Thy do. See this Mercruiser documentation (bulletin):

http://www.boatfix.com/merc/Bullet/94/94_02.pdf

Change that could be to Am wrong...lol.. Still stand by the prop recomendation

Does the IV have a limiter?

Only on Merc Racing (blue) engines and its an add on component that can be removed.

Maybe sometime I will get a Bravo drive.
But for the Alpha, 3 blade will be the way...and thanks for the why of it.

It's raining in NH today,so no boat.
I hope they don't postpone NASCAR at NHIS.
I have to drive right near it to get to the lake and I have tomorrow available.

Best prop I ran on my 20ft Searay was a Tempest. Good bow lift without the stern lift like a mirage +. On my 23ft stingrays, a 4 blade offshore or Vensura has been a great overall prop. If I were you, I'd get my hands on a 21p. I have a 21 rev 4 that I'd get rid of, just have to see what I have into it. It's in great shape, but I think the Offshore/Vensura would be best overall.
Oh, are you running the xm262/4 cam? 5-5100 should be good from. The XM276HR made peak power from 56-6000rpm in my 383.