Same size prop replacement - Cavitation now

waterside

Cadet
Joined
Jul 10, 2007
Messages
9
WTF. I own a 1996 26' Bayliner Cierra with a 5.7L Mercruiser and Alpha I outdrive. I damaged my prop and bought a Mercury replacement with the exact same diameter and pitch (16"x16 pitch). On my test drive, the prop would begin to cavitate at various acceleration rates. Most noticeable was when I jammed the throttle wide open to quickly get on plane. The boat RPMs would start increase at a normal rate, then accelerate rapidly. The boat would then slow its acceleration as the RPMs increased. I can get the boat on plane with slow throtlle manipulation while listening for the sounds of cavitation and backing off on power. My old prop did not cause the boat to cavitate, or if it did, it was never noticable by the RPM gauge or make a distinct sound. The new prop is the "flow through" design that does not have a solid splined hub like the original prop. Can anyone tell why a direct size replacement prop would cause this problem. I did not know my exact WOT PRMs before I bought the new prop, but now they are at 4200. I belive my boat should be at 4600-4700 WOT. Can some tell me the size prop I need. My research says I should go to an 18 pitch. What about three blades vrs. four blades. I want to stick with AL. Any suggestions on which manufacturer? Can I run the new prop for the rest of this season and not cause engine damage? Sorry for the long thread, but this is extemely fustrating. I just spent two boat bucks for nothing.
 

Don S

Honorary Moderator Emeritus
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Aug 31, 2004
Messages
62,321
Re: Same size prop replacement - Cavitation now

Moving to the Prop Questions and Topics forum
 

waterside

Cadet
Joined
Jul 10, 2007
Messages
9
Re: Same size prop replacement - Cavitation now

Thanks Don, I just realized my mistake as I read an old thread on props and saw your response it needed to move to the correct forum.
 

thelightning

Cadet
Joined
Jul 22, 2007
Messages
28
Re: Same size prop replacement - Cavitation now

use a four blade for fast accelleration, also good for skiing and stuff.

a 3blade is faster... but your at 4200 and u can be at 4700 you should decrease the pitch.. not increase.

Its upto you what you do with the boat. adjust your prop to that.
 

MikDee

Banned
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Jun 6, 2007
Messages
4,745
Re: Same size prop replacement - Cavitation now

Was this a new prop, or reconditioned? are you sure it's cavitating? something don't sound right, like it's slipping on the hub instead of cavitating. Or, your drive coupling off the engine maybe slipping, maybe this happened when you damaged your old prop.
 

waterside

Cadet
Joined
Jul 10, 2007
Messages
9
Re: Same size prop replacement - Cavitation now

got it, decrease pitch to increase RPMs. I use the boat just for cruising, so best speed for the best fuel burn rate is my desire. Any reason why the direct replacement size prop cavitates while the old one did not?
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Same size prop replacement - Cavitation now

Along with the prop did you also buy the correct adaptors required?

The only things you should reuse from the old system is the thrust washer and the prop nut. The thrust washer is the @ 3" dia washer that goes on before the prop is installed.

Then the correct plastic adaptor @4" long, fits inside the prop hub, the new prop, the new spline adaptor which is also the propnut washer.

I suspect one of these parts is missing and the prop is acting like a slipping hub.
 

MikDee

Banned
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Jun 6, 2007
Messages
4,745
Re: Same size prop replacement - Cavitation now

The new prop is the "flow through" design that does not have a solid splined hub like the original prop.

Are you saying you have a new style prop with a separate core? Then somethings missing, or broken with it.
 

waterside

Cadet
Joined
Jul 10, 2007
Messages
9
Re: Same size prop replacement - Cavitation now

Yes, thanks for jogging my memory. I believe you are correct, I think I did not take off the old thrust washer and now have a doubled up washer condition. This would explain why I had a small gap between the prop and the lower unit housing while the prop was tight and free of play. Duh

I'll checK this as soon as I get home, but I'm sure I did not take a thrust washer off with the old prop.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Same size prop replacement - Cavitation now

The problem you have if you indeed doubled up the washer is that exhaust gas creates bubbles behind the prop blades causing the cavitation. This is the same effect one gets by ventilating a prop which consists of drilling a hole behind each blade to aid hole shot. You have created what might be described as a "super ventilated" prop.
 

waterside

Cadet
Joined
Jul 10, 2007
Messages
9
Re: Same size prop replacement - Cavitation now

Yes indeed, makes perfect sense................that's why I'm a Cadet and getting my problem answered by Admirals and above :)
 

wcsparky05

Petty Officer 1st Class
Joined
Feb 5, 2007
Messages
204
Re: Same size prop replacement - Cavitation now

what is cavitation, again? boat listing to one side?
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Same size prop replacement - Cavitation now

Although cavitation and ventilation are very different, the two terms are often and incorrectly used interchangeably. The problem here is ventilation where exhaust gas has created small voids behind the prop which causes the prop to lose bite. It has nothing to do with the boat listing to one side or another.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Re: Same size prop replacement - Cavitation now

Cavitation is water boiling in an area of low pressure. As most people know water will boil at lower temps when the atmoshereic pressure is reduced. Lower the pressure enough and the water will boil at 0 degrees C. The same thing happens underwater, in low pressure areas on the prop blades, hub, and behind protrutions on the gearhousing. When these vapour bubbles collapse it is like a tiny jackhammer and will, over time erode the metal.
 
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