Sonars do take some thinking on the part of the operator. It has to do with having to wait for one pulse to go out and reflect back before putting another pulse in the water. The speed at which this happens (sampling rate) is controlled by the range setting. If you are trying to look long range, you might entirely miss a small reflector that is between 'pings'. If you can limit your range requirement you get a faster sampling rate and increase the chance of returns from obstructions. This is also tied to vessel speed. If, for example, you are stopped dead in the water or at anchor
, the constant returns from a stationary target can look like a wall.
Boat speed can also affect signal-to-noise at the transducer. Bubbles will of course screw it up everytime, no matter what the manufacturer says. mechanical noise
in the right freq. band can also do it, unless your system is sophisticated enough to use chirp technology, Kalman-based filtering, and platform motion correction.
The filtering has to be good enough to grab the correct return ( the first one back) and ignore all the reflections that come rattling in right behind it. This is called multi-path distortion.
Organic growth on the transducer will reduce snr, as will a layer of paint
or especially silicon. If your transducer goes through an oily patch, you need to use some good old dish soap to wash it off. Need a good acoustic coupling to the water. the soap will 'wet' the transducer.
You gotta think like a sound wave when operating a sonar.