Several years back, I read of a fellow who set up a sensitive AM
receiver "wide open" on a freq not occupied by any standard broadcast.
This is similar to the cheap "storm detectors" which amplify distant
lightning noise. He reported that most boats and ships generate a
significant amount of RF noise which can be picked up and amplified to
provide audible warning. However, this doesn't help much in a busy
harbor with lots of noise around any way.

Rufus

Derek Weston wrote:

In a previous thread I sought advice on optical and hydrophone-based
ship detection systems. I have since done some experiments with
hydrophones which may be of interest to others.

I built the hydrophone described at
sonar-fs.lboro.ac.uk/uag/downloads/bender2.pdf
and connected it to a preamplifier similar to that used for the low
noise PIN diode laser receiver described at http://www.k3pgp.org/ This
was connected in turn to a simple LM386 audio amplifier driving a speaker.

Generally, the loudest noises were those from the clicking shrimps.
Ships travelling at low speed some 500 metres distant could often be
heard, and some travelling at higher speeds were audible up to about
1500 metres away. Rain on the water was suprisingly loud, and heavy rain
masked all other sounds.

Mulling over the impracticality of trailing a transducer, the
depthsounder transducer was connected to the same amplifier system. This
transducer proved far less sensitive to audible frequencies than the
"Bender" hydrophone, by an estimated 15 dB. It also seemed to have much
poorer response at lower frequencies. An additional disadvantage of the
depthsounder transducer as mounted in our yacht is that underway the
noise from even small waves striking the hull was very loud and masked
most other sounds.

Even with DSP I doubt anything I've tried so far could be part of a
useful ship detection system.

So far its looking like Larry, Armond and others' advice to just
generate more power and use the radar may indeed be the best way for me.
The hydrophone systems will probably be relegated to the role of
eavesdropping on cetaceans.

Derek Weston
Talking Depth Sounders http://www.alphalink.com.au/~derekw/mit/