Why are all prop shafts water lubricated?

Pioda, you misjudge and possibly underestimate me. Life is one long learning process and the more you know about something the more you realize that you know so little.
I must admit that I am not unbiased, for I have a degree in mechanical engineering: what I learned at the university is not easily put aside. But marine engineering was not my specialty, so I proceed very carefully.
The title of this thread is not correct, the overwhelming majority of all prop shafts is oil lubricated: outboards and sterndrives, some surface piercing drives and (I was told) many shafts from large ships. The ones that use water and stuffing boxes are a minority, but I agree that low cost and easy maintenance are quite valid reasons for doing so.

I also saw bearing damage in gearboxes; I may even have caused some when shifting gear. The particles that cause damage are never primarily from the bearings but from the gears or are introduced by carelessly adding oil. My construction has only 3 bearings and my intentions are to assemble it absolutely clean.

Using stuffing boxes instead of neoprene seals is an interesting suggestion, I have to think about it. Do you think there is a packing gland available for the oil-air barrier?
Seal wear doesn't really bother me, it is more the channel that a seal can grind in the 316 steel if abrasive parts are around. But I think a have a solution, for which I will start a new topic as soon as I have test results.

 

Taking prices from the Lancing Marine pricebook 2007, that solution costs $950,00 plus taxes. My solution costs $105,00.
The Python and PSS take up 19", where I only have 10" available. My solution needs approx. 9 1/2". These are the differences.

 

Python and other products

Hello FCFC. Thanks for the pointers.. some of us may not be aware of those products.

I think the goal here is to find a design for a Do-It-Yourself Surface Drive that uses off-the-shelf parts as much as possible. This started with CDK's question about use of water-lubricated bearings at the propeller end of a shaft/tube configuration. (Which we're still debating ! )

The Python units combine engine coupling, CV joint and thrust bearing very nicely, and would be good for a typical inboard installation with a prop shaft coming thru a shaft log etc. And the PSS is a 'better shaft seal' with carbon wear surface, usually replacing an older design packing.

The configurations we have been thinking thru do combine most of the Python and PSS ideas into a single-tube arrangement. And they use a standard straight shaft with no special machining.

It's not surprising that similar problems have people designing similar solutions

 

CDK I have been through university too. It was Saturday afternoon on a bicycle.

I would like to enter a drawing in the CDK draw a prop shaft competition but my wife will be home soon and I have to go to look at kitchens.

But I will run my idea past you shortly

Poida

 

Per ABYC, diameter tolerance for propeller shafts is + .002 -.001, total variation .003 = 0.076 mm.
Per SKF catalog, shaft tolerance for ball bearings fit is a class 6 tolerance : 0.013 mm.

I have not checked shaft roundness , surface roughness or hardness to fit technical seals because I have not found such specs for standard marine shafts. These requirements do exist for technical seals. Required roundness for paulstra joints is 0.005 mm.

For outboards and Z drives, which do have ball bearings and seals and the whole thing lasts with not too much problems, the propeller shaft is not a standard marine shaft, but a fully purpose engineered and machined mechanical part.

So you will need shaft (precision) machining, unless hammer and chisel are mechanical tools to fit ball bearings and seals.


Another point to note. Thrust bearings either python or these ones http://www.aquadrive.net/AquadriveInstallationManual.pdf are shown and sold with silent blocks. And both have requirement for silent block installation. So there is a strong probability that your thrust bearing should be soft mounted on the hull bottom or transom.
I would guess that these products do contains a spherical roller bearing, or a combination of self aligning bearing + spherical thrust bearing. because when you check SKF misalignement constraints on other bearings, it is next to impossible to reach them trying to align separately mounted parts. If you want to keep "ordinary" ball bearings, you would have machine the stern tube in a whole, with both ends aligned within specs. And then you will have to soft mount your stern tube in your boat. Unless you want all prop vibrations transmitted thru ball bearings to your hull.

Now, everybody is absolutely free to make its own experiments and have its own ideas. I am just looking at similar products, trying to understand how they have done and why they have done like that.

