Thanks for all the great advice everyone!
Unfortunately, we are chartering, not sailing our own boat, so I can't take any option that involves modifying the boat. But your suggestions still were helpful in clarifying some of my expectations.
We had chartered SVG last year, and generally were not too worried about boardings then, nor are too much now. Watching CSSN postings from the past year, it does not appear that there is high threat level (maybe not much different, really, from risk of a break-in at our land home, though we do live in a pretty quiet, small town). We also do watch for where problems have been persistent (e.g., Canouan) and generally have been able to anchor
last year (and plan this year) in places that have had little to no reported problems.
But we may sleep a bit more relaxed having some form of warning system.
Taking it all in, I'm going to proceed with the unit I had in mind -- very portable, and easily adapter to mount pretty much anywhere on the boat. From comments everyone has made, I suspect placing it in the aft quarter of the cockpit, low in the cockpit (really need only to detect feet) and facing the companionway
, may work just fine and minimize chances of false alarms. Hopefully, that would avoid any objects that might swing from the boom or lifelines
, seagulls (so long as they don't invade right to the cockpit floor), etc.
While this unit reportedly has a VERY loud siren, it also has a chime mode, which we could use to test it. Possibly, the chime and the bright strobe might be enough to ward off a boarder. I completely understand and respect the concern for a loud siren waking everyone in the anchorage -- i've always hated car alarms for that same reason. But a genuine boarding probably should wake the anchorage, or there may be some risk that the boarder might try another boat being scared off ours.
Thanks again! We sail second half of February into early March. I'll report back here with results at that time.