Why are all prop shafts water lubricated?

Discussion in 'Inboards' started by CDK, Sep 5, 2007.

  1. CDK
    Joined: Aug 2007
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    CDK retired engineer

    For my repowering project (see the forum Surface drives "from sterndrives to jet to surface drives") I plan to use ball bearings, a stainless steel tube and ss propshaft with neoprene seals both outside and inside, so the oil in the tube will stay there. The inside bearings will also take the thrust, so I can use universal joints between the shaft and the close mounted gearbox.

    But wherever I look for parts, I only find water lubricated constructions, stuffing boxes or ricidulously priced patent shaft seals.
    So I am confused.
    As far as I know, all of this worlds outboards and sterndrives use ball bearings and oil; they keep the water out directly behind the prop with a simple but highly effective neoprene seal. On the various boats I owned over the past 45 years, I never needed to change the seal.

    Why are all other prop shafts water lubricated?
    Water in my opinion is an extremely poor lubricant, especially seawater that harbours all kinds of seeds from oysters and other shells that start a new generation of abrasive life forms in every quiet tube, chamber or orifice. They are the main source of impeller wear in cooling systems. And why let the water almost reach your bilge, only to be stopped by a gland in a stuffing box?
    The stuffing box - in my opinion - belongs to another century, where steam engines powered every ship that had no sails and sometimes even the ones that did. Why is it's modern replacement so expensive?

    I have of course no problem in obtaining the parts I need from other sources, but in the boating world, oil lubricated prop shafts do not seem to exist.
    Am I overlooking something?
     
  2. TerryKing
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    TerryKing On The Water SOON

    Simplicity??

    CDK, I think the simple answer is that water-lubricated bearings, at least for typical inboard shaft-strut configurations, work well, are cheap and have long life. My rubber strut bearing on a 1 inch stainless shaft is running fine for 30 years now! My shaftlog is bronze on the stainless shaft, is greased regularly, has no side loading, and has lasted 30 years also.

    Especially for that configuration, a separate oil-lubricated ball bearing would need 2 good seals AND some way of venting or handling expansion of the oil.

    Outboard/outdrive units need gears and bearings that handle high sidethrust that are close to the shaft/water interface. So oil/grease lubrication of the gears drives the requirement, and the use of those seals. And I believe they vent pressure into the upper body, above the waterline.

    I'm sure ball or needle bearings could be fitted to a strut. But keeping the outer diameter small is important for drag reasons. For a into-the-hull bearing on a skeg or sailing hull the diameter is not a restraint and a rolling bearing and seal would be pretty easy..

    If you're building a surface drive you have a tube from above-to-below waterline so using ball/needle bearings and available seals, oil lubrication and pressure venting above the waterline should be fine...

    Let us know what you do and how it works out!
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Hi TerryKing,

    Apart from my gut feeling that oil and ball bearings are superior to water and rubber, there were some other design considerations.
    In your setup, you will probably have a flexible coupling between shaft and gearbox, where the gearbox bearing takes up the thrust. That implies a fairly stiff mounting of the engine/gearbox. I closely studied a Volvo turbodiesel in a 37 ft sailing yacht that uses the same construction. A beautiful machine, much better than I could possibly build from a VW car engine, mounted on 4 rectangular rubber pads. But the feet offer very little longitudinal flexibility because they must withstand the thrust load and as a result vibration is transmitted to the hull over part of the rpm range.

    I intend to use quite flexible circular rubber pads and mount a double universal joint between gearbox and prop shaft. They have a flange on one side and a splined hole on the other, so only engine torque can be transmitted.
    For that purpose, the prop shaft must have its own thrust bearing; the shortest possible construction is to include it in the shaft tube.
    Regarding the tube diameter and drag: I use a fairly large tube with the same diameter as the propeller hub, without a strut. It looks good on the drawing and I know from experience that a 2,5" thick walled ss tube is very, very hard to bend, so I expect it to be be strong enough.
    You are correct about the need to vent oil expansion. Unlike outdrives I have no upper gearcase, so my plan is to use a small oil jar above the waterline, connected with a 1/4" copper tube.

    Unfortunately, my age doesn't allow to tell you if it did last 30 years, but I will keep you informed a bit earlier.
     
