I would mark the shipping channel with a race note (NOT to be avoided by optimal routing).
Then configure an alarm in Exp that is triggered when the Time or Distance to race note channel falls below a certain value.
In case your route passes multiple race notes this alarm may also get triggered for those race notes where you don't want it to be triggered, but otherwise it is the best option I can think of.

Another option would be to set an alarm when LON or LAT passes a certain value but this would be harder to configure and not works as well depending on the orientation of the shipping channel in relation to your course. 



 

Having said the above, I cannot set it up with a stable latitude; I think it may be defaulting to where the boat "is", as it isn't currently connected to a GPS.  Nick, I probably need your advice as there isn't anything in the Help file.
Cheers

I've never actually used simulator, just haven't had the need. I note that the Help menu says it is launched from Apps, but it doesn't appear as a choice here in the office .  And as I wrote earlier, when I enter my preferred latitude (N-S track) in Alarms and press OK, on re-opening the Lat has gone to 33N or so ... I want 33S!  Any idea why that would be?  No GPS position?
Failing anything else, I'll try for real tomorrow.
Cheers

Thanks, will fix the input.

Duncan - I can't see anything in the help that refers to the old simulator.

You can just configure the alarm and close the window.  The alarms gets triggered as soon as the condition is met.  You can link e.g. a sound file to the alarm to get an audiable warning.