Surprise!! Shaft wear by packing nut

[Rich Stidger]

I'm trying to get ready for the summer season. In 2019 (my last sailing season) I was having some difficulty stopping the dripping at the shaft log when at rest. My last packing renewal was June 2016 when I removed 4 rings of packing but replaced with 3 rings. I don't remember the details but I must have had a problem getting 4 rings to go in.

So I removed the 3 rings- it went surprisingly easy. But before I install the new rings, I observed some wear areas on the shaft. So I measured with my digital caliper.

I measured the shaft itself. Then I measured at the wear area at the top of the packing nut; and finally at the edge of the threads where the shaft enters the log.
See the picture below for the measuring points.

I measured as follows:
Shaft 1.25
At top of packing nut = 1.22 (in 2016 this measured 1.23)
At edge of gland threads = 1.18 (no 2016 measurement to compare)

So experts, what now? Do I need a new shaft? Am I likely to have success replacing with 3 rings?
Any other alternatives?

I hate finding issues like this at the last minute.

 

[dlochner]

Time for a new shaft. Shaft strength is related to the cross-sectional area of the shaft. This means small changes in diameter at the outer edges have a greater affect on strength than the same change closer to the the center.

You can easily calculate the loss in cross section and use it to calculate the loss of strength. Find the cross section area with a diameter of 1.25" and find the cross sectional area with a diameter of 1.18 ". Subtract the latter from the former and you have the area that has been lost. Divide that by the original area and you have a rough indication of how much strength has been lost.

Mechanical and materials engineers may have a more precise way of measuring this, however, this method is close enough.

 

[Justin_NSA]

If it were me, I'd replace the shaft. Maybe even install a dripless seal. In fact a new dripless seal might ride on a different part of the shaft and avoid the need for a new one? But it's your boat, your money.

 

[jssailem]

Fresh packing at least 3 and possible 4. Properly cut and install with the cuts spaced out around the shaft. You can fit 3 in then compress then and open the stuffing box and add the forth. Close up and then test the unit.

 

[dLj]

I don't think you are in danger at this point, and you should be able to use packing to seal your shaft. But I'd start budgeting for a new shaft to replace this one at the end of this season. If I were about to start crossing oceans, I'd change the shaft now. But if just coastal cruising, I think you are OK for this season. That's just my 2 cents worth...

dj

p.s. sail a lot, motor as minimally as possible...

 

[shemandr]

By my calculations - and I'm not an arithmetic guy - I get a cross sectional difference of 0.07 square inches or I guess expressed as a percentage 7%.
I think the bigger question is if you can stop the seeping of water. Paraphrased ... "You're gonna need a bigger packing..."

 

[Richard19068]

I think the loss of cross sectional area (pi times the square of the radius) is closer to 11%. What loss of strength that results in I don't know.
If you are worried about strength rather than water intrusion you might want to contact a prop shop and given the horsepower, shaft material, shaft speed, propeller diameter and possibly some other factors they should be able to calculate the required shaft diameter. The info to do that calculation is available on the internet but it is a bit technical and confusing.

 

[Rich Stidger]

First, thanks to all for your inputs. I agree that a new shaft may be in my future. I really hate the thought- probably a new coupler, machine shop work to fit the coupler and prop, likely need to pull the rudder to get the shaft out, etc. These jobs tend to snowball.

I did the calculation per @dlochner and I came up with a strength loss of just under 11%. Maybe that is a concern, but I really want to stop the dripping at rest.

I measured my packing nut and the depth was 1 3/16". Four rings of 3/16 is 3/4". It turned out that all four rings fit just fine. I can't imagine the cause of the problem I had in 2016 of not being able to get all four in.

In any event I will splash and see what I have. Hopefully I will be able to get the required drips at running and at rest.
I have one more day to wrap up all the jobs so that launch can occur in a couple weeks. In those couple weeks my wife and I will be on a road trip vacation, so all boat work must be done. Upon return we will have a week to clean and prep for a 7 week Maine cruise. Our summer is booked almost solid. Go away black cloud.....

 

[shemandr]

Relying on my memory I used the wrong Pi value. So 11% it is.

 

[Tally Ho]

I am not an engineer and would not argue with those who are calculating the material lost and the resulting weakness of the shaft...but, I pulled my old bronze shaft a few years ago because of what looked like circumferential cracks between the stuffing box and the coupler. When I got it home and inspected it, the wear on the shaft was even worse than I imagined...

My point is, I would probably run that shaft this season (assuming you can keep the leaks to an acceptable rate) and replace it over the winter 2022 if you think it needs it.

I probably don’t run 30 hours on my engine over the course of my sailing season...if you run a lot of hours, maybe your situation is different.

I did replace mine, and the coupling, and the stuffing box hose while I was at it.

I will be pulling my stuffing box after this season to check for wear...I wonder if any shaft movement (vibrations, the engine shaking, etc.) translates to the stuffing box rubbing the shaft ?

I am still on the fence regarding a dripless system....maybe I will try a Volvo lip seal.

Greg

 

[dLj]

@Rich Stidger - let us know how it goes. Very interested here...

dj

p.s. When will you be in Maine?

 

[JamesG161]

Thus the reason for the only non-shaft contact sealing method.

PYI Inc. | PSS Type A Shaft Seal

Get one and no more shaft wear.
Jim...

 

[Rich Stidger]

@Tally Ho - I normally put about 130 hours on my engine each year, but when we do a Maine cruise that adds another 100 hours or so. We sail when we can and motor when we must.
We plan to leave Narragansett Bay around June 6 and return July 24th. We take a week to reach Portland and a week to return to Narragansett Bay. So we will be in Maine for the middle 4-5 weeks between Portland and Northeast Harbor.

 

[Tally Ho]

@Tally Ho - I normally put about 130 hours on my engine each year, but when we do a Maine cruise that adds another 100 hours or so. We sail when we can and motor when we must.
We plan to leave Narragansett Bay around June 6 and return July 24th. We take a week to reach Portland and a week to return to Narragansett Bay. So we will be in Maine for the middle 4-5 weeks between Portland and Northeast Harbor.

Good luck...getting a shaft replaced at the last minute is a bummer.

Greg

 

[Ralph Johnstone]

I pulled my old bronze shaft a few years ago because of what looked like circumferential cracks between the stuffing box and the coupler.

Always enjoy your videos. Orderly and very well presented.

You took the cracks very nicely in stride. My compliments. I would've had an apoplectic fit seeing the size and likely depth of those cracks. And with the root of those cracks acting as ideal stress raisers .......................

It always surprized me that boat manufacturers were using bronze shafts even as late as 30 years ago. I can remember there were some pretty exotic SS alloys back then and I can't imagine a very large difference in price for the stock material.

You dodged a bullet and can put that all behind you with a new exotic SS shaft.

 

[dlochner]

I've use Deep Blue Yacht Supply for drive train parts. They were pretty quick when I needed a shaft replacement and the price was competitive.

Boat Propellers, Prop Shaft Seals, Shafts, Zincs, and Marine Hardware

Lowest Prices, Largest inventory of boat props, inboard prop shaft seals, zinc anodes, inboard shafts, cutlass bearings, and marine hardware.

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[RoyS]

I doubt that slight and smooth changes in shaft diameter result in significant reductions in strength. Sharp grooves will result in stress risers that may propagate into a crack but smooth reductions in shaft diameter like yours are probably not a concern. The likely stresses on your shaft in a sailboat propeller application are not as serious as, say, an axle and wheel application. Seal and sail on IMHO.

 

[JoeWhite]

I agree with RoyS. To reduce dripping I put lanocoat before I screw on the nut. Lanocoat softens and fills voids when propeller runs and when stops it cools and seals leaks. It also provides lubrication, no wear on my shaft. I replace the flax when on the hard for bottom paint once every 3 years.
I can not think that engineers would design a shaft that would breake if diameter is reduced so slight amount.

 

[MitchM]

not to get into Gieck's engineering formulas in detail, but : the engineering formulas for compresssive strength of a beam, whether loaded as a simple beam fixed at 2 points or a cantilever fixed at the gearbox , free at the cutless bearing, aren't what should be used in this situation. compressive load on a beam is indeed proportional to area-- but this isn't compressive loading. the prop shaft is loading is not in compression but rather is a load on a solid shaft rotating in torsion. shear stress in torsion increases as the gearbox drives the shaft to rotate faster at various rpms. shear stress due to this torsional force T from the gearbox is proportional to the power (ie rpms ) and distance from the shaft's surface to its center of mass, ie radius. so a difference of diameter 1.22 original to 1.18 worn , which gives a difference in radius of . 61 inches original to .59 with wear, is not an 11 % reduction in strength . you d need to know a whole lot more data on this shaft to accurately calculate the small difference in shear stress due to the small reduction in shaft radius. i'd put more wraps on the flax packing till the drip stops, and go sailing.

 

[dlochner]

not to get into Gieck's engineering formulas in detail, but : the engineering formulas for compresssive strength of a beam, whether loaded as a simple beam fixed at 2 points or a cantilever fixed at the gearbox , free at the cutless bearing, aren't what should be used in this situation. compressive load on a beam is indeed proportional to area-- but this isn't compressive loading. the prop shaft is loading is not in compression but rather is a load on a solid shaft rotating in torsion. shear stress in torsion increases as the gearbox drives the shaft to rotate faster at various rpms. shear stress due to this torsional force T from the gearbox is proportional to the power (ie rpms ) and distance from the shaft's surface to its center of mass, ie radius. so a difference of diameter 1.22 original to 1.18 worn , which gives a difference in radius of . 61 inches original to .59 with wear, is not an 11 % reduction in strength . you d need to know a whole lot more data on this shaft to accurately calculate the small difference in shear stress due to the small reduction in shaft radius. i'd put more wraps on the flax packing till the drip stops, and go sailing.

I'm not sure to whom you are referring with the reference to Gieck's engineering formulas, however since I was the one who posted suggested evaluating the strength of the shaft, I'll respond.

The method I recommended, i.e., looking at the the loss of strength based on the reduced diameter of the shaft was derived from reviewing the breaking strengths of shackles made by a company that produces high quality shackles. Shackles are typically not placed in a torsional stress, nor are they placed in compression. Shackles are typically placed in tension. The main point of my post was to promote an understanding that even small amounts of wear can have a large impact on the strength of fittings.

The OP reported that his shaft had worn from 1.25 inches to 1.18 inches, a .07 inch reduction. This is only a 5.6% reduction in diameter, however, the cross section area was reduced by 11%, about double. This seems more significant.

Since I am not a mechanical engineer I can't and won't make a statement about the relative safety of a prop shaft that has lost about 6% of its diameter. However, based on a review of empirical evidence, the loss of strength may well be greater than one might assume and that may have significant consequences.

The article below explains in greater detail how I have come to this empirically based conclusion.

The Grind | Good Old Boat

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