I have been using marine goop, 3M adhesive , plumber putty to glue my fishfinder transducer inside my kayak .
They all failed at some time.
How long ? It depend on your luck , maybe 8 months maybe 1 .
I have instead used these magnetic transducer mount for many years and am very please with it.
Flip it up, no water drag,
Flip it down, fishing...
https://www.fishfindermounts.com/pro...ucer-mount-kit
These are super strong magnet, really need some force to separate them apart .
It hold my Raymarine Dragonfly 7 transducer secured @ 5MPH under Hobie Mirage turbo drive no problem !
Well worth the investment .
I have multiple kayaks, and transfer the fishfinder between these are a piece of cake.
I just run the wires under the front hatch, no drill any holes..
I double epoxy the metal surface of the magnets so no rust can grow on it.
Check it out .
TL
Danny