"Matt Lang" wrote in message
om...
- Does the transduce just have to touch the water with the its bottom
or does it have to be submerged?
The transducer doesn't even have to be wet! It is possible to install the
transducer by glueing it on the inside of the hull. Such a technique
generally cuts down the effective range of the sounder, but it can sure make
installation a lot easier.
- for the sonar part I need to know things like:
- what makes the return big? is it object size or density?
The "ping" is not a laser sharp beam, it is more of a floodlight. The beam
spreads as it goes down, and by the time it reaches the bottom it can be
covering a fair amount of area. If the bottom is perfectly flat, smooth and
hard the returning echo will be fairly consistant and sharp. If the bottom
is jagged and rocky you will get early returns from the closest objects and
later returns from the farther away objects.
The type of bottom can also cause the return to get wide. hard rock will
provide a clear, crisp return. The ping can penetrate soft mud and bounce
off a harder layer underneath. A layer of grass growing on the bottom can
also provide an early return.
- what shape of object yields what kind of return? apparently fish is
archs but thats rarely the case in real life it seems ...
The depth sounder makes a guess that it is a fish when it gets an early weak
return. A fish will only reflect a small amount of the energy, so the
returning signal will be weak. If it occurs significantly before a strong
return it makes the assumption that it was some sort of object suspended in
the water. Since most things either float or sink, something half way in
between should be a fish. The stronger the echo, the bigger the fish.
- 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.
- 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.
Rod