Derek Weston wrote:

We shorthand frequently on our sailboat, and electronic means to
automatically detect vessels in our area 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 as we're primarily solar panel
powered. Our radar detector works well, but ships often have their
radars switched off.

Two ideas I've been mulling over to which others may be able to
contribute useful ideas:

1) Nav light detectors
I think it may be possible to devise an electronic system which can
detect red and green coloured lights, and distinguish these from the
moon and the stars.
I'm imagining a rotating system with a cylindrical lens and both
red-filtered and green-filtered detectors, and triggering an alarm on
the basis of the magnitude of the difference between the two sensor
outputs. With appropriate relative gain settings, the moon and the
stars could give a near null result, while coloured nav lights may
give a greater output sufficient for the alarm trigger.
This may be close to the limits of present technology. I can imagine
the sensors may need to be cooled (with consequent condensation
problems).

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

Any thoughts, previous work, intelligent comments....

Derek Weston
Talking Marine Instruments
http://www.alphalink.com.au/~derekw/mit/apps.htm

Years ago a marine publication recommended buying a standard traffic radar
detector. Seems that they are so broadband that they will pick up nearly
everything in the radar band. Owing to philosophical objections to the
practice, I've never owned one, but it might be worth testing.

I should think that anything of 'size' at night would be equipped with, and
using, radar.