pontoon prop blowout,cavitation,vent.?

[shadetree918]

I have a 91 Lowe 24' toon with a 78 Evinrude 140hp running a 13.25 x 21p prop (not exactly sure about the dia. but pretty close). Problem is the engine will act like I threw it in neutral at full throttle or the prop fell off. Really does this quick if I hit the gas hard but if I ease into it slowly it still does it but not as fast. RPM's shoot way up to about 6000 or so til I cut the throttle back. I am actually going slower that a toon with a 90hp. should I try a higher pitch or what? Thanks.

 

[WillyBWright]

Re: pontoon prop blowout,cavitation,vent.?

Sounds like a spun hub, and a 21 on a toon is crazy. I'd think a 17 would be far more likely. 21 is for planing boats. Your motor needs to turn 5500rpm at full throttle under an average load. I'll bet you're nowhere close when it's working right.

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

how can I tell if it is a spun hub? why is a 21 crazy? I can never get to full throttle, "blows out". I may be able to get to half running about 3000 but not sure if the tach is correct. By the way the prop is a 12.75x21 aluminum. How low should the prop be setting in reference to the toons or whatever? There are existing 1/4" holes in the top of the anti-cav plate from one of the hydrofoil doohickys. Could this be a problem? Do I need to repair the holes.

 

[Bondo]

Re: pontoon prop blowout,cavitation,vent.?

Scribe a Line across the end of the Prop, marking the prop body,+ the Hub.....
Then Run it,+ induce the Slippage.....
Shut it Down,+ Recheck your Line,.......

Chances Are,.... The Line won't line up anymore.....

If Not, Replace the Prop,+ Try Again........It'll probably be All Fixed......

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

Ok, will try that as soon as I can get it back in the water. Thanks for the help.

 

[kp913]

Re: pontoon prop blowout,cavitation,vent.?

Have some cup added to the propeller, this will help alleviate the blowout problem you are having.

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

Do I still need to replace the prop with a lower pitch like a 17 or so? I have a 21 now and it is brand new. About how much does it cost to add cup to a prop?

 

[kenmyfam]

Re: pontoon prop blowout,cavitation,vent.?

You need to check for that spun hub first. Then verify the tach and see what your WOT rpm is.
Chances are that with a 21" pitch on the toon you are way overpropped. I would be thinking a 15" pitch to try out first.

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

Well, I tried turning the engine over with the prop while in gear and it turned the engine. Didn't seem like a spun hub although it does "give" a little, supposed to do that right?

 

[Frank Acampora]

Re: pontoon prop blowout,cavitation,vent.?

Your prop has a rubber insert between the hub and the prop body. This hub and rubber insert combo is pressed in with a big hydraulic press and plenty of lubricant. Its purpose is to "cushion" the prop shaft and help prevent damage in the event of a prop strike. ---Maybe---
"Spun hub" refers to this rubber insert wearing out or deteriorating and no longer gripping the prop body tightly. Thus when you apply too much power, it slips inside the prop body and you lose thrust. So, if you scribe a line as directed above, you will quickly see if the hub is spinning within the prop body. If your 21 prop is brand new, take it back if it is indeed slipping and see if the dealer will repair it. Don't despair if he won't though. Rehubbing is not too expensive.

 

[Frank Acampora]

Re: pontoon prop blowout,cavitation,vent.?

BTW even with a spun hub, turning by hand, you cannot generate enough force to make it slip.

 

[kenmyfam]

Re: pontoon prop blowout,cavitation,vent.?

Have to give it true "load" conditions to check for the spun hub.

 

[rickdb1boat]

Re: pontoon prop blowout,cavitation,vent.?

Could be cavitation too. Try trimming the engine in when it does this. You may have it trimmed out too far and the prop is "Blowing out". I would have no more than a 15" pitch prop on that set-up....Do you have a tachometer on the boat? If not, adding one would be a good idea...

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

Does it with no trim on the engine. I have a tach on the boat but not sure how accurate. Going to get a portable tach and add to engine to see if there is a difference between the two.

 

[walleyehed]

Re: pontoon prop blowout,cavitation,vent.?

Just to add a note to this..I have a customer with the same toon with a newer 115 (Same as your older 140) and at 3200ft it will turn a 14X13P at 5700 RPM...set up by myself. at or near sea-level, I would guess your correct prop to be in the 15P range to get the proper RPM.

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

What diameter prop should I use with the 15p or 17p prop? I have a 12.75d now. What about those "hydrofoil" and "dolephin" add-ons to the engine? Had one on there and removed it when I moved the engine to the toon. Thanks for the help.

 

[walleyehed]

Re: pontoon prop blowout,cavitation,vent.?

If the foil would help in eliminating venting then yes, but your diameter of 12-3/4 sounds like an older omc prop. If it were me, I'd be looking for a 14X15, or 13-3/4X15, (both cupped) to test..I don't think that will be far off from 5700-5800.

 

[shadetree918]

Re: pontoon prop blowout,cavitation,vent.?

the 12-3/4 prop is brand new, I guess it could have been an old brand new prop. I will ask my prop shop dude if he has a 14x15 prop I can try. Will that size fit on my 140 rude without hitting the plate? Thanks again for the help.

 

[walleyehed]

Re: pontoon prop blowout,cavitation,vent.?

the 14's designed for V4's will clear just fine.