Derek Weston wrote:

... electronic means to
automatically detect vessels ... are of great value to us.

We have a radar with a watch facility which is great, except it
draws 30-odd watts, more that we can afford ...

In general your solutions appear to be a bit more complex than is absolutely
necessary. Just my point of view, of course.

For example, my small Furuno "watches" on a scheduled basis and draws very
little. You might want to consider a more energy-efficient radar.

... Our radar detector works well, but ships often
have their radars switched off.

Are you sure of this, and if so, how? You are suggesting that these
commercial vessels are operating illegally (which of course is possible,
but, en masse, unlikely).

1) Nav light detectors
... [details snipped] ...

I am not an electronics designer, but this setup seems beyond the financial
and maintenance abilities of a typical small boat sailor.

2) Hydrophone systems
Perhaps a ships prop or other ship noise can be detected
electronically.

This leaves out vessels not propelled by machinery. In any case a small
sonar seems like a good possibility compared to the optical device above.

--
Good luck and good sailing.
s/v Kerry Deare of Barnegat
http://kerrydeare.tripod.com