  4. Frosty

    Frosty Previous Member

    CDK . A thrust block with a cv joint enabling the engine to be on very flexible mounts has been done . Its available from a few manufactures . It works very well.

    I think its called 'Aquadrives'
     
  5. TerryKing
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    TerryKing On The Water SOON

    I didn't PLAN on it lasting 30 years, but I'm glad it did! At 67 I'll be really lucky to see another design thru 30 years :)

    I'm very interested in building a DIY Surface drive, and I'm trying to absorb what everyone else has been learning. Question: Thrust - (say 1000 pounds or more, on a good-sized installation). What type bearing are you planning to take the thrust? I know ball-cage bearings can take side-thrust, but supposedly tapered roller bearings are better. And some marine drives have a flat needle-bearing thrust bearing. What have been used on surface drives successfully? And was the thrust bearing at the propeller end or the top end?

    I have to read thru all the surface drive posts here...

    Please keep us informed about what you decide and how it works!!
     
  6. fcfc
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    fcfc Senior Member

    There are things I do not understand.

    For me, in typical applications, from propeller, you have water lubricated bearing in the strut, floating stuffing box or seal at the inside end of the stern tube, rigid coupling to gearbox end. The engine on "hard" silent blocks.
    The shaft rotates on two bearings : the strut and the gearbox end. It may limit the shaft length.

    A variation of this is to use thrust bearing, like aquadrive or python drive. After the stuffing box/seal, the shaft is rigidly connected to one side of a thrust bearing. The other side of the thrust bearing is connected to the gearbox output thru a CV joint, or similar. And the engine uses softer silent blocks.

    In both cases, the stern tube is short : it goes from hull bottom or skeg to stuffing box. The shaft is exposed between strut and hull.
    If you use a skeg instead of a strut, the stern tube is longer, and there is no more exposed shaft but bearing and seals are same.


    The probability any floating fishing line/rope is caugth in the stuffing box/seal is rare. It may be caugth in the stern bearing, but will not cause any leak since all is in ambient water.
    Even the seal is more or less accessible from inside of the boat for any maintenance or emergency.

    Sterndrive do not have this problem since any leak will be contained in the leg. There is no chance to flood the boat from a shaft seal leak.

    So I think you want to trade one water lubricated bearing, one oil lubricated bearing (gearbox end or thrust device) , one seal/stuffing box and partial stern tube with full length stern tube containing two oil lubricated bearings and two seals. Will the friction difference between oil and water bearing cover the losses of an additionnal seal ?

    A shaft does not rotates that fast, and radial loads are not that big. witness the overhang of one leg struts.
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    "A thrust block with a cv joint enabling the engine to be on very flexible mounts has been done . Its available from a few manufactures . It works very well."

    I'm very glad to hear that. My cv joints are already there, I purchased them at $200 in the USA for the Berkeley jets that are about to be dismantled. They have not more than 20 service hours. The 10 spline hole is a nightmare of course: I must find or make an adapter to fit the 1" prop shaft, but rather that than throw them away.

    "What type bearing are you planning to take the thrust? I know ball-cage bearings can take side-thrust, but supposedly tapered roller bearings are better."

    I use SKF ball bearings at the prop end to take radial forces, twin tapered roller bearings back-to-back at the other end to take the thrust. Because roller bearings for a 25 mm (1") shaft are a bit small for the forces I expect, I use the next larger size with 30 mm hole and make a ss sleeve, pressed on the shaft while hot and fixed with 4 set screws.

    "So I think you want to trade one water lubricated bearing, one oil lubricated bearing (gearbox end or thrust device) , one seal/stuffing box and partial stern tube with full length stern tube containing two oil lubricated bearings and two seals. Will the friction difference between oil and water bearing cover the losses of an additionnal seal ?"

    The full lenght stern tube in my case will be only 2 ft long, all submerged. With a heavy welded-on flange it will be secured to the transom, or in fact the beginning of a tunnel that ends at or slightly behind the transom. This is still a problem area: hydrodynamics calls for a smooth surface, the mechanical strenght requires a large flange area.
    Yes, I think an oil filled tube with bearings will have less friction than with water, most certainly after a few years in the sea. My hydraulic cylinders from the former sterndrives were useless after each winter season because of stone hard marine growth that I used to remove with hydrochloric acid.

