transom bottom paint

When I bought my first boat the bottom paint was applied right up to and overlapping the transom plate for the drive by an inch or so. Not knowing any better, I continued this practice every year for a decade, but didn't paint the drive itself. I never saw any negative result.

I believe the reasoning is to do with the copper in the paint setting up a galvanic cell and causing corrosion to the drive. Not sure if there is anything to this theory or not, but as I said I didn't have a problem. This was in freshwater.

If it is a copper based paint, cuprous oxide, then you need the 1 inch gap from the aluminum and other metals so that it does not formed a galvanic path. Some paints, especially many newer ones, are not copper metal-based. Those paints do not need the gap.

Realize that the aluminum outdrive should NOT be painted with copper based paint as the aluminum becomes an anode to the copper and starts to corrode. Therefore generally people use non copper based paints such as Trilux or Hydrocoat ECO on the drive and any standard copper based paint on the hull. As stated, the 1 inch gap is to keep the copper adequate distance away from the aluminum.