Regarding the Nissan problem: I also have a Nissan 6 hp motor on my Ariel. The manual states that the screw is designed to adjust the amount of force required to turn the engine, but also states that it is not designed to lock the engine in a particular position. So at low speed under power, the torque will cause the engine to wander even if you do tighten that screw fairly well.

It would be nice to remove my motor during a long sail and place the well plug in the well so that I could keep the sea from coming up into the well, and reduce drag. But for me that is not feasible, nor desirable

I use a lifting davit to lift my motor into and out of the well when I am at the dock. I have to use a tabernacle system to pass under a bridge on my way to sea, so there is no way I am going to lift that 60 lb motor by hand out of the well and carry it into the cabin with the mainsail up except in an emergency. I probably sail slower because of that. I really don't want to trip over a motor when I am below for that matter. I have pulled the motor at sea with the davit. The davit installation IAW the Ariel Assn Manual places the davit in the path of the boom, so if had to lift the motor with the mainsail up, it would be a poor time for an accidental jibe. In an actual emergency, I did pull a previous 80 lb engine on a close reach with an experienced helmsman. It worked, but on a close reach and heeled over, pulling the engine was no fun.

My solution to keeping the motor somewhat stable is to run a large diameter length of bungee cord from the forward port leg of the pushpit three times around the OB Motor handle (below the throttle) and then around the forward starboard leg of the pushpit. When it is in place, there is tension on both ends of the bungee, but I can adjust that tension by slipping the bungee on the motor handle. The bungee holds the motor pretty much in a straightforward position in line with the keel. I take that bungee cord off and lower the motor handle to close the hatch when I am sailing. I keep the screw on the motor tight while sailing. Sailing in conditions from 1 to 30 knots in variable ocean swells on all tacks the motor stays in place fairly well. At least I have never considered any minor wandering to be a problem.

I have thought of building a simple U shaped device that would slip onto the forward edge of the lazarette hatch and lock in place there a thumbscrew similar to the one used to secure your motor to the forward edge of the well. The motor arm could then be placed in the U, and would not wander. Of course that would only work if the lazarette hatch were open.