Locking the Prop?

[Breck]

I dove down this morning to attempt a removal of the propeller on my Pearson 30. I put the Atomic 4 into forward gear, but it continued to spin freely. This prevented me from being able to torque the nuts that hold the prop in place on the shaft.

When in gear, is it expected that the Atomic 4 will lock in place like my car, or is it supposed to spin freely?

[Al Schober]

Breck,
When you put the reversing gear in fwd, you're engaging some clutch plates that couple the output shaft to the engine. There's nothing to keep the engine from turning. I think you're just turning the whole engine.
When tightening or loosening the prop nuts, I just put a piece of wood between the hull and a propeller blade. How thick a piece depends on your prop to hull clearance.
Did I read correctly that you're doing this under water?
In a proper installation, the prop in held on the shaft by friction, not by the nuts. Friction also handles the torque. If the installation was faulty, the key handles the torque and the nuts keep things in place. To install the prop, you use the big nut to push the shaft up the taper. Push it up far enough, and it will stay there! Then the big nut is removed and the thin nut is installed against the prop - finger tight. Then the fat nut is reinstalled (slots aft) and run finger tight against the thin nut. The fat nut is then backed off a little to install the cotter pin. Then the thin nut is backed off to contact the thick nut, then the thin nut and fat nut are tightened against each other.
Commercial propeller practice these days uses a finer taper than we use (1:20 vs 1:12). These propellers use no keys. The propeller is advanced on the taper using hydraulics. During advancement, there is also pressure pumped between the hub and shaft to expand the hub. Once advanced, the hydraulics are removed and a seal plate installed - there is no nut at all!

[joe_db]

You can turn the engine over by hand with the prop. Take a piece of 2x4 to jam the prop against the strut to tighten the nuts

[CajunSpike]

Another possible option(if you have it) is to use the engine hand crank to stop the motor from spinning. Insert the hand crank, tie it down so it can't move. Turning engine ccw at least, it should not turn with this setup.