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It sounds like you have multiple problems, so you will need to isolate each humminbird unit from each other and also the trolling motor from them because it could be introducing noise also. At least until you can get them to work individually and then you should be able to tie them back together or network them. The first thing i would do is make sure the units have their own battery source, this will help with noise if there is any. Sometimes my 787 will lose the bottom also but only past 100ft. Check to see if the bottom depth is not set manually above the actual depth of the lake. Also check for the amount of keel offset you have programmed.
Third try setting up your units to see fish in around twenty feet and you should be in the ballpark. Hope this helps some...
from South Central Mississippi. This sounds like you are trying to run both sonars at the same time. The sonars bounce sound off the bottom and it comes back to the transducers and if the boat is at a certain distance from the bottom, one sonar will pick up both or the other sound and cause all kinds of interference. So try just running one sonar at a time. The Di and SI units the boat has to be moving at somewhat speed(best 2-4 mph) to work right. Now 2d will work while sitting stationary but then the fish will have to swim tru the sound cone to show up as being there. This youtube will tell you why fish show up on sonar and why they fail to show up. Hope this give you some help with your sonar problems.
