prop size

[birdhook]

What's the correct prop size for my 1971 CC 140hp I/O mercruiser? I've had a 21 pitch alum 3 blade Michigan prop on it and have been told a 19 pitch might be better. I have a feeling the motor is working to hard for the 21 and the motor wants to stall at a very low troll speed. Gets up on plane O.K. I forget what top speed and rpm's readings are but they seem pretty good. Thanks, Keith

[birdhook]

I guess everybody with 20CC converted their I/O to outboard.

[stevebrownell]

Will,

I'm one of the few that still have a 20 SF with a Mercruiser 140 I/O on this board. I run a 4 blade 19, which I think is also a bit much, although some recommend that prop. as a good compromise for the drive/engine/hull combo. This year, I'm probably going w. a 3 blade 19, as the 4 blade just does not allow the engine to rev. to the sweet spot. It does hold well, but my top speed is around 28-30 mph. on a calm day. I think it lugs the engine, which is a no-no.

And don't let anyone talk you into hanging an ugly outboard on your boat, the lines of the inboards and I/O's really are a tribute to the design.

[birdhook]

Thanks for the reply Steve. My name is Keith. My boat is called the WildWill after my younger brother Will who died last year from complications of Spinal Bifida. He was 35. I told him that the SeaCraft will always be named the WildWill. I think I will try a 19 pitch also sometime this year. The 21 pitch that is on now might have been a prop from our other SeaCraft that had two 165hp I/O's. I have a feeling it's the wrong prop. Sometimes after running to spot to fish and shut her down she acts as though she's flooded when you go to start her again. I had a rebuilt carb put on that helped the problem but I think the engine is luggin also. I'll let you know what the numbers are when I get her in this year. I love the I/O's except for the lack of cockpit room but that is not enough reason for me to switch to outboard with bracket. Thanks again, Keith

[stevebrownell]

Keith,
Your welcome, sorry to hear about your brother, but that is a fitting tribute.

good luck w. the propping - it is a bit of a trial and error proposition getting it right.

[birdhook]

Steve, I had the boat out for the first time this year and got these readings. She puts out 4200 rpm's at full speed (32 mph). That's with 2 guys, 1/2 tank gas and fishing gear. What should the max rpm's be for my 140hp mercruiser? Thanks, Keith

[stevebrownell]

Will,
Those are higher than mine, but I am using a 4-blade prop, which I believe is too much for that engine. I will likely try the 14x19 three-blade this spring, but the new console and re-wiring is standing in the way.
4200 sounds pretty good, anyone else want to chime in on WOT numbers for the mercruiser 3.0?
I'll check the book tonight, but I think those are about right.
FYI - I get about 29-30 mph w. the 4 blade, but my engine is lugging down to 3500-3600. lighter load however.