Prop size

[Steven Medina]

So I bought a 1981 21 ft cuddy cabin and it came with a 15" x 17" prop. Worked fine but seemed rpms were pushing 5000 when WOT said 4600 so I put a 14.5" x 19" prop on it and it ran like shit. Still was saying my rpms were high. Any suggestions as to what size prop is best for bigger boats ?

 

[Chip S]

So I bought a 1981 21 ft cuddy cabin and it came with a 15" x 17" prop. Worked fine but seemed rpms were pushing 5000 when WOT said 4600 so I put a 14.5" x 19" prop on it and it ran like shit. Still was saying my rpms were high. Any suggestions as to what size prop is best for bigger boats ?

The OEM prop for that boat was 14.5 x 19 stainless steel ( https://owners-resources.searayweb..../1981 Sport Boat Sport Cruiser Prop Chart.pdf ). It's not uncommon for people to go down on pitch to compensate when the engine gets "tired". Given the age of your engine I'm guessing that's what the PO did. I would recommend going back to the 15 x 17 prop and just watch your rpm.

If you want to make a big project out of it you can have the compression of your engine checked to see if the engine is getting "tired". There's no point checking the compression, though, if you're not willing to rebuild/replace the engine. I would just switch back to the 15 x 17 and go boating.

 

[Motoguy2158]

From SeRay Prop Chart:
1981 210CC
260 OMC
14 1/2 X 19 SST
4600 RPM

The 260 Mercruiser option used a 15 1/2 X 19 Cup
4600 RPM

So from what I've read.....the 1/2 diameter you took away should add approximately 150 to 200 rpm,
the increase in diameter would net you 300 RPM for 1" pitch so 600 rpm.
600 - 150 = 450 reduction in RPM approximately. 5000- 450 = 4550 RPM...
So when you say it ran like shit ? Not gettin on plane good, revving the motor but not going anywhere,
no speed? Based on the Sea Ray prop chart you should have been close.
I do know heaver bigger boats like bigger diameters props for lift and bite.
Maybe a 15" x 19 would be a better option as it would drop the rpms by about 600 and give you
the larger prop for lift and bite. some prop shops who should be able to dial you in will have test props you can try.