I have used a short(15") shaft, 4hp., in flat water, that pushed my Commander just fine.
I have also been caught in a tide rip where standing waves had
me at a near stand still wth the boat hobby horsing and about two
feet of green water coming over the deck, I fired up my 15hp, long shaft,gained weigh,
and was able to bear off to get out of tha hole. I never cavitated the prop
once. Its safe to say that you will never cavitate with a long shaft (20"), never.
I bet my 4hp. would have burned up and possibly not gotten me through the rip
Tacking as I would have had to do would have lenthened the time I was taking solid water over the bow,
and as I had only the main up with little way would have been dicey at best. Go with a long-shaft if you anticipate
rough water.
Cheers, B.
Commander#215