On Wed, 11 Jun 2008 19:49:31 +0200, Salomon Fringe
wrote:

Thanks all for all the replies, couldn't reply until now because I am
cruising. Hadn't expected such a great response either ;-)
Of course, about zero depth, I am not referring to water just a few
centimers deep (...) but to water that is close to 0 below the
transducer. The thing is that my more than 20y old unit that I had
wanted to replace *is* able to measure as little as 15cm below the keel.
My trawler-style boat has about a 25cm difference between fore and aft
so if I were to mount it at the best (highest) position I would still
have a gray zone of 25cm with the transducers I am looking at (50cm min
depth), and I don't like that. There are quite a few harbours where I
have less than 50cm below the keel, and if so I want to know how much
exactly e.g. to predict if I am going to hit the bottom in a falling tide.

A Furuno rep actually told me I should look at a simple smart sensor
depth-only device (similar to advice given in this thread) so I guess
the big improvement I am looking for (depth graph) is not in the cards
for me. Can't believe that sounder technology has made no major
improvements in this sense for more than 20 years.

Something else is that I have two sea chests with transducers, one with
the old digital depth meter I am using and the other from a Lowrance
X16. That unit has been enjoying its retirement somewhere on the sea
bottom for years now but the transducer is still there and apparently
works at a funny frequency, 196khz. Is there any way to detune any brand
of 200khz sounder to work with this unit? If not I have no other option
than to have the transducer replaced.

Some people said I am asking for the impossible when I asked for a
near-zero-water-below-the-keel sounder, but when you do the math it sure
doesn't seem impossible, at 200khz one cycle takes 5 microseconds, the
speed of sound in water is approx. 1500m/s so in principle you should be
able to measure down to 0,75 cm. About 1/3 of an inch. Perhaps you can't
make the pulse that short or something, dunno.

Holy smokes - an X-16? Dude - you need to move up to the 21st
Century. :)

What kind of unit did the 196 KHz transducer connect to?