On Fri, 24 Sep 2004 22:20:25 -0700, "rmcinnis"
wrote:


"Matt Lang" wrote in message
. com...

~~ snippage ~~

- I often have all sorts of returns and speckles and want to know what
it is that creeps under my boat

It isn't necessarialy a fish. It is possible to get a reflection off of a
thermal layer in the water. The fishfinder can sometimes be fooled by noise
or vibration coming from other sources, like the engine or turbulent water.

You can also get interference from other sonars in proximity to yours.
I've seen that happen on lakes with heavy fishing pressure.

- What effect has frequencey on the echo and return. MY finder has 50
and 200 khz, why does certain frequencies get used and how does
frequency correlate with echo angle?

A higher frequency is easier to focus and will have better resolution on
depth. Unfortunately, the higher frequencies also get absorbed faster so
they tend to not be as far reaching.

- 200khz being pretty fast how does boat speed change the return?
- up to what speed is the finder useful for identifying objects in the
water?

The ping frequency doesn't determine how fast the sound travels, that
remains the same for all frequencies. You would have to be going pretty
darn fast and have a very narrow beam to to move out of the beam width
before the echo can come back, sound moves pretty darn fast, especially in
water.

What is more significant is the interference that the speed might cause.
What really messes up a depthsounder is getting pockets of air in the path.
If your boat "porpoises" and causes air to get trapped under the hull then
the depthsounder will go nuts. Obviously, if the spot in the hull where the
transducer is located comes out of the water you will have trouble.

At certain speeds vibration from the engine, shaft or rudder can cause
problems. If the transducer is mounted behind a thru hull, strut, or
similar irregularity in the hull the eddy currents and possible cavitation
can wreck havoc on the transducer.

Nice post Rod.

Although I defy you to desbribe putting a through hull transducer in
the Ranger as easy.

The only other "issue" I would include is water density and speed. As
water density changes, the actual speed and quality of the echo can be
affected. Some finders use an algorithym to deal with that - that's
why there's a temp sensor on most of the newer models. On other
models, there are additional algorithyms to adjust for SOG (meaning
the Speed Over Bottom rather than Ground in this case).

The newer digital finders can do all sorts of interesting tricks with
signal processing where you can filter out noise, interference and
such, but you really have to be knowledgable about sonar to make the
translation from fishing guyese to technicalese.

Once you get the hang of it, you would be amazed at what you can see
on the bottom. I have a new Raymarine color finder that I just
installed on the Ranger with all the new DSP goodies - it's pretty
amazing, I'll tell you what. :)

But again, interpretating the results is the real trick. Depending on
where the transducer is mounted, you are looking pretty much back in
time rather than current time - what was rather than what is. Once
you get the hang of how your target species relates to what you see on
the finder, then it all starts to come together.

Good luck.

Later,

Tom