Diesel prop size?

[NYCruiser]

I own a 1977 33 FBC with Yanmar 315’s. Specs say it should be propped to reach 3900 rpm at wot, which is 100 over the max 3800 rating. My boat hits 4400 at wot, so it is way under propped. Wondering if anyone with Yanmars knows what diameter/pitch prop is good for this arrangement. Thanks. Btw it looks I have 1.5” shafts.

[dougl33]

Welcome aboard.

Can't really answer this question without more info. What's the current prop size (diameter, pitch, blade count) and what is your trans ratio?

[NYCruiser]

Thanks for replying. Sorry for the slow response, but didn’t know the answer until I hauled out to clean the bottom. Trans ratio is 2.040:1 (ZF) and current props are 21X20 three blades. Seems like there is plenty of room for more diameter.

[dougl33]

I'm not sure I'd go up on the diameter, but you could certainly add an inch of pitch or add cup (which is like adding pitch). Cheaper to try those 2 things than adding diameter!

[Aquaman]

NY Cruiser
I have a 1987 Sportfish with Yanmar 6 LYA 350's my prop size is 21 x 24 3 blade with some cup. Hope this helps