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As far as watch keeping aids are concerned I would think that a passive sonar unit would be just the ticket. Sound travels faster and further underwater than in the air and a ship of fishing vessel would be audible for some distance.
Any one hear of such a thing for small boats?
Reply;
In fact, at one time I had a financial backer who was interested in just such a device. However I am convinced it is a very tough technical problem if you use just one mic or sensor. This is because sound is distorted so much through water, you may hear a ship 20 nm away clearly but not the one right next to you. An array of sensors at different depths would be needed.
Or it may be possible to link a net of sensors electronically.
I also served in the USN on Destroyers. One of our main jobs was hunting subs. Our sonar was state of the art, but when it came to detecting surface ships, it was nowhere near reliable, it would sooner or later but when was a question and getting accurate range could be a problem.
Of course the same was true for subs, even though the sonar dome had many sensors. But we did a search pattern and did it over and over if necessary, as well as deploying both radio drop sones from a helo that would go to different depts and towing a powerful array at different depths. With all of that in action and also links to other ships and underwater listening stations we could detect subs before they could detect us...most of the time.
Of course a sailboat can't do all that. So in other words yes it could be done but the answer is not simple or cheap.-Ken
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