Yes. I did it with an older Paper chart recorder with lots of power. I put
the TRansX at about 45 degrees under the bow in front of the keel. It
needs interpretation just like radar though.
You compare the bottom of the normal sounder and the FWRD looking. When
the slope of the bottom gets steep and is less than or equal to the depth
of the normal trace, you have an obstruction approx the depth read in
front of you on the frwrd modded trace.
This is so because the _rate_ of _change_ from the 45deg trans~ is high
when the bottom is shoaling in front of you; ie, each foot of frwrd
movement is approx the same as the rate of depth change when there is an
object in front of you at your depth. So, when the trace is very steep it
is because your forward motion is what causes most of the change in the
trace, assuming the obstruction is solid and large enough to provide most
of the echo. If there is no obstruction the rate of change of the trace is
very low or nill.

A string and a fixed point will show you all possible positions for a
single echo.
ASIDE: i found that using the standard sounder at twice the expected depth
allows you to read the second echo (looks like a second bottom below the
actual bottom trace). the rate of change of the second echo is similar but
not as sensative as the modified transducer trace. It will steepen rapidly
when the bottom is shoaling quickly. when the two traces meet you have an
obstruction right in front of you at the depth indicated.

I suspect it is a consequence of the wide cone of radiation from the
transducers that allows this. All you read on a simple depth sounder is
time of return on the Y-axis and elapsed time on the X-axis. If the
sounding is ahead or below it gives the same return time. Those little
dots below the surface line are reflections that came back a little later
than the ones straight down and that are likely from rocks at an angle in
front or aside the lone of travel... you can't tell which! that is unless
you use the second echo becasue it has the element or rate of change
asociated with it so any change to shalower includes you frwrd change as
well so the line steepens with forward movement to a fixed
echo...presumably at lower depth.


NOW, if i had time i would edit this down to the 4 lines necessary to
convey my thoughts...or even the one word..."YES".
rick



On Mon, 22 Dec 2003 17:18:57 +0000 (UTC), lomaxelectrical
wrote:

I know this this might seem daft, but, I was idly wondering whilst
gazing
at my carnation shaped propeller if a second, narrow angle depth gague
with
it's transducer shooting horizontally forwards just below the water
surface,
would give advance warning of an impending prop repair? (alarm set at,
say,
40 feet). How would the sonar cope with seeing (hearing) the water
surface
and waves? would 2 sonars interfere with each other?
Steve





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