The fleabay example above has a 45 degree arc. Not sure if that's 22.5 forwards and 22.5 back I expect so. If it is then my high school trigonometry (unused for a while) tells me to see 3m forward the depth needs to be 7.24m. if the angle is 45degree forwards it drops to 3m but even that's deep enough for most ribs..
Other way to think is what your safe draft is and how far ahead you can see in that. Not sure what it is for you but the example device is limited to 0.7m.
At 22.5 degree forward angle I get 28cm forwards. That's not far.
A couple of years ago I had fitted my plotter but hadn't fitted the sounder to the transom. I screwed it to a bit of 2x1 and ventured up the forth. Was really handy when venturing into literally uncharted waters with my crew holding it over the bow.
Above comments re survival of it at speed +1 from me, but I did wonder if an NMEA 2k one could be plugged in as required using a data Bulgin buckaneer and set the plotter to show NMEA depth when signal present. (ie when the sender is plugged in)
Having said that I've had one use in umpteen years of ribbing so I haven't done anything about it.
Batteries lasted about 10 days worth of boating (rechargeables) so not too bad. Its just gone with the boat that I sold so shall be buying another for the next boat for sure.
Handy little piece of kit for the money, I would'nt use the mount it comes with for a transom but glueing it to the hull will be fine. Plenty of cable supplied too.
I made one from a plain battery box. Sonar, Switch panel, sealed (water tight) cigarette lighter for lamps, etc.. to just plug-in. As well as mounting external Power Posts (Brass threaded 5/8") with all fused connector within the battery box except the switch box which contain it's own quick-change fuses for each of seven lit rocker switches. Works absolutely great.