    The picture explains why I replace the jets. It was taken 2 years ago when the boat was 3 months in the Adriatic sea.
     

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  8. TerryKing
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    TerryKing On The Water SOON

    Thanks for the details...

    OK, that's about what I envisioned. What force do you expect? What Horsepower/KW? I'm looking at a lower range that you are, I expect, perhaps 120 HP.
     
  9. Guest625101138

    Guest625101138 Previous Member

    The difference between your situation and an outboard, stern drive or even saildrive is that your shaft breaches the hull. Leaky seals will result in the boat eventually sinking or at least becoming flooded if it has solid buoyancy that keeps it afloat.

    By contrats a shaft gland can be repacked and tightened up to reduce leakage. As a child it was one of the jobs I did on the workboat my father operated - loosen the gland at the start of the day and tighten it up in the evening. The packing and adjustment even accommodates shaft wear.

    If the boat will noramally be out of the water it is not an issue. If the boat is intended to be in the water for long periods then you should address the risk of sinking. The seals need a lot of care to ensure protection from growth on the shaft and any errant rope or fishing line that may find its way wrapped around the shaft. The prop on outboards provide a mechanical labyritnth to protect the seal. But outboards are rarely left for long periods in a situation where hard growth accumulates anyhow.

    I had a bad experience with the prop shaft bearing on a saildrive when the boat was moored for long periods in a current. I soon leant that it needed to left in gear to avoid accumulating the equivalent of thousands of miles of travelling. The bearing just wore out and the seal lost contact.

    I have thought about the issue before and came up with the idea of pressurising the bearing housing with a head of oil. Providing the oil is a bit higher than the water level outside the hull (you need an allowance for lower oil SG) the water will drain out before water gets in and damages the bearings. A small site guage on the oil column would provide early warning of a leaky seal. Hopefully plenty of warning to effect repairs before there is any risk of sinking.

    Rick W.
     
  10. Guest625101138

    Guest625101138 Previous Member

    correction .. the OIL will drain out before water comes .....
     
  11. TerryKing
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    TerryKing On The Water SOON

    Good Idea...

    I like this idea.. It also handles the expansion/contraction over temperature which needs to be handled/vented somehow anyway. A minor amount of leakage thru the seal would no longer be a problem, and slow decrease in the oil level would be a warning to replace the seal..
     
  12. CDK
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    CDK retired engineer

    Theoretically, with 25 Mph and 70 HP, the thrust would be 1050 lbs for each engine, so about what you estimated. In a former life the boat had 2x140 HP Mercruisers and did 35 Mph with a clean hull. With the current jets it did a lot worse: 20 Mph at full throttle.
    BTW: How do you connect to the internet from your location? Just curious...
     
  13. Poida
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    Poida Senior Member

    CDK

    I don't think that something that is old (belonging to another century) should be discarded only for that reason. I would have been discarded years ago if that was the case.

    I have worked with many pumps, including those pumping sea water, fitted with gland packings. The ease of removing the old gland packing and replacing beats trying to remove a rubber seal where normally the unit has to be dismantled to press the seal out.

    A standard seal on a bearing is not designed for pressure, any excess pressure from grease is easily forced out through the seal.

    An outboard motor has a small shaft and with sealing you have to look at the peripheral distance as that provides extra leakage .

    The only modern alternative that I can think of that would replace the very efficient and inexpensive gland packing is a very expensive spring loaded ceramic seal. Once again I wouldn't like to have to install or replace one on a prop shaft.

    Poida
     
  14. TerryKing
    Joined: Feb 2007
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    TerryKing On The Water SOON

    OT: The view from Here..

    I am in a modern high-rise with cable TV and a Motorola Cable Modem.. (For What Its Looks Like Here, see: http://www.terryking.us) The internet speeds are good. Of course the weight of the Great Wall does keep some things from working, like BBC and Wikipedia, but I'm peeking over when I need to...

    It's great era to live 'overseas', in many ways. But I have to work hard on the language barrier...

    And the BOATS are so cool and so different...!
     

  15. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    China... the closest I even came was a Chinese restaurant. Holland has them in even small villages.
    My situation is explained in www.puntakriza.com . We have no cable TV, just a telephone line of nearly a mile long that gets hit whenever lighting is near.
    But wikipedia we do have.
     
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