Is this a Cavitation Problem?

S

Steve O

Active member
Motoring along the other day at top speed. Suddenly, while the motor continued at top revolutions, the boat slowed way down.

I turned down the throttle, and proceeded slowly for a while, then accelerated and got up to top speed again no problem. Later, the same thing happened.

Does this sound like the engine is mounted too high, or could it accidentally be slipping into neutral, and the forward progress was just coasting? This is a very old motor (I think from 1985)

I'm a real motor dummy, so It'd be great to hear from some folks with motor knowledge. Thanks in advance for any knowledge you care to share.
 
Steve,

If everything has been working fine before this and you have made no changes, then I would suspect the prop hub is slipping. The prop has a rubber insert between the inner hub ( the splined part) and the outer hub(the part with the blades). This rubber insert may have deteriorated and is slipping under full load. A prop shop can fix it right up for you.
 
Steve,

If everything has been working fine before this and you have made no changes, then I would suspect the prop hub is slipping. The prop has a rubber insert between the inner hub ( the splined part) and the outer hub(the part with the blades). This rubber insert may have deteriorated and is slipping under full load. A prop shop can fix it right up for you.
Click to expand...

Ditto
Lou
 
I think Dave got it right. This exact thing happened on my 9.9 a few weeks ago when I was getting ready for the season. Left the ramp, through the no wake area. Everything was fine I throttled up , got to speed, let off, throttled up and nothing just bubbles. It would just let me go at an idle speed. Got her to the shop and I had spun the hub on the prop. Since the prop was so small and pushing another hub in was a pain I just replaced the prop.

Bill
 
Yup! Same thing happened to me on my 9.9 last year. I suggest just changing the prop before you go to the expense of putting it in the shop. New prop fixed me right up.
 
Thanks folks. This doesn't sound too bad. Even if it's not the problem, I'm guessing it's a good idea to have an extra prop along in case one hits a rock. So there doesn't seem to be much harm in trying a new prop.
 
,Dave, I have a 6 hp evinrude in the barrel i tested the foward and reverse all works well ,but when on the open water I'llget up to top speed then it seems to jump out of gear, I'll idle down put it in gear go full foward and does the same thing jumps out of gear do i have the same problem{spinning hub} thanks ,Cas
 
[font=Verdana,Arial,Helvetica]Cas

Your motor problem sounds like worn gears. There are four separate "gears" in a lower unit. Three of them are constantly engaged and turning any time the engine is running. The fourth "gear" is a slider which slides back and forth on the prop shaft. When it is shifted to the center position, it does not engage any other gear so this is neutral. When it is shifted away from the center position, it will engage one of the gears (previously mentioned) which are turning all the time. Shifted one way you have forward, shifted the other way you have reverse.

When the slider and the two gears which engage the slider, become worn, they will "pop out of gear" under load. This is what is happening in your lower unit. Only fix is to replace these worn gears. Typically this is a job best handled by a professional mechanic with the right tools.

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Steve,
Had the broadbill out today and thought you jinksed me. Same thing you discribed happened.
I pulled up the motor and there was a ball of weeds on the lower unit!! I was in about 15 feet
of water in the middle of the bay.
Go Figure!! John
 
Sorry to jinx you. I went out yesterday and the thing worked fine with no fix. If it is as suggested, or if it is the worn gear version of the problem, I imagine I'll have to get it fixed soon.
 
Dave thanks for the reply,YOU da man ,I new you could help me out thats what is so grate about this site pefect strangers helping other strangers out in time of need. Thank You Cas A.
 
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