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  #1  
Old 05-04-2012
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Prop Cavidation: Vibration-Caused?

Back in 2008, Abracadabra spent most of her time in her slip due to what turned out to be a Martec prop problem. Or so we thought. For various reasons, Abracadaba saw little use in the following three seasons. Taking Abracadabra over for haulout, last fall, the problem was back: Attempting to accelerate too hard or exceed a certain speed (based on headwinds and water): And she'd "break free." There's no way that Martec prop has enough time on it to need a rebuild again.

But there is this...

When we had the stuffing box hose replaced by our yard, season-before-last, the yard noted "Did you know your prop shaft is bent?" Me: "Uh... no?" Yard: "It is. Not badly, but it is bent."

Here's my theory: In order for a prop to work, there must be attached flow on its blades. Cavitation is what happens when you lose that. My theory is the excess vibration and disturbed motion of the prop is causing the problem.

The yard pulled the prop shaft earlier this week. Turns out that, not only was it bent, but it was out-of-round and, as the yard put it, "just plain worn out." Turns out the cutlass bearing was done for, too. (Me: "Huh. The yard that swapped props for us said it was fine, then." Them: "When was that?" Me: "Two or three years ago?" [Turns out it was four.] Yard: "There's no way that cutlass bearing was any good two or three years ago." Interesting.)

Our plan, so as to get her in the water and actually, you know, sail, this season, is to put the Michigan Wheel on the new prop shaft (we don't trust the Martec any more), live with the light air performance hit, and, once our finances have recovered, put a Gori folding prop on.

Thoughts?

Thanks,
Jim
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Old 05-04-2012
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Re: Prop Cavidation: Vibration-Caused?

I had prop cavitation at cruising rpm due to a bent shaft strut.
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Re: Prop Cavidation: Vibration-Caused?

Sounds like a plan, although a 'sound' Martec folder shouldn't be a big issue with vibration (but always a bit more than a fixed prop). A Campbell Sailor prop might be an attractive alternative. Fixed prop with less frontal area than a michigan wheel.

FWIW we are thrilled with our 2nd hand Max prop.. having had a Martec folder I can vouch for the improvement of the Max - in all areas, drag, reversing, thrust... A less costly alternative would be the Kiwi feathering prop, though there have been reports of overpitching in reverse that can stall smaller engines.
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Re: Prop Cavidation: Vibration-Caused?

To me, the bent shaft, and the worn cutlass bearing and the excessive vibration go hand & hand. Any use of the engine depending upon how badly the shaft is bent could quickly trash the bearing. And the bad bearing will also quickly trash the shaft.

I too, had excessive vibration, and a worn cutlass bearing. It turns out that my 25 year old bronze shaft was less than perfectly straight, and I had a bad motor mount. I replaced the motor mounts, the shaft, the cutlass bearing, and added an improvement (PSS Shaft seal) while I was at it. Look here for the details; Drive Train Maintenance

Is it at all possible that the shaft could have been bent either before, or shortly after, launch with the new prop? If so, you may be throwing the baby out with the bathwater.

I suggest fixing ONE problem, before potentially introducing another. I would fix what is known to be broke (shaft & cutlass bearing) and take a close look at the motor mounts and alignment, before I switched back to the old Michigan prop.
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Re: Prop Cavidation: Vibration-Caused?

Quote:
Originally Posted by PBzeer View Post
I had prop cavitation at cruising rpm due to a bent shaft strut.
This was more than that. The shaft was bent, out-of-round and, it turned out, mis-aligned with the engine to the point that the bearing surface in the shaft log was turning the shaft down and the cutlass bearing was wearing unevenly.

Quote:
Originally Posted by Faster View Post
Sounds like a plan, although a 'sound' Martec folder shouldn't be a big issue with vibration
Perhaps the excess vibration was causing premature wear on its bearings?

Quote:
Originally Posted by Faster View Post
FWIW we are thrilled with our 2nd hand Max prop...
That would be sub-optimal in our waters. Lots of weeds. I mean lots of weeds. Floating golf-course-greens-sized patches of 'em later in the season. That's one of the advantages of a folder, in our environment: Won't catch the weeds.

Quote:
Originally Posted by eherlihy View Post
To me, the bent shaft, and the worn cutlass bearing and the excessive vibration go hand & hand. Any use of the engine depending upon how badly the shaft is bent could quickly trash the bearing. And the bad bearing will also quickly trash the shaft.
Plus the shaft/engine alignment was way off, it turned out.

Quote:
Originally Posted by eherlihy View Post
Is it at all possible that the shaft could have been bent either before, or shortly after, launch with the new prop?
We're thinking it might've happened either at our first haul-out, but hard to say.

Quote:
Originally Posted by eherlihy View Post
I suggest fixing ONE problem, before potentially introducing another. I would fix what is known to be broke (shaft & cutlass bearing) and take a close look at the motor mounts and alignment, before I switched back to the old Michigan prop.
The P30 doesn't have motor mounts. Motor's mounted directly to the engine bed. They're going to have to shim to fix the engine/shaft alignment problem

As for the Martec: I'll take that under advisement, but we just don't want to miss another season because of either personal sickness or boat sickness. That's why we figured on just putting the Michigan Wheel on, for now, and being done with it.

Thanks for the follow-ups, everybody.

Jim
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