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Thread: Marine stern bearing design and material

  1. #11
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    Thanks to all for the feedback and engagement on your visits to NZ. It is a magic place.

    Some more information after discussion with the owner.

    He beached his yacht not that long ago and drained the stern tube thru the gubscrew hole that holds the bearing. The water/oil mix (was very white) from the stern tube. He tells me that caught every last drop (being an ecofriendly dude) while beached on Great Barrier Island. Oil was replaced with new SAE30 by putting the fresh oil into the header tank.

    After replacement of the oil all vibration disappeared and full revs could be reached. However this only lasted for a few days of motoring through the day and anchoring at night. The vibration gradually came back so that a week later the vibration was much the same as before.

    Another point not mentioned is that the oil header tank, approx 2.5m above the stern tube, never gets any oil/water mix in it so there we have it there is no oil circulation, convection or otherwise.

    There are two tubes coming from the header tank to the front end of the (100mm diameter) stern tube. One is at the bottom and one at the top of the stern tube. This is clearly not enough to create a convection.

    The yacht has a swing keel of 2 tonne. He tells me that during hull scraping exercises while beached he has subsequently removed the grub screw and there is again milky oil. This does mean the rear seal is not totally effective; thought that may be exacerbated by a worn bearing.

    The owner has suggested the possibility of running a separate tube from where the oil enters the stern tube then along the inside of the tube to the rear bearing, together with maybe installing a low pressure oil pump. The idea here to put positive flow the bearing at stern and from front tube to header tank. This would allow monitoring of the change in oil, the temp as well.

    The previous owner has maintained that the stern tube running through the diesel day tank currently creates some cooling provided the day tank is full enough to cover the stern tube. Keeping the day tank close to full is a pain as there are floor covers that need to be removed to monitor the filling of the day tank. To keep the tank full it needs the diesel pumped from the holding tanks every 3-4 hours. This can be a bit fraught in a challenging seaway and probably doesn't get done as regularly as it should to match the cooling requirement.

    The day tank has enough capacity for 48 hours of steaming so it would be good if this requirement could be eliminated.

    Yous have got us going on our private discussion and we are thinking that if a tube (sort of brake line size) could be run along the inside the stern tube (there is a 20mm gap to the shaft) we could get positive oