Jeffboat - the video is very cool, nice boat! Did not realize it was more of a crusier than a go fast but that doesn't change much.
Here's the problem with what Rolla wants you to do in getting 22" props in my opinion - you have the little B series engine 5.9L as as you keep increasing dia with the surface props the drag the props have when fully submerged gets to be soooo great you simpley do not have enough cubic inches to make enough low end Hp to break those things loose (I know vent tubes, etc) and if you do because of lower gearing then you won't have enough RPM available to compensate to make the speed.
The dia as far as I'm concerned is strictly to develope ft/lbs of thrust which has no bearing on speed at all. Speed is from pitch - distance traveled. But as you increased dia to make a prop more effiecent at slower RPM you then need to increase the pitch to overcome the loss in prop speed and pretty soon you have a tug boat propeller - lots of thrust no speed.
I 100% agree with 18" dia and 1.15 gears, we ran close to that set-up in 1990 (20 years ago) and you can run into the low 70's no more than that. 17" / 17.5" props 400 hp 1.15 gears, heavy, heavy patrol boat and into the high 60's. With a lighter boat same package up the pitch and you hit 75 mph.
To go faster than that you need prop speed and I say this on here over and over and I will find old pictures from 25 years ago over the next day or two to demostrate that. If you have the Hp and want the boat to go faster you need to go to smaller dia propellers and spin them faster.
Looking at your boat if you get into the 70's with that power you can't ask for much better, it is a big, heavy, sqaure boat not like a 42 Fountain that is low, light and shorter (doesn't make either bad just different)
I assume your problem is you went backwards with the Cummins from where you were at? You wanted to buy a package and not peice together drive, trans & engine?
I would also guess the Yanmar turns a little faster than the Cummins (by the specs) and when you look at prop speed of the Yanmar at 1.28 vs the Cummins at 1.15 they might be very close?
I think you are on the right path, smaller and faster the question is how much and that is trail and error for the most part - just don't let people tell you how BIGGER is better it's bulls**t!
One word of caution, the smaller and faster the props are the more top speed but less cruising speed and you need to be going faster to stay on plane with a big boat like yours - it is a balance.