Neither the transducer nor any ground plate should be painted. Some owners have been able to successfully apply a VERY thin coat of antifoulant paint to a transducer without ill effects, and they do sell special paints for transducers as well. However, in most cases they should be unpainted.
If there is a ground plate on your boat (and if so, it most likey would have been owner added), it would need to be bare of paint in order to maintain continuity to the water (paint would insulate it and render it pretty much useless to ground the boat)
I would expect a ground plate, if it was to provide lightning protection, to be located very near under the mast, to one side or the other of the fin-keel. But again, it would depend on what the owner that installed it was trying to ground (or protect?).
Is there any kind of wire or metal strap connected from inside the hull near the "bump" behind the sail-drive? That might be a clue as to whether that was/is a grounding plate.
Is the sail-drive original? The OMC Zephyr, 15 hp 2-stroke sail-drive was installed in many 25s as a factory option, but O'DAY also offered a couple of different VIRE brand straight inboards (with a strut-supported shaft and prop) prior to the introduction of the Zephyr sail-drive. If your boat originally had a straight inboard the strut would have been about 18-24" forward of the rudder and that "bump" could be where the hull was glassed over after removing the strut if the original inboard was replaced by a sail-drive (gas or Diesel). Later 25s had a Diesel inboard (straight inboard) as an option....... slim possibility it might have been replaced by a sail-drive?
