As stated above about using 5200...NEVER below the waterline, it is not designed for that use. 5200 is removable, use acetone in a small sports bottle to spray it over the area and use a scraper to get between whatever you want to remove and with a little finesse it does come off. 5200 is the only thing you should use to seal any screw holes below the waterline, only other thing I would use is a good epoxy, but that might be even harder to get off. As far as a piece of starboard, as long as the starboard is cut to match the foot print of the transducer mounting bracket and does not voerhang at all it should not prodice much turbulence, but good luck getting starboard to match that foot print nicely, for many it is a tough material to work with and do neatly. If you cut at the wrong rpm it will bond back together and edges are hard to keep neat, usually requires a routered edge to make it look nice, but that would then create a lip and turbulence for your application. Most transom mount transducers I've installed all had adjustable mounts that allowed for some movement up and down, if your location is correct, that is all the movement you would ever need, personally I think adding the starboard is more work then necessary, especially if your trying to avoid drilling into the hull. If you said you wanted to epoxy the starboard on and then mount it to avoid drilling holes in the hull I would understand that but you have no fear of drilling into the hull, so not sure of the big benefit. I would NOT use just a silicone sealant for under the waterline, it will break down over time and will allow water in and over time ruin the transom, 5200 is the best thing aside from epoxy to prevent this from happening, no sense in cutting corners and having expensive repairs down the road.
