Depth on fish finder wrong when underway

[jayelwin]

so my 12 year old sea ray 220 has an eagle cuda 350 fish finder i use for depth. and trust me, on the shrewsbury river its shallow in lots of places. The sensor is transom mounted. when I'm not moving its very accurate. and when I'm trolling along it shows me a pretty good tracing of the bottom. but on plane it is wildly inaccurate. it often shows strange depths that have a repeating number like 33 ft or 66 ft and its inaccuracy is always displaying too deep. on plane in a known 9 ft it will often say 66 feet. and on plane the "bottom" tracing ends up being a whole lot of speckles on the screen. i inspected the sensor and it is parallel with the waterline and bottom, its not tilted. it had some sea life on it and i was hoping thing would improve when i scraped it clean but no real change.

i know its an old unit but i feel like it worked better earlier in the season. any common issues that this rings a bell?

 

[src]

I had a similar problem with a transom mount sounder that I installed parallel to the bottom/waterline, and found I just had to adjust the angle of the sounder so that it aimed a little more forward. Remember, your bottom isn't parallel to the waterline when you are on plane.

 

[Tonka Boater]

The manual for my Lowrance fish finder/depth sounder said that results can be inaccurate at high speeds because the transducer can't return the ping fast enough to get an accurate reading. In most cases the ping should be instantaneous but when on plane it could take a fraction of a second longer which can result in very inaccurate readings.

Another cause could be turbulent water.... this is a quote from my Lowrance manual "The transducer may be in turbulent water. It must be mounted in a smooth flow of water in order for the sonar to work at all boat speeds. Air bubbles in the water disrupt the sonar signals, interfering with its ability to find the bottom or other targets. The technical term for this is cavitation."

 

[1990-270-da-lover]

+1 on turbulence and also the air bubbles raised along the bottom of the hull when on plane disrupt reading the transducer is getting.