 

The shafts are already here, so I could do some measuring. Unroundness is indeed a problem, so I cannot use them as they are. But fortunately there is plenty of material: my micrometer found 25,28 to 25,35 mm. Not enough to cut that on my lathe but once I have it centered, grinding the small area for the outer bearing and seals to 25,0* shouldn't be too difficult. The inner (thrust) bearings will be mounted on a sleeve, so no problem there. The whole stern tube cannot be machined on my lath because of its lenght, so the carrier for the inner bearings has to be made seperately and flanged to the tube.
With prop vibration being transmitted to the hull you adressed a topic I have not considered at all I'm afraid. How serious is it?
I've experienced vibrations from the damaged prop on one of the stern drives, but I guess you mean something else. My boat currently has jets: there was no vibration whatsoever after I installed them 2 years ago, but there is some now, without doubt caused by the barnacles that have settled on the impellers. These are both caused by radial unbalance; is there an axial component as well?
I've seen the silent blocs on the python drive and assumed they are used because exactly aligning a seperate thrust plate in a hull is very difficult if not impossible.

 

These are just broad generalities.

http://www.nautica.it/superyacht/527/tecnica/vibrations.htm
http://www.nautica.it/superyacht/517/tecnica/propeller.htm

 

Thank you fcfc, that was very enlightening.
Aerodynamics was briefly part of my education and I see some faint similarities, but the fact that water cannot be compressed makes it also very different.
There is a controversional issue I have some problems with. This is the last sentence of the propeller article:

"In conclusion, the more blades a propeller has - since the frequencies will be greater and the vibratory amplitudes less - the more certainty there is of avoiding hull vibration."

By itself a logical conclusion, but the word "amplitudes" reminds me of something.

I know from personal experience that reducing amplitude can be achieved by accurate guidance of a revolving shaft. Printed circuit boards are drilled with very hard drills at high rpm. When properly guided in a spindle they last incredibly long, but the same drill in a simple machine like a Dremel tool is shattered in the first hole because the amplitude is larger than the brittle drill can handle.
But I also once visited a factory where they made industrial mixers for agressive liquids. Very long, tall shafts with some sort of prop at the end, totally unsupported, yet running at several 1000's rpm without vibration. The same applies to the Asian boats, propelled by a V-8 and a long, unsupported shaft.

TerryKing's suggestion to use a water filled tube with an oil filled section for the thrust bearing is an alternative I'm still considering.
Btw, what I plan to build looks a lot like this: http://www.performanceimports.co.nz/saro.html

 

You need to run a propeller in line with the flow to avoid unbalanced bending forces on the shaft. If the shaft is inclined then you inevitably have an oscillating force that will cause damaging vibration if it can find a natural frequency in something to excite. The higher the frequency of the oscillation the less likely it is to cause a problem because water provides effective dampening. So more blades gives higher frequency. Higher revs give higher frequency.

I can deliver 1HP through an 8mm diameter aluminium shaft that is 4ft long and unsupported at the prop. It is no problem in water. In air it is hard to get it through the first mode of vibration - dampening is much less.

Rick W.

 

Thank you Rick Willoughby, exactly the answer I've been expecting. Although I instinctively would step back from your aluminium shaft experiment, my guess is that even with a barnacle on the prop, the shaft will find a new center of gravity and rotate around that as long as the shaft's flexibility allows it..
I'm glad you explained it, not me. I already offended someone by admitting I went to school. My reputation for being a stubborn old man needs no further emphasis.

When asked about vibration in a submerged object I would have said there isn't any. That's why I stated that I never gave it any thought when designing my weird oil filled stern tube. But these Italians from www.nautica.it are no fools. Or are they? The articles came under the heading 'superyachts', so most probably the rich owners already start worrying if they hear any engine or propulsion sounds at all. For my project I'm working on two VW turbo diesels, world famous for their distinctive noise......
I do recall the Germans who wanted to approach their enemies unnoticed, had some vibration problems with their U-boats, around 18 Hz if my memory doesn't fool me, but let's leave it at that.

No I hesitate to say it, but is the bearing behind the prop really necessary? Or is it only there in case the prop is so unbalanced that it tries to live a life of its own and that on a properly maintained boat it doesn't have to do anything at all? If so, that would explain why the water lubricated rubber bearing perform so well after TerryKing's 30 years.