Brighten up that hull with some sandpaper, clean with acetone and then use 3M 5200 adhesive to apply your transducer bracket with 4 stainless screws into the transom. From that point on just use shorter srews to install into the Star-board material and you won't have to worry about water intrusion if you have to tweak your ducer position. The liberal amount of 5200 on the back of the plate will seal the 4 screw holes.