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Old 06-24-2014, 09:42 AM   #1
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Prop Shaft Diameter Sizing

When a new propeller shaft is made how close to the size in question should it be? There must be a tolerance of how much less than the specified size it can be. I'm sure the stern (cutlass) bearing only comes in one size and the shaft is supposed to be fit to the stern bearing. One wouldn't want it tight of course as it would probably heat up and self destruct. One wouldn't want it loose enough to "rattle" around in the bearing causing excessive wear and noise.

Should a "clunking sound" be heard if one tried to move the shaft back and forth? I do hear a clunking sound and I think it's not right but how do I know how loose or tight the stern bearing to shaft fit should be? My shaft is 1.25" dia. How many thousands of an inch tolerance should be maximum? As the prop shaft becomes smaller how small is too small? I had a new prop shaft made and I think it's smaller than my 10 year oil shaft. I want to call the yard boys on this or be assured "that's normal". And I don't want to cause the yard any unnecessary trouble and expense. But I definitely want the stern bearing to fit the shaft properly.
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Old 06-24-2014, 10:00 AM   #2
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From ABYC P-6 for a 1.25" shaft:

The OD tolerance is +0.002" and - 0.001"

The bearing clearance is 0.003" - 0.007"
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Old 06-24-2014, 10:10 AM   #3
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Runout is the test of a cutlass bearing clearance. Grab the hub of the prop and push from side to side. It shouldn't move more than 1/32 to 1/16". Any more than about 1/8" and it is time to replace.

Although some cutlass bearings have more clearance than that and cause no trouble, ie vibration.

And I would doubt that your prop shaft is too small. They are made from cold rolled alloy and the diameter is tightly controlled. It would take a huge error in manufacture to have one with more variation than ABYC allows.

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Old 06-24-2014, 10:12 AM   #4
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No, there should be no slop, so side to side movement.. Are you talking front to back or side to side?

Go to the hardware store. You can buy or rent a micrometer to check it yourself. Did you replace the cutless bearing at the same time? Did they make the shaft from micing you old one which may have been worn? When I replaced my 1 1/2" 9' shaft last year it was undersized but that was from the Taiwan Boat yard 30 years ago. I replaced the shaft and both cutless bearings at the same time with standard sizes.

If it is front to back clunking, you have a completely different, and potentialy dangerous issue. Your coupling could be loose, your trans could be failing, and if your shaft became disconnected, you shaft could slide back and drop off, depending on your configuration, I've seen that happen first hand.

Good Luck
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Old 06-24-2014, 12:44 PM   #5
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Terry, David and Spy,
Thank you all very much. Love this learning.

Terry,
Yes new cutlass and new shaft. And I'm sorry. I can't believe I didn't make it clear it was side to side movement. It barely moves but the shaft does make a sound in between thudding and clunking. Off the top of my head I would think it shouldn't move enough to make any sound at all. So there's definitely clearance (a gap) between the shaft and the cutlass bearing. I'll go and try to measure it.

David,
For years I've been reading your excellent mechanical expertise. Spy is quoting the ABYC standards and the min OD of the shaft is only .001 under 1.25". The bearing clearance being a max of .007 means there should be a max clearance "slop" between the shaft and the bearing of .007". Since 1/32" is .306" I assume you must be talking about much bigger boats and shafts. But if 1/32" slop is fine I'm in good shape as I'm sure there's not that much clearance. Probably 1/64th or less. I need to go measure.
As a boat shaft is made from "shaft stock" I would assume it would be very very close to 1.25". But when it is machined into a completed shaft for a boat I think they "polish" the shaft after the machining is done. If they polish like prop man grinds the dia will be reduced. So the fact that I can feel some movement dosn't mean I haven't got a good fit nor does it mean I'm going to hear the shaft banging against the bearing.

