Prop Hub High RPM

ronmax99

Cadet
Joined
Feb 10, 2005
Messages
20
My engine will run fine at about 4000 RPM for awhile, and then it will slow down and the RPM shoots up to about 6000. If I throttle down and slowly accelerate, all is well again. I am assuming that this is being caused by a defective prop hub. My prop has big nicks and I am going to replace it anyway. Is there anything else that could cause this RPM/speed problem. Also, how do I select the proper replacement prop.<br />Thx.
 

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
Re: Prop Hub High RPM

Hi ronmax.<br /><br />Sounds like it could be the hub. Either that, or the prop is cavitating/ventilating at higher RPM's while underway. But you would know if the hub was spun, by the way it behaves during hard acceleration from a standstill. It would rev under a load without really moving the boat if it's spun. As for prop selection, I have a 70HP Rude on my 24' toon and I run a 13" cupped prop. I'm betting you would also do well with the same pitch on the 90 HP motor. It depends on what RPM's you are getting now at Wide Open Throttle. You would want to shoot for 5800-6000 top end RPM's with a light/normal load on the boat. 13" pitch would be a good place to start....<br /><br /><br />Edit:<br /><br />After re-reading your post again, I'm more inclined to think your prop is losing it's bite on the water, rather than being a hub problem. A little more info about how it takes off from a standstill and how you have your motor trimmed(In/out) would tell more of the story. Also, has it always done this or is this something that just cropped up as of late?<br /><br /> ventilation
 

ronmax99

Cadet
Joined
Feb 10, 2005
Messages
20
Re: Prop Hub High RPM

I have tried various trim degrees but nothing seems to help. If I try to go from stop to full thottle the problem is there right away, high RPM no accel. This problem seems to have been always there. I bought the boat around 2years ago. Last year I replaced the prop with a re built one, and the problem I beleive got worse. I think that the pitch on the prop is 22, I do not recall. however, I can check. Full RPM is about 5000 at 22RPM when all is well.
 

imported_bjs

Petty Officer 1st Class
Joined
Jun 21, 2005
Messages
368
Re: Prop Hub High RPM

take a white or something you can see make a mark on your hub going across to your prop nut take and run your boat then when you take boat out of water see if mark has moved if moved your prop hub is spun.if not you have other issue my boat did same thing turned out the prop was just wore out to many nicks etc. etc.
 

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,721
Re: Prop Hub High RPM

Gotta determine if it is a slipping hub or ventilation. A prop shop could check out the hub for you.
 

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
Re: Prop Hub High RPM

If I try to go from stop to full thottle the problem is there right away, high RPM no accel.
Sure sounds like the hub to me, although I can't explain why the problem might have gotton worse after you got another prop(Although being re-built could have been the problem). I would do as the others suggest and either mark the shaft and hub, then run it and see if the marks change, indicating a spun hub or take it to the prop shop for diagnosis. BTW, 22 pitch on your toon would be WAY out of line. Double check that pitch and get back to us. If you're hitting 5000 RPM's, you are about 1000 below where you need to be..
 

ronmax99

Cadet
Joined
Feb 10, 2005
Messages
20
Re: Prop Hub High RPM

The prop indicates 13 X 14. I do not recall what these numbers mean.
 

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
Re: Prop Hub High RPM

Probably a 14" Diameter X 13" pitch... Pretty close to what you need, as I said in my previous post... Your Tach could also be off a bit, but you should be running more RPM's than indicated... Speed sounds good, though..
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: Prop Hub High RPM

Numbers figure out to about 28% slip at 5000 and 22mph with a 13P....maybe slightly high, even for a 'toon...I would think 23-25% would be closer, but then again, we may have that much wear on the prop, and if it's slipping (Hub), which it's sounds like it IS...that may be the difference. If it IS slipping and 5000 is all you get at WOT, we need to get a lower pitch on there and get the RPM up past 5500...5700-5800 would be best.<br />According to your statement that it slips or "Revs" when you hit the throttle, indicates pretty well your hub has cut loose.
 

ronmax99

Cadet
Joined
Feb 10, 2005
Messages
20
Re: Prop Hub High RPM

Thanks for the info. I bought a new prop 14X11, I hope I went in the right direction. I have the boat out of the water right now for minor maint. I will follow up when I get it back in the water to let you all know how the new prop is working.
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: Prop Hub High RPM

V4, 13 spline would be 14" Diameter in those.......11 and 13 pitch. At 15P, diameter drops to 13-3/4...<br />I think he's on the right track..
 

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
Re: Prop Hub High RPM

Yep, 14" X 11" pitch sounds good... good luck and let us know how she runs...
 
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