Wot rpm's too high

[Snaponman]

Hello everyone, I am a newbie to the boards(and boating) I just bought a used 1988 ringer 186captiva. 4.3mercruiser 175hp 2bbl. Alpha1 outdrive. I am very happy with my new purchase, but a bit concerned that I may be revving too high. Had it out yesterday,(first time) 4adults(approx. 800lbs) 15gallons of gas aboard. Boat planes out in about 30seconds. I'm pretty happy with that. But at wide open throttle I'm achieving 5500 rpm@50 mph(gps speed) I think It may have the wrong prop. The one that is on it has these numbers: 832832a45 21p. Any suggestions would be greatly appreciated. I just checked mercury marine's prop selector, and it is recommending a 13.375x25pitch part# 48-78126a45. This sounds like a pretty big jump to me??? Any help would be GREATLY appreciated. Thanks in advance,Bob

 

[steelespike]

Re: Wot rpm's too high

Using 1.81 gear ratio and 12% slip 21" prop produces about 5200 rpm.
If the numbers are accurate it would appear a 23" prop would produce about 4800 rpm.
Check on the outdrive for the gear ratio.

 

[Snaponman]

Re: Wot rpm's too high

Using 1.81 gear ratio and 12% slip 21" prop produces about 5200 rpm.
If the numbers are accurate it would appear a 23" prop would produce about 4800 rpm.
Check on the outdrive for the gear ratio. Thank you for trying to help me. I left the boat up north at my cabin, and won't be able to check that until next weekend. I will try to repost next Sunday. Bob

 

[Bayou Dave]

Re: Wot rpm's too high

I have a '87 Rinker v190 with the same engine and outdrive. My gear ratio is 1:84 and I got a 14.25 x 21 prop. That keeps me in the 4400-4800 rpm range and gets me on plane in about 10 seconds. Not sure what speed is at WOT as the speedo isn't working well and have not checked it with GPS. It does pull a skiier quite easily. I run about 27-2800 rpms on plane with 3 onboard and 19 gallons of gas.

 

[Tail_Gunner]

Re: Wot rpm's too high

Always verify the rpm gauge there usually off.

 

[Snaponman]

Re: Wot rpm's too high

Thanks for that, But how would one go about checking the tachometer for accuracy?

 

[SteveMcD]

Re: Wot rpm's too high

Thanks for that, But how would one go about checking the tachometer for accuracy?

I abuse a digital timing light. I haven't seen any auto parts store that don't carry them.

 

[limitout]

Re: Wot rpm's too high

I abuse a digital timing light.

?????????????? please explain?

 

[steelespike]

Re: Wot rpm's too high

?????????????? please explain?

I believe that is a spell checker error. supposed to be "use"