Ripple,
I have the 3M20A in my C30. I live on the left coast of Florida @ John's Pass. The tides can run pretty strong at the pass and during full & new moons, really pick up speed. At such times, I may be making 1/2 to 1 knot speed thru my tight pass.
My old 5411 In sea conditions was very weak. Remember 11HP (nominal), is at the flywheel and only at max RPM. You'll lose HP down the shaft to the prop so your actual HP will be 50+/-%, sometimes less.
I moved to a 3-blade prop which helped but it was very hard on the engine.
Not the greatest power, but it all depends on your location. Located further North, I would think that your area has alot of tidal current and maybe sea conditions to deal with.
I assume you are looking at early '80s versions when the rule of thumb was 2HP per ton (always thought this to be quite conservativley stupid). This is quite insufficient for areas of strong tidal movement & seas.
It took me forever to go anywhere or keep up with more powerful boats. I was always the last to motor in. The storms in my area can be very sever at times & I remember times where the chop was high and only a boat length apart. In these conditions, running at only maybe 1-2 knots. The prop would leave the water and my speed would drop to a standstill. Motoring was a mute point and I would need to storm-sail my way through. And, this was because my engine didn't have enough "ass" to plow thru at a higher speed.
I'll bet some will disagree with me on this, but everyone has his/her own opinion, and that's what makes this a good site for the exchange of ideas.
It was for me a sore spot until I repowered.
In looking for a boat let me say that any catalinas built from 1990 on up is like 100 years difference in engineering/construction/thinking. There wasn't great detail back in 1980 in quality control & assembly as the newer models posess. Just something to keep in mind pal, and not be conservative in choosing engine power...
CR