Shaft-Stern Tube Interface

Discussion in 'Metal Boat Building' started by H.E.Hope, Sep 29, 2011.

  1. H.E.Hope
    Joined: Sep 2011
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    H.E.Hope Junior Member

    Hi I'm new to this forum and if I've put this in the wrong place let me know.
    We have a custom built steel trawler which has developed a small leak (drip) between the shaft and the stern tube. When we investigated I could not see a recognizable (to me) stuffing box. It looks like a sealed bearing in the end of the stern tube. Part way down the stern tube is a fitting that feeds grease into the tube. I did pump some grease into the tube and the drip slowed considerably.
    My question is, has anyone seen this configuration before and if so, do you have maintenance recommendations?
    I've attached a series of pictures in a pdf document to perhaps explain better what the configuration looks like. Sorry about the picture quality but it is extremely difficult to get a camera down there.
    Thanks
    Dave
     

    Attached Files:

  2. michael pierzga
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    michael pierzga Senior Member

    I dont see a " lip Seal " on the end of the stern tube ? All the oil or grease filled systems Ive used had a lip seal. Is there a bearing in the tube or are we only looking at the stuffing box ? Who made your boat ? What country "
     
  3. H.E.Hope
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    H.E.Hope Junior Member

    Thanks for responding. This is my first true inboard so lots to learn. Not sure what you mean by a 'lip seal'. Can one be put on? What is in the picture is what there is. Looks like a bearing face to me. The vessel is not young but this has never cropped up before. I hadn't seen that part of the boat before as it is really hard to see. I had to cut away part of the flooring in the stern head to find it:( Now that I have, would like to fix it properly, once I know what that is.
    From everything I've read about 'stuffing boxes' there should be a gland nut or something but not here. I can't see anyway to adjust/move any part of that assembly. The boat was made by Heinz Nedling in Hamilton Ontario Canada. We are the third owner.
    Where can I find more information on grease filled systems? New to me.
    Appreciate your input.
    Dave
     
  4. michael pierzga
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    michael pierzga Senior Member

    Lip seal....typical. like the seal found on any pump.

    Also is there a bearing ? Does the shaft pass thru a bearing that is set into the stern tube ? What kind of bearing...material ?

    If your boat is out of the water its a very good idea and a typical maintenance task to pull the shaft. When you pull the shaft for inspection you will learn of all the system components.

    The system Im presently using has two roller bearings, two thrust bearings inside a shaft log ,sealed by a lip seal on the engine room side and a mechanical seal outside .
     

    Attached Files:

  5. H.E.Hope
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    H.E.Hope Junior Member

    Thanks. The picture you sent does resemble what I see. The vessel is in the water (we are live aboard) and hadn't intended to lift the boat before we got to Florida. Can the lip seal be changed while the boat is in the water? How does it come out? If I measure the shaft diameter and the inside diameter of the stern tube would that help?
    Thanks
    Dave
     
  6. michael pierzga
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    michael pierzga Senior Member

    Well, technically you could replace the seal in the water but I wouldnt. One mistake and you sink. Besides its probable that your system is leaking because components are worn. Best to haul and service.

    Look at the diagram of a typical grease packed stern tube. serveral wear components like bearings as well as the shaft itself becoming out of round. I would just pump her full of grease and live with it until you can haul and fix correctly.

    http://www.spw-gmbh.de/stern-tubes.html
     
  7. pdwiley
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    pdwiley Senior Member

    It looks to me like an oil seal (grease seal) on the inboard end too. As my system has something similar that I built myself, I'm familiar with the parts.

    There are possibly bearings inside there too.

    If you're getting water dripping then the outer seal is shot, assuming there is one. If there are ball, roller or thrust bearings in that tube then they are exposed to salt water and they're not going to last. In fact they might already be totalled. The only way to find out is to either get the original manufacturer to tell you what's in there or pull it apart and have a look. This is a slipway job.

    Given the amount of crap shown in the pictures there's no way you can get that seal out and replace it with a new one anyway. You'll tear up the new seal while installing it. If there's an outboard seal and bearings, replacing the inboard seal is not going to fix the problem, it'll only stop the leak of water that (maybe) should not be getting that far in the first place.

