Thanks for the replies. Guess no-one has a good word to say about 3D
then.
I can't get my head around how the 25=BA beam works, given that the mix
of data from the bottom of the cone at any one point is displayed on
the screen as only a single data entry. Let me explain - if the depth
of the water at the extreme right side of the cone is, say, 100ft, and
the depth at the extreme left is 120ft, what does the screen show? OK,
I can understand that if there's a fish anywhere in the beam, that will
appear on the screen, but what about varying bottom data?
Also, I'm a bit confused about the recommendation for a dual frequency
transducer, I thought the dual frequency was for only v deep waters
(specs typically say 'up to 2,500ft'). This is what
fishfinder-store.com have to say about it:
"If you need a deep water finder this is the right one, however, if you
are fishing inland waters a dual frequency unit is the wrong choice,
you should select a single frequency unit because on high sensitivity
it has a 60 degree 200 Khz cone verses the dual frequency that only has
an 8 degree 200khz cone. The 200khz is the only frequency that sees
fish.
The Dual Frequency is great in deep water and very poor in shallow
because the 50khz in the dual frequency shows deep bottom structure but
not fish."

DSP - wot that?

Any light shed would be greatly appreciated!