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Discussion Starter #1
If there is a "Prop 101" thread, I'd love to read it first before I ask for anyone's time - out of courtesy. Please point me in the right direction if there is such a link.

'92 Hawaiian 21' open bow. OMC 351, Cobra drive. Current prop is 21P.
(other numbers on the prop - 48 832832 445 21P )

The boat seems to top out at 35mph (per GPS, don't have a speedo in the boat). It'll pull up to 5,500 before I am overcome with fear of blowing the motor - there's a tag on it that says max RPM is 4,000-4,400.

Am I in the right ballpark with this setup, or is something screwy? I'd like to think the boat should go a little faster than 35, is the prop my limiting factor?

Ultimately, I'd like to have two nice props - one for pulling a skier/wakeboarder, and one for when the wife's back at camp, and the boys are out finding someone to race. That being said, I know who NOT to pick on, so let's not get carried away by turning this into a "junk that OMC and replace it with a BBC and Bravo drive" thread. Would love to do that, but pretty sure my wife would leave me.

I'd like to avoid spending a ton of money buying props, putting it in the water and saying, "well, that's not it. Let's try again."

Thoughts?
 

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How many blades does the prop have? I agree, 35mph isn't right for your boat. My Ski Centurian tournament boat runs about 45mph(and tourny boats are not known for speed), and it also has a 351 W. Has the prop ever been damaged?


Darrell.
 

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I just did a prop slip calculation. If your gears are 1.5 it's 52%. If they are 1.62( not sure about OMC ratios) it is 48%. Either way, something is not right. Your boat should be running around 45-50 easily.


Darrell.
 

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Has anyone played with the outdrive?

You either have a slipping torque coupler or prop hub, or soemone changed the gear ratio, even with the lowest gear ratio available, something is FUBAR, 5,500 RPM should get into the very high 60's to the low 70's...:)bulb

GT :)hand
 

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Started brainstorming after my post....You need to make sure and use the right hardware on the Mercury propeller on an OMC shaft.

Pull the prop and if it is spun the evidence will be a scarred thrust washer on the taper of the shaft and washer. You could also put the shifter in reverse and rotate the prop forward with your foot and see if it slips (wear good shoes), may need to lean into it pretty hard. Just be mindful that the engine may turn over...

It is also imperitive the nut gets torqued to 60 ft/lbs.

I would put hard earned cash on a spun hub.....Find someone with a good used prop for an OMC and try it. Either an aluminum or stainless would work for this test.

Do not run that small block ford harder than 4800 RPM, you will hurt it. These motors in the OMC's are not as tough as you might think (automotive crate motors).

BTW, that boat should run in the high 50's, maybe 60, but high 50's for certain.

GT:)hand
 

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Let's hit the water!
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Discussion Starter #6
Thanks for all the info. Will look into all of it.

Would the fact that it's a 14 1/4" prop have anything to do with it? 3-blade prop, too, for the guy who asked.
 

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Thanks for all the info. Will look into all of it.

Would the fact that it's a 14 1/4" prop have anything to do with it? 3-blade prop, too, for the guy who asked.


The prop you have should work great with that small block. Problem is, your speed is off by about 20mph. I think GT Jets called it right. Check that prop hub. Good news is, if it is the hub, it's a cheap fix.



Darrell.
 

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Let's hit the water!
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Discussion Starter #8
So I went out to take a look-see, and I'm going to go out on a limb and say that finger-tight probably isn't quite tight enough. Pulled the cotter pin out, pulled everything apart, and looks like all the splines are ok. Put everything back together, tightened her down to 60 ft. lbs. We'll see how she does today. Will report back. Thanks for all the input.
 

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Discussion Starter #9
So now I'm thinking it's a whole different issue. I think my tach is off. It shoots up to 5,000 rpm like nobody's business, but really doesn't sound like it's screaming the way a small block Ford does at 5K. So that's next on my list.

Boat ran great today, even "past" 6K rpm, or so my tach tells me. Forgot the GPS at home, so no idea how fast we were really going. Didn't stay at "6K" too long anyway, cop boat seemed to be wherever I was.
 

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So I went out to take a look-see, and I'm going to go out on a limb and say that finger-tight probably isn't quite tight enough. Pulled the cotter pin out, pulled everything apart, and looks like all the splines are ok. Put everything back together, tightened her down to 60 ft. lbs. We'll see how she does today. Will report back. Thanks for all the input.
Sounds like two things...tach is set for 6 cyl and I would have the hub in the prop checked.

The splines rarely if ever fail, the hub is a rubber shock mount that is vulcanized to the center section of the prop and pressed into the prop with Carrow syrup (I know, sounds weird, but totally true).

Finger tight is no bueno....FWIW still should be in the high 50's .

GT :)hand
 

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Let's hit the water!
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Discussion Starter #11
Prop is tightened down to 60 ft. lbs. now, no worries there.

Checked the tach, set for 4cyl. Corrected that. We'll see how she does now that I won't be terrified of "6K" rpm.
 
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