"Salomon Fringe" wrote in message
...
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.

S

My lowrance x65 and later lms332 both seem to go below 2 feet. The pulse
length is about 3 inches. You might try a cheap eagle like the 168 which
sells for under 100 dollars complete. The one I use on a canoe also
reads quite shallow.