Best size prop for my 1.84 Mercruiser with 3.7L 470 ?

Hi, I have a 21 foot cuddy cabin boat with a 1.84 gear pre Alpha drive and an early 80s Mercruiser 3.7L 470 4 cylinder engine (170 hp). My boat weighs approximately 2150 pounds. What would be the optimal prop size and pitch for this setup? I have no intentions of zooming over the water...just pleasure cruising.

It was missing when I got the boat and motor/drive.

Ayuh,.... A 19" pitch prop is 'bout middle of the range, so I'd start there to see what numbers she'll pull,...

Even if it's wrong, it'll make a good spare to get home on,...

You shoot for the Rpms, at Wot, with yer average load, 'n average fuel,...
Ya won't be runnin' those kinda Rpms often, but the motor's got to have the ability to run those numbers,..
Otherwise, the motor is luggin' All the time, 'n that's Bad,...

I have a "Prop Finder" spreadsheet, courtesy of Rod Stewart, Canada. I can't seem to upload it. Using your boats "loaded weight" of 2500-2650 lbs. and engine HP of 170 HP at 4600 RPM it calculated a prop pitch of 23. Bondo said start w/a 19 pitch because it will make a good spare. Couldn't agree more. I would add to buy a refurbished 19 pitch aluminum prop...much cheaper...and go from there. You can expect a 200 RPM change for each 1 pitch change. Go down in pitch to bring the RPM up and vice versa. A stainless prop will give you better performance, esp. out of the hole. It is also 3-4 times more expensive. Buy refurbished from a reputable shop and save about 20-25% of the cost.

As a reference I have a 1983 19' Grady White dual console with a 3.7 Mercruiser and the 1:84 outdrive. I have a 21 pitch aluminum prop that works just fine for me.

15x15 is where you should start.

IMO, you need a 19" pitch prop.