Spy,
The .003" tolerance for the shaft is clear. However I'm confused about the bearing that appears to have .008" tolerance. ......... Oh I think I see the light. Neither the .003 or the .007 is in any direction. It's just clearance for the shaft to bearing fit. I have a caliper that's one revolution per inch but I've used it a lot and and can measure down to about .002 to .003 w a magnifying glass. My problem will be how to hold the caliper so I can repeat the measurements and confirm that my measurements are correct.
OK thanks much all I'm going to measure.
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Old 06-24-2014, 12:56 PM   #6
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Keep in mind that this standard is for design and construction. Which may be appropriate for a new shaft as in your post.

It is not realistic to think that these clearances are not going to grow during wear in, normal operation, and then to failure.

Loose is a relative term. A better indication if cutlass bearing wear is a visual inspection of the groove depth.

Keep in mind that none of these standards were used during Willy's construction, so the shaft removed may have been oversized relative to the new shaft.

You could always set up a dial indicator and push pull the prop to measure your sloptivity.

Troubleshooting often comes down to the simple children's game of "one of these things is not like the other" as taught on Sesame St.
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Old 06-24-2014, 01:41 PM   #7
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A clunking sound would be surprising even with loose bearing. There is just not that much room. Are you talking moving the prop fore and aft or side to side and top to bottom? Is the prop tight on the shaft??
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Old 06-24-2014, 04:08 PM   #8
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Bayview,
It's not a metal to metal sound. It's like an axe head thumping on a log. We're not yet at the boat. Had to stop at Starbucks. Blonde coffee and a double chocolate brownie. Then I'll be ready for the boat. The prop's not on the shaft.

Spy,
Wasn't the original shaft. I had this one installed 10 years ago. Aqua met 22. And I think the stern bearing fit was tighter last week when I wiggled the shaft. Don't have a dial indicator. I'm taking along a plastic slip stick to see how close I can get. If it's different every time I measure I'll get the dial indicator somehow.
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Old 06-24-2014, 07:44 PM   #9
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Measured several different ways and on about the 3rd or 4th try I did manage to measure consistently .015" total up and down movement. May have put a bit more force than necessary upwards so the clearance could be down to about .012".

That's a lot closer to the specs Spy came up with than what David was talk'in. I think I'll chill out and mention it to the yard guys and get them to say it positively won't make noise underway. I don't think it will actually but I really like to have things like this "unquestionable" at launch because hauling back out is so expensive. And the PSS is pumping water through the tube and the flutes of the cutlass bearing. The water under pressure should act as a cushion and that may be an advantage of the PSS. The only reason I didn't go back to a stuffing box was that the stuffing box requires much better alignment and any misalignment puts pressure on the stuffing box.

Another reason I'm OK w a bit more clearance (if in fact there is more) is that when I shook the old shaft the bearing probably wasn't perfectly aligned and that should make it feel tighter.

Thanks everybody for the assistance. Really was helpful.
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Old 06-24-2014, 07:57 PM   #10
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Eric:

I had a sailboat which probably has a similar power train to your slow speed trawler. It also had a typical cutless bearing housing bolted to the deadwood.

It had 1/8"+ of slop at the cutless bearing and didn't give a bit of trouble.

BTW 1/32" is about .03 inches, not .3 inches as you stated.

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Old 06-24-2014, 07:58 PM   #11
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Eric,
I have always worked on .001" per inch of shaft diameter.
This is for white metal bearings.
Wear can go out to a few thou but once it gets a rattle then it is time to inspect the bearing and shaft.
Ideally one should be re-metaling the bearing before wear is pronounced on the shaft.
The shaft can then be dressed with and oil stone and new bearing fitted.
Not sure on cutlass bearings for smaller boats as haven't fitted one for a long time but they do come in a standard size so shaft must be in good condition and have a good percentage of bearing surface.

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Old 06-25-2014, 12:04 AM   #12
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David,
Yea I'm not good w numbers and it's out of the bag now.
And I suspect on many boats the shaft will lay to one side of the cutlass bearing and not rattle at all. One may get more thrust from one side of a prop pining a shaft to one side if the intermediate bearing is pressing down, or from the weight of the shaft and prop. Or most likely because of water flowing at an angle through the blades like w a steep shaft angle. But some rattle away like it was their destiny.

Benn,
I thought about having the shaft metalized to a perfect fit. Would be commonly done if it was necessary or of great advantage but stern bearing fit isn't a subject like shaft alignment. Not that important it seems so I'm sure I'm "good to go".
White metal? ... Kinda like "Babbitt"? Soft metal married to hard metal for min wear? Re bearing surface my stern bearing is 5" long.
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Old 06-25-2014, 07:39 AM   #13
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Cutless bearings are not a tight fit. A little slop with new bearing and new shaft is normal. You measure 0.015" play, that is fine. Once run in, it will increase a bit, still ok. I consider 1/16 fine (0.0625). When running, hydro forces on wheel load the shaft against the bearing in one spot, so it tends not to clunk around. I've seen things worn to 1/8", and there were no complaints. Just noticed when hauling for paint. Good to change then, though.
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Old 06-25-2014, 10:09 AM   #14
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Ski,
It feels and sounds like the bearing is in perfect alignment w the shaft as the contact seems sudden when I jerk the shaft back and forth. I'd rather have .005" slop but I'm quite sure this is going to last for years. The last setup went 8 years w practically no wear .. should be fine. And yes I've seen boats w lots of clearance and the skipper is say'in "what slop?".

And of course another opinion in that direction is a plus.
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Old 06-25-2014, 10:24 AM   #15
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Eric: How bad was the old shaft and bearing? Only 8 years out of your old set up, what made you decide to replace both?
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Old 06-25-2014, 10:58 AM   #16
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Larry,
Good question and my omission. I had the yard remove my shaft to replace the PSS as it was 10 yrs old. The intermediate bearing was "froze" on and they cut it out/off. But they damaged the shaft. Ordered a new shaft before even talking to me. I would have liked one 1/4" longer. The old shaft was AM22 and I'm not sure what the new one is. One could say things coud'a gone better. I've got too much bad knee and arthritis for some boat jobs. Could have done it but w much pain.

But I'm OK and Willy is too as long as they did the set screws and keys properly but for awhile I'll be a tiny bit apprehensive w lots of throttle in reverse. I really like to do most thinks myself.
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Old 10-28-2014, 10:28 PM   #17
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Question Shaft sizing

I have a 97 34' mainship motor yacht with 454 gas burners. I have a little vibration coming from one and the service guy says I have a bent or warped shaft. No one knows what size so the mech recommended we pull he boat out of he water and pull shaft measure reinstall and order. Once it comes in we would do it again. I'm trying to eliminate as much down time as possible. Does anyone know anything about this.
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Old 10-28-2014, 11:34 PM   #18
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Sure it's a bent shaft? Our Mainship 37 (but with the same 454's) had a slight vibration problem on one engine, turned out one spark plug wire was loose.
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Old 10-29-2014, 12:05 AM   #19
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No I'm not. There was a small amount of water coming in around the packing where the shaft penetrates the hull, I called him to look and he thought it was a bent shaft. I called the previous owner and asked if he had maybe hit something. He said it had always had that vibration and was not aware of hitting anything. We keep it on a reservoir here and there are stumps if you are not familiar with the lake.
I'll check the plug wires in the morning though. The problem i also have here is there are only two mechanics that have the means to pull the boat out of the water, so I'm a captive audience and I don't have enough experience to tell myself. Any pointers?
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Old 10-29-2014, 12:21 AM   #20
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Is the mechanic competent with inboard engines, gear and shafts or is he just guessing?? It could also be just out of alignment?? Which can be corrected by readjusting the engine alignment to the shaft. You need someone who is familiar with aligning shafts to check it out in the water, prior to hauling the boat out. If it is a shaft problem, then the shaft needs to be sent to a shop for repair or replacement. It could also be a dinged propeller. That will also cause a vibration.
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