    Until you know what's in that tube I wouldn't plan on taking that boat anywhere other than to a slipway. If it's just a grease filled tube with a plain outer bearing cooled by seawater & grease lubricated, likely no problems other than a small leak into the inboard spaces. But, as I said, if there are ball, roller or needle thrust bearings that have been and continue to be exposed to salt water, they are going to fail. The only remaining question is, when. That's where Murphy's Law applies.

    If it were my boat I'd pull it all apart.

    PDW
     
  8. H.E.Hope
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    H.E.Hope Junior Member

    Thanks. I've tried to find the original builder to no avail. I cannot find any markings on the stern tube or the seal. The 'crap' you see is grease, handfuls of it which I've been cleaning out. My hesitation in pulling the boat is finding someone here that is familiar with the configuration and knows what they are doing. The rudder post is right behind the prop with little room and pulling the rudder looks like a major exercise. It could be quite simple if you know how - therein lies the dilemma. Suggestions?
     
  9. michael pierzga
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    michael pierzga Senior Member

    Boats are plenty of work. Shaft seals on both the rudder and prop shaft don't last forever. If the boat was built by a pro then all these tasks are doable.

    Lip seals are cheap. Bearing stock is cheap. Fixing the corrosion in the machine room bilge caused by a year of dripping sea water is expensive.

    In general prop shafts are pulled and serviced every few years.
     
  10. CDK
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    CDK retired engineer

    With a grease or oil filled stern tube the first sign of a water leak means that the seals have been compromised and must be replaced. Not doing so results in bearing damage and increasing water intrusion until things get out of control.

    I made such stern tubes myself and use oil reservoirs above the waterline to make sure water cannot enter if the outside seal fails. The oil level is marked, should it drop suddenly I know it is time to pull the shaft and replace the seal.
     
  11. H.E.Hope
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    H.E.Hope Junior Member

    Thank you both for your input. We've decided we'd best pull the boat and get it done. Today will be trying to find some place that can do it in the time frame we have. Also try and find parts. Any hints on how it is put together?
    I really like the idea of having some kind of indicator should the seals fail. Could you send a diagram? I've had a really good look for any damage caused by the tube leaks. There is a bit of rust (can almost rub it off) but only surface so I think we've caught this before and real damage. I have a friend who is a certified marine surveyor that I can ask to have a look at it.
     
  12. CDK
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    CDK retired engineer

  13. H.E.Hope
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    H.E.Hope Junior Member

    Thanks, I wonder what's inside ours.
     
  14. Poida
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    Poida Senior Member

    Hi CDK

    I have looked at your drive tube and if I am understanding your drawing correctly, and I can sometimes do that, the external bearing appears to have both seals pointing inwards to prevent grease from excaping.

    Would it not be an idea to have the outside seal facing out to prevent water intrusion.

    On of the advantages of the old packing box is, you can tighten the gland nut up to squeeze the packing against the shaft or take out a couple of the gland packings and replace them without removing the whole seal. Of course water will get in during this process but not more than your bilge pump could handle.

    N.B. I have never done this on a boat but have done it on a pump gland packing with a few thousand litres of liquid with a 5 metre head. Much more pressure than would be on a boat packing.
     

  15. pdwiley
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    pdwiley Senior Member

    No because the internal oil tank is above the WL sufficient to provide a positive pressure head against the outer seal. Oil will leak out, not water leak in if the outer seal fails.

    I liked that idea so much that I built my own version of it. Main difference with mine is that I don't have a big enough stern tube to use ball bearings on the outboard end so I'm using a chunk of Vesconite machined to take the oil seals.

    To the OP, I think you're wise pulling it now. You'll know what's there and if you take lots of pictures, measurements etc the next owner will thank you for it. Also, generally all bearings, seals etc are readily available from industrial suppliers, usually at a fraction of the price of the same thing from a marine supplier. All you need is the bearing & seal numbers & brands.

    PDW
     
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