ok, so I did my homework and read the threads
My boat is slim but heavy (8tons) and I've thought was borderline underpowered with the A4. My rudder has an ovoid aperture so folding is not an option. I'm not too worried about reversing - I already have impossible prop walk... but I do care about losing power.
Running a 2 blade means higher revs? My engine seesm happiest around 2000-2500 so that's where I like to keep it.