Which Sonar Frequency ?

[Radiorick]

I have a Garmin GPSMAP 172C with a GSD21 sounder. It has a dual frequency sonar: 50khz and 200 khz. I'm using a transom mount transducer. My question - which frequency is bettter or should I use it in dual mode?

The boat is in SW Florida - mostly sandy bottom and shallow water, rarely over 20 feet.

I frequently lose the bottom (flashing depth indicator) when going fast but when I slow to idle or slow speed it reacquires quickly.

Would a thru hull transducer work more dependably?

Thanks,

Rick

[GordMay]

Greetings and welcome aboard the CF, Rick.

You might be most satisfied with the higher frequ. 200 KHz setting, in shallow Florida waters.

Depth sounders generally use 50 KHz frequency, and a broad beam to measure bottom depths. Features of 50 KHz sounders are; deeper depths, wide cone angle around 35 degrees, less definition and more noise.

Fishfinders generally use higher frequencies (192 or 200 KHz) and a narrow beam (around 6 to 22 degrees) to produce a higher resolution echo. This higher resolution is better at locating smaller targets. Features of higher frequency sounders are; shallower depths, narrow cone angle, better definition and less noise.

Dual frequency sonars can show both the high and lower frequencies at the same time. The reason for dual sonar is that the 50 KHz signal has much greater depth capability, while the higher frequency will show more detail.

[Radiorick]

Thanks for the quick answer. Now the second part of my question . . . what contributes to losing the bottom at high speeds? Is it more likely to be cavitation around the transducer? Would a thru hull transducer work better? I've checked the transducer to be sure there aren't a bunch of barnacles or chitters growing on it and it's very clean.

I have two depth finders on my big (34' Sea Ray) up north and they never lose the bottom. Both use thru hull transducers.

Thanks, again, Rick

[GordMay]

Originally Posted by Radiorick

. . . what contributes to losing the bottom at high speeds? Is it more likely to be cavitation around the transducer? Would a thru hull transducer work better?

Yes (sort of), & yes, though high speeds are usually problematic, wherever the transducer is located.
You want to try to keep the X'ducer out of disturbed water.
Of course, it has to be IN the water, sometimes problematic with a planing hull.