Just 2 cents worth,, not change needed. :roll:
Bob makes an excellant point on the line of sight/ field of vision needs.
I have a '2006C chart plotter about the helm wheel, below the windshield and a RayMarine C80 mounted under the shelf just to the right of the center window.
Each unit does have a GPS attached for backup. I have lost signal from one and not other on occassion. A portable unit would be a great addition to be used here and when biking/hiking.
I seldom use the C80 as a general nav aide, but do use the RADAR display; unless I'm using the A/P to track to a WP. My visual scan to the RADAR is really focused in collision avoidance and WP Tracking so I'm not having to lookup too long and away from looking out for ........ The screen size makes easy for me to see small targets at a glance.
The Garmin '2006C , with its Fish/Depth Finder, has evolved into my primary nav tool because of the ease of managing the menus and display without having to reach up. Each has its strengths and I also find the size of the display very comfortable for the speeds I travel.
The CD25, let alone a CD16 or CD22, does not have enough space to mount these units at the helm. The market and some e-bay looking could expose some good deals for the unit that meets your desires. Do look for one that can be updated and supported by the manufacturer.
As many backups as I have on the boat, I have not figured out a way to have a backup sonor setup because of the universal frequences used. Forward scan would be a plus for the many reasons covered in other threads.
I hope I didn't lose the focus of the question here. :roll:
Art