Thanks, John. The 3GMD was the first version of what became the 3GM30. It was produced 1981-1983 or so, at which point it became the 3GM30. The D is rated at 20 hp sustained running at 3400, with 22.5hp at 3600 for one hour max, while the 30 version ups the rating to 24 hp at 3400 sustained and 27 at 3600. Other than whatever they did to up the ratings, it appears to be almost exactly the same engine and transmission. My concern is that both manuals make a specific point of saying that the 3GM uses a different tranny fluid than engine oil. I believe that my new to me 1985 Marshall 22 catboat has engine oil in the tranny, based on its light golden color, and the concern is that it seems to be contra to what the book calls for, although it runs just great. Based on the year of the boat, I assumed the motor was a 3GM30, until I found a plate on it saying it is a 3GMD. It is certainly more than enough power for a 22' sailboat
, easily pushes it well past hull speed of 6.1 to a bit over 7 knots, and makes it feel sort of like a motorboat as the bow lifts. Not an efficient way to cruise, but fun now and then.