I have a 2011 Basstracker that I wanted to put a HB 997 unit on. I found that the holes that were drilled and tapped for the previous depth finder were not in the correct position for the 997. This meant that i would have to DRILL NEW HOLES into the hull. Not being exactly sure where the new location would be, I opted to place a section of CORIAN counter top material on the lower transom to mount the transducer onto.
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This allows me to, if i need to, make changes in location of transducer with out making more holes in the transom. Just remove the corian, drill and tap new holes and reattach the corian.
Any plate material that is thick enough to accept threads to secure the transducer mounting bracket.
I foolishly applied silicon adhesive to the back of the corian plate before mounting. Now when I want to remove to drill different holes I have to "break the seal" of the adhesive to remove the place. All you need to do is make sure that the screws that mount the plate is waterproof.