60% Prop Slip????

[boatinjoe]

I have a 1971 Nylox Cabin Cruiser. Recently acquired by someone who was a hack mechanic (so nothing is off the table). The boat has a 215HP OMC Stringer. The motor is running pretty good at this point (smokes a bit when running at 2000RPM or above but not bad). I've been slowly diagnosing problems on the boat, and replaced the prop due to a spun hub. I bought a 14" pitch prop and hit the water. Runs nice, motor doesn't seem to be working very hard to push it along, but i'm having to over rev the motor to get on plane and once I'm there I'm spinning the motor pretty fast for the speed.

5000RPM - 22MPH
With the gear ratio of 1.16:1 I'm calculating about 60% slip. This just doesn't seem right at all.

I've been playing around with all of the different gear ratios from similar years for this drive and I found one combination (16:28 Upper with stock 15:23 lower) that calcs out to be 10-11% slip...... Is it possible that this boat has the wrong upper unit on it?

Am I going down the wrong path here?

If not - anyone know if I can just swap gears? Where might I pick them up?

Help! (yeah i know stringers are old crap drives... but too late to change that.)

 

[sw33ttooth]

Re: 60% Prop Slip????

i wouldnt say its crap, you just cant get a lot of parts and there reallllllllllllly old. i've seen a lot of stupid and wierd odd ball things, every thing is possible. i dont know anything about gear ratios but if the last owner is what you say and the bolt patterns match up the same, its all possilbe he had a problem and that was his fix.

 

[Howard Sterndrive]

Re: 60% Prop Slip????

OMC never made a 16:28 upper for V8's - it is only for outboard powerhead setups under 100hp and it wont bolt up to anything you have and you'd never find one anyways
but I think you need to verify the tachometer accuracy with a shop tach first. Then change props to get the correct rpm - probably 4400 @ wot would be a good target for that engine
I think you'll find your tach is the only problem.

 

[Don S]

Re: 60% Prop Slip????

You are way off on your gear ratio, what you have is a 2.04:1 gear ratio for a 90 hp motor.
You should have 21:16 upper and 15:23 lower with an overall gear ratio of 1.16:1 for your 215.
Then you find the correct prop to get your WOT rpm within spec.

 

[boatinjoe]

Re: 60% Prop Slip????

OMC never made a 16:28 upper for V8's - it is only for outboard powerhead setups under 100hp and it wont bolt up to anything you have and you'd never find one anyways
but I think you need to verify the tachometer accuracy with a shop tach first. Then change props to get the correct rpm - probably 4400 @ wot would be a good target for that engine
I think you'll find your tach is the only problem.

Didn't think of bad tach. Seems to be working - real stable idle measurements, moves smoothly as you rev. But you could be rigfht. I'll be checking that out first thing

 

[boatinjoe]

Re: 60% Prop Slip????

looks like a bad tach reading.... bought an optical tachometer and marked both the crankshaft and prop. using reasonably stable reading at fast idle I got 1.19:1 gear ratio... close enough to be right.

now to get a working tach on the boat.

 

[Howard Sterndrive]

Re: 60% Prop Slip????

might be a dial on the back of it for 4/6/8 cyl, or different terminals to connect the grey wire for # cyls