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Discussion Starter #1
Good afternoon!

I have a early 90's 8HP Mercury Mariner. Overall the engine runs really good, however I have noticed when I run anywhere from 75%-100% throttle, it begins to slip. I can hear the engine grow louder, but it begins to slow down.

I typically slow down when it starts to do it and putt my way in. It also happens periodically. Could it be my prop? I appreciate any advice or suggestions.
 

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No shear pin, must be a thrust hub prop.

http://www.crowleymarine.com/mercury-outboard/parts/496_210.cfm

If the pressed in rubber hub wears out, it'll slip.
 

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Post a pic of the prop. Sounds like the prop may be damaged causing cavitation. I'm thinking if the boat typically planes at 75-100% throttle (less torque required) and it aint slipping out of the hole (most torque required) then the rubber hub should be good. Did it ever run without slipping or has it always slipped?
 

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A picture of the Propeller, before dissembling.



Top nut off.



Second "washer" nut off.



Propeller off.



Final bottom washer off.



All pieces shown.

Is it strange that i'm not seeing a rubber hub what so ever? I will say this is a cheaper replacement propeller (Solas). When I first replaced it, it never slipped. However over the past 6-7 times running full, it has began to slip. I appreciate any advice.  Please let me know if I should take any additional photos.
 

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Front of Propeller



Back of Propeller

The more I look at it, the more it looks real beat up. Is this a case of buy it cheap, buy it twice? Or is there a fix?
Thanks guys!
 

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From the wear marks. Looks like hub slip to me. As in the once enough torque is applied the rubber hub will allow the the shaft to spin in the the prop. That's why you hear the RPMs raise and speed decrease. Any prop shop can press a new hub in. Or it may be cheaper to buy a new one somewhere. Or upgrade to a SS prop.
 

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I'd use a little paint or scratch the surface of the metal to mark the prop in relation to the hub. Run it. Then disassemble and see if the mark still lines up. If not then you know the hub is slipping.
 

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I went ahead and ordered a new prop. Mercury factory this time.

Do you think its necessary to apply more grease to the propeller pole (Shown w/ blue grease in the picture)?
 

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Discussion Starter #10
Oh man.

Well went ahead and ordered a new propeller, Mercury this time. Installed it and took it out on the lake for spin. Well lo and behold, this brand new propeller began slipping at full throttle. It seems like the engine begins to vertically shake some then RPM's fly while the boat slows. Very frustrated. Fellow gentlemen, any other ideas of what the problem may be? Prop shaft? I can also take more pictures if needed.

Thanks for all your help so far.

Whooper
 

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Discussion Starter #12
I was also looking into possible Cavitation issues. The only reason I doubted this at first however was because the boat ran fine many of times without the slipping/bogging. I could literally run WOT for a good 15-20 mins and then it would happen. Or yesterday for example I would run WOT for 2 mins before it began happening.

Has anyone had issues with Cavitation?
 

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Discussion Starter #14
Did you move anything around in the boat? Maybe a gas tank, or battery.
Only a small gas tank (3 gal) and no electronics on board. The boat literally weighs less than 300lbs. I will post pics when I get home this evening.
 

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Discussion Starter #15
Well thanks for all the tips.

I took it out yesterday and it ran fine at around 75% throttle. Honestly the engine is an old dog, and last thing I want to do is dump another $400 for repair. Ill just rock 75% till she dies!
 

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How deep does the lower unit go into the water. The anti-ventilation plate, (AKA) cavitation plate, should be about a half inch below the bottom of the boat.
Frank-S
 
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