The current thread running in the P235 forum, apart from arousing feelings of jealousy that they can lift their props out of the water for sailing and parking, led me to wonder what P21 owners have done.
The original engine mount board in the well doesn't seem to be in the right place. The previous owner had a Mariner 5 2-stroke and had made up a new mount of rather complicated construction that bolted through the sides of the well rather than the aft-facing surface that seems to have been drilled for the purpose.
I now have a Mercury 4-stroke which didn't fit on the modified mount - it was much too far aft and the engine top fouled the back of the well. For the Mercury I have had to make a new mount, screwed to the original holes in the aft-facing face. It took a fair number of experimental versions from old bits of wood to get the motor in the right place and I made the final version out of 30mm Perspex which seems to do a good job.
My other problem is that with the "sailing" prop on the motor it doesn't easily go through the hole. It can be done and I did it once but it's much easier to take the prop off to get it up and down. Unless you happen to be at sea. So it's not really an option to take the motor out for racing though our handicap suggests that we ought to.
What have other 21s done?
Geoff 21/30 Cygnus