I use a Dallas made 3 blade steel on my 21' Howard , same hull as yours. I have used the stainless also, but never have taken one off and changed to the other to get a good comparison on the speed. I don't think there is much difference in the two. The steel is stronger but the stainless stays rust free. Maybe you should try a 11 1/2" dia. with the low gearing you have, you can always have it cut down if need be. Get rid of that hook and don't turn that bronze prop any more than your are turning it now, they will break.