A friend has 22' saltwater boat with Lowrance HDS 7" unit on center console. Transducer is mounted in sump. I grabbed transducer and it is very securely "epoxy/glued" down. Any guess why HDS 7 unit looses depth when on a plane?

Air (turbulent water) passing over the hull where the transducer is located.

I have a through hull transducer mounted about 1’ from the back of my boat in the center of my hull. It is on a 3 degree slant on the hull well within the allotted 8-10 degrees. After I get on plane and reach 20mph, I loose my depth. It sure isn’t because it’s not in the water. It’s not because a rib is affecting the water.
I’ve given up trying to keep depth on my boat. I’ve used the transom mount, in hull mount and the through hull transducer all without showing depth above 25mph.
The best one so far has been the transom mount one, it might on a good day show depth at 30 mph while on plane.

Their is only one reason that a transducer looses bottom at speed. It is getting air underneath the transducer and sonar cannot read in air. Is this issue new or been like this from the beginning or has it ever worked? When installed did you test it before gluing it down?

Should be able to get sonar, but not side or down imaging while on pad. Speeds have to be slow for those to read. Put you screen on 3 options…sonar, down, and side image views simultaneously.