I tried a dual frequency sounder with 3 Kw@38 Khz
and a tunable 82-88Khz.
After using the both I just stuck with the 38 Khz
as with the 82-88 as the picture was not as good.
I can get a bottom reading at 15 kts in 1200 fathoms
with my current sounder and fish life down to 800
fathoms. That’s good enough.