theoretically cross your path in a couple of hours, if you both hold the exact same course and speed, which you won't?

Pete

 

[mainsail1]

Doesn't that mean you get alarms for ships many many miles away, which will theoretically cross your path in a couple of hours, if you both hold the exact same course and speed, which you won't?

Pete

i am sure you are right if I used alarms all the time but I only use alarms in fog which I encounter maybe once a year. I have never found the quantity of alarms to be a problem as not that many ships are going to pass within a mile and my radar will probably not ID them until they get in to 10 miles anyway.

 

[[70521]]

So, what are good settings for CPA and TCPA, especially when crossing the channel? Setting the CPA too high gives lots of false alarms for bats that are going behind, but too near and it can be quite frightening. Similar, setting TCPA too high and you're forever cancelling alarms and in danger of missing something, too low and no to time avoid.

I am not sure you get bats that far offshore

Personally, I don't set alarms. if I am that close the the big stuff I will have spotted it a long time ago. AIS is brilliant for seeing what a vessel is called and where it is going. One day I might get round to adding the data feed to my chartplotter, but it is well down my list of priorities.

 

[westhinder]

Personally, I don't set alarms. if I am that close the the big stuff I will have spotted it a long time ago. AIS is brilliant for seeing what a vessel is called and where it is going. One day I might get round to adding the data feed to my chartplotter, but it is well down my list of priorities.

I do not set audio alarms either, but there is a visual alarm for CPA<0.5 mile, at which point the symbol on the plotter starts blinking. That is plenty of warning for me.
If you finally do add the data feed to your plotter, you will wonder why you have waited so long. That is, if you have your plotter in the cockpit. Having all the info available in the cockpit makes sense, as that is where you run the boat and make your decisions. I used to navigate my chart table around the seas, having the plotter in the cockpit has transformed my sailing. I would not want to go back.

 

[[70521]]

I do not set audio alarms either, but there is a visual alarm for CPA<0.5 mile, at which point the symbol on the plotter starts blinking. That is plenty of warning for me.
If you finally do add the data feed to your plotter, you will wonder why you have waited so long. That is, if you have your plotter in the cockpit. Having all the info available in the cockpit makes sense, as that is where you run the boat and make your decisions. I used to navigate my chart table around the seas, having the plotter in the cockpit has transformed my sailing. I would not want to go back.

To be honest there is so little shipping about, we are at the western end of la manche, I usually consider putting an entry in the log when a ship is spotted.

 

[horatio_nelson]

Only just got AIS on my boat and really appreciated it on recent channel crossings. My B&G Vulcan has it (from V50 VHF) which mirrors it on a Sony tablet in the cockpit with full functionality. It's fantastic

Haven't bothered setting alarms yet though.