23 cc inboard prop size question

[Highway12]

Just purchased a 23 cc 350 Mag MPI 300 HP. The boat is way under propped. Can anyone recommend the correct prop size for this setup?

[MarkR]

tjudgejr-

Mine is a '75 Sceptre inboard, Marine Power 350 - 4B Carb 260HP

16 x 16 cupped

2700rpm - 20knts (most economical cruise - I think)
4200rpm - 30knts

[joshz]

On my old 23 inboard cc I started with 16x16 cupped. This prop would not get the max wot so I went to a 16x15 cupped. This prop got the max wot for the engine. My numbers were 22kt cruise and 31kt wide open with a carbed 350 merc. Hope this helps, I played with a few and this was the best for that boat.

[Highway12]

Thanks guys, Sounds like 16 x 16 would be a good staring point.

[MarkR]

The reason for the cupped edges is it helps reduce the noise generatoed in that tunnel.

[Highway12]

Thanks The cup also adds pitch which should put my RPMs right where they should be. I was testing a 16 x 16 yesterday and blew the raw water pump impeller so I never got to full RPM to tell if it was correct or not.

[Highway12]

Anyone know what prop to start with?

[aldo]

I'm running a 16x15 cupped as well on my 350/260hp inboard with throttle body injection.
I experimented with quite a few others but get the best numbers with it.

[Highway12]

Does anyone have a recommendation on motor mounts?

[Gary Hill]

I have a 16 x16 prop and 315 HP Marine Power engine And tweaked it here in Tidewateer VA to turn 4600 RPM.