AIS alarma

[KREW2]

Firstly just to say what a show Cherbourg put on for the summer solstice, anyone else there?

However our return journey was a bit tense with thick sea mist, visibility varied from poor to non existent, and the traffic in the channel was very heavy.
Settings on my SH 180i were active range 10 Nm, CPA 0.8 Nm TCPA 15 minutes.
The CPA alarm was almost constant, and was giving me warnings of boats 25 miles away, mostly vessels anchored or moored up in Portland Port.
I thought the AIS would not pick up targets outside my active range, but reading the instructions this morning they are displayed, but not active, however the CPA alarm flagged them up almost constantly for 4 hours.
Googling revealed and old thread on this subject.
A boat 20 miles should not be an imminent threat. I had my TCPA alarm set to off. My understanding of it now is you have to have the TCPA alarm set to ON, and this will stop the CPA alarm going off with boats outside the active range.
I will obviously try this next time out, but would be interested to hear from others how they prioritise their alarm settings.

 

[LittleSister]

I have the same problem on my CP300i. I've ended up just keeping a visual eye on what's approaching on the screen, as well as on the horizon.

My understanding (perhaps mistaken), from experience and a previous thread, was that it will allow only additional, not combined, alarms - e.g. you get everything that meets your set CPA plus everything that meets your set TCPA; you don't get only those that meet both your set CPA and your set TCPA.

In busy waters this is a real shortcoming in an otherwise great bit of kit, and I seem to recall that someone rang SH to confirm this was the case. As SH have pulled out of the plotter market I doubt it will ever be addressed.

If I've misunderstood, or there's some cunning work-around, I'd be pleased to hear about it.

 

[KREW2]

Your take seems to be the same as mine.
I still don't understand why it gives out an alarm when the target is outside the set range. When I approach Portland there are usually a few large ships anchored in Weymouth Bay and more tied up in the harbour.
On days of good visibility i would turn off the alarm and just keep a an eye out for anything that looked threatening, then follow up with the on screen info.
Yesterday I darn't turn off the alarms, so had to cope with anchored boats 25 miles away coming up as a constant threat.

 

[LittleSister]

I still don't understand why it gives out an alarm when the target is outside the set range.

Because (as I understand it) it meets one of the other criteria - CPA or (unlikely, given distance) TCPA.

If you have CPA and TCPA and Range on it will give you an alarm for every vessel that meets the set CPA or TCPA or in Range.

Of course, what we actually want is only those vessels that meet CPA and TCPA and in Range.