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Old 01-01-2011, 10:40   #1
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Pitted Prop Shaft Repair ?

Yes, I know I should just replace it. I'm trying to save some money where it's possible, my financial outlook isn't good now but in a couple of years it will be better, I'm just tired of waiting to go cruising and trying to get started sooner rather than later.
My prop shaft is pitted in the area inside of the shaft log. It's not in a sealing area or where either of the bearings mount. It's between the shaft packing and the cutlass bearing. The largest pits are about the same mass as 3-4 bb's, most about like 1-2 bb's and there are a couple dozen pits spread out along 12" of the 1 1/2" shaft. Can these be repaired? Should I ignore them?

On the Porsche racecars that I build, we can routinely run 300hp through a 1" shaft without any problems as long as the driver doesn't think he's a burnout king. 1 1/2" seems plenty strong enough for 50-60 hp even if it's strength is a little compromised.
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Old 01-01-2011, 10:48   #2
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Afraid I have no answer for your question, but as an aside.... Have your priced replacing it? My friend just replaced his and I was shocked how cheap the part was ($160). Might be cheaper than you think.
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Old 02-01-2011, 09:03   #3
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The problem is you can't tell how deep the corrosion goes into the shaft. I looked at that second picture and knew right away that if it was my boat I would replace the shaft.
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Old 02-01-2011, 09:24   #4
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The reason this has occurred is the lack of oxygen to the stainless steel. I know it sounds counter-intuitive, but stainless steel needs oxygen to help prevent it from going active.

One solution would be to get a drippless shaft seal with a nipple that allows pressurized sea water from the engines cooling system to be injected into the area between the cutlass bearing and the shaft seal.

I realize this is the last thing you want to hear because of the cost but it is something you could add in the future when replacing your corroded propeller shaft.
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Old 02-01-2011, 09:27   #5
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Quote:
Originally Posted by sww914 View Post
On the Porsche racecars that I build, we can routinely run 300hp through a 1" shaft without any problems as long as the driver doesn't think he's a burnout king. 1 1/2" seems plenty strong enough for 50-60 hp even if it's strength is a little compromised.
Yes, but when the driveshaft on a Porsche fails, you aren't left in the middle of an ocean with no motor.
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Old 02-01-2011, 09:32   #6
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Originally Posted by sww914 View Post
Yes, I know I should just replace it.
You won't got any argument from me. Yes, you should just replace it.
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Old 02-01-2011, 09:37   #7
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The limiting factor for a propeller shaft is torque and not horsepower. These are two very different things.

The way that it works is you can run a propeller shaft at 6000 RPM with little torque and produce a given horsepower. Or you can slow down the RPM, increase the torque and have the same horsepower produced by the propeller...essentially by increasing the pitch and/or diameter of the propeller.

Or in other words, in order to produce the same horsepower, when a shaft turns slower, the torque must be increased. When you increase the torque, the diameter of the shaft must be increased.
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Old 02-01-2011, 10:43   #8
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Quote:
Originally Posted by sww914 View Post
Yes, I know I should just replace it. I'm trying to save some money where it's possible, my financial outlook isn't good now but in a couple of years it will be better, I'm just tired of waiting to go cruising and trying to get started sooner rather than later.
My prop shaft is pitted in the area inside of the shaft log. It's not in a sealing area or where either of the bearings mount. It's between the shaft packing and the cutlass bearing. The largest pits are about the same mass as 3-4 bb's, most about like 1-2 bb's and there are a couple dozen pits spread out along 12" of the 1 1/2" shaft. Can these be repaired? Should I ignore them?

On the Porsche racecars that I build, we can routinely run 300hp through a 1" shaft without any problems as long as the driver doesn't think he's a burnout king. 1 1/2" seems plenty strong enough for 50-60 hp even if it's strength is a little compromised.
We discovered similar damage to our prop-shaft when we did our bi-annual haulout in January '09. I discovered that a repair--grinding out the pits, building up the shaft with weldment and then re-machining the shaft to size at a local machine shop--would have cost about as much as a replacement shaft with no warranty.

FWIW...
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Old 02-01-2011, 10:54   #9
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Welding on stainless may weaken the shaft. All you might do is to make it look pretty again with no gain in strength...possibly a loss in strength. I'm no expert at welding stainless but I do know that welding metal has the potential of weakening the metal or making it more brittle. You will want to consult with an expert on this.
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Old 02-01-2011, 12:38   #10
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I'd be afraid that pits of that size are a sign of unseen "crevice corrosion" which leads to sudden and complete failure in stainless parts. It will be way cheaper to replace that shaft now, than to call a tow and deal with all the consequences of a failure away from the dock.

Personally, I'd go with the new shaft. The old one can be tested, but between the cost of tests and repairs...cheaper faster easier better to just replace it. Shop around while the need isn't urgent.

I'd disagree about a dripless shaft seal, that's another expense and simply using a GoreTex dripless packing material should do very nicely with a new clean shaft.
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Old 02-01-2011, 12:44   #11
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I have had nothing but good luck in over twenty years I have been using drippless shaft seals.
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Old 02-01-2011, 13:33   #12
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So how about moving the location of the shaft seal forward onto a new and clear section of shaft. Put an extender tube on the shaft log to move the shaft seal forward.

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Old 02-01-2011, 13:44   #13
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No question replace it. As for shaft seals and injecting water yeah that would help but only when the engine is running. That is a very small time compared with the time the boat is in the water without the engine running, so it really does no good for this problem. Check for galvanic current flow just to be on the safe side.
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Old 02-01-2011, 14:11   #14
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I have had nothing but good luck in over twenty years I have been using drippless shaft seals.
Luck should have nothing to do with it.<G>

I have nothing about patent seals, but note that some folks have had them fail in ways a plain log simply can't. And that buying and installing them is of course an expense.

When the dripless seals were first invented and marketed--great idea. IIRC the dripless packing (GoreTex string or teflon putty) didn't come on the market until quite some time later, but it is available now, and much cheaper than a patent seal.
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Old 02-01-2011, 14:19   #15
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Yes, but when the driveshaft on a Porsche fails, you aren't left in the middle of an ocean with no motor.
Unless, of course, you're driving along a Californian cliff top road when the draftshaft fails on your twitchy rear-ended Porsche - then you may well find yourself in the ocean with no motor ;-)
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