Prop pitches - '72 OMC

[southkogs]

Hey y'all - lookin' for some thoughts as I'm playing with fine tuning the boat. I'm not moving terribly fast on this as I still have some trim issues in the ole' Stringer drive, but I'm considering props too:

The boat is a 17', weighs in at 2,000#, generally has 5 on board (2 adults & 3 teenagers). The drive is an OMC 165 (Chevy I6) and is supposed to be WOT at 4,600 RPM. Right now, I've got a 3 bladed 14x16 aluminum prop and we're showing WOT closer to 5,400 RPM at 32MPH (GPS). Hole shot is pretty sluggish.

This drive was recommended (as I understand it) originally with the 14x16 or 14x18.

I'm playing with my trim (figuring out if the motor's gone kaput again or if I've got a goofed up jackscrew), but do you experts think there's any value in getting the 14x18 prop? Or perhaps even go the other way to a lower pitch?

I'd like a better hole shot, but I don't think I want the boat to go much slower. If I could get 40MPH outta' her at 4,600RPM that'd be pretty cool ... but speed isn't that important terribly important to me, gettin' up on plane quickly is more my push.

Let's leave tabs and tails outta' it for the moment (not that I've ruled 'em out).

 

[steelespike]

Re: Pro pitches - '72 OMC

Re: Pro pitches - '72 OMC

Do you know your gear ratio?

 

[southkogs]

Re: Pro pitches - '72 OMC

Re: Pro pitches - '72 OMC

Do you know your gear ratio?

1.39 overall (I think).

 

[jestor68]

Re: Prop pitches - '72 OMC

Something is wrong here.

The numbers you publish equate to a 46% slip, which isn't likely for your boat.

It may be time to verify the accuracy of your tachometer, and/ or verify the gear ratio.

 

[steelespike]

Re: Prop pitches - '72 OMC

I agree 32 mph with a 1.39 gear ratio and a more typical 12% slip indicates about 3400 rpm.
But at 3400 you would have to go to a 11" pitch.to get to 4600.
1.39 seems extreme. We know its a 16" prop and the speed is 32 If we assume 12% slip with a 1.80 ratio
rpm would be about 4350.
It all comes down to we have to confirm the tach and it does seem suspicious. at 5400.
We could also try to confirm the gear ratio.
Its a real pain but we could count the ratio by removing the plugs and turn the motor by hand to count revolutions for one
revolution of the prop.You'll have to mark prop and crank pulley to be accurate.

 

[southkogs]

Re: Prop pitches - '72 OMC

I agree 32 mph with a 1.39 gear ratio and a more typical 12% slip indicates about 3400 rpm.
But at 3400 you would have to go to a 11" pitch.to get to 4600.
1.39 seems extreme. We know its a 16" prop and the speed is 32 If we assume 12% slip with a 1.80 ratio
rpm would be about 4350.
It all comes down to we have to confirm the tach and it does seem suspicious. at 5400.
We could also try to confirm the gear ratio.
Its a real pain but we could count the ratio by removing the plugs and turn the motor by hand to count revolutions for one
revolution of the prop.You'll have to mark prop and crank pulley to be accurate.

The tach is a suspect, probably even over the gear ratio. I thought I had overcome that point, but maybe not. Like I said, this isn't a front burner issue ... just starting to work on it.

I'll go back to messing with the tach ... might just have to replace it, but can't find anything that'll fit in it's spot.

Thanks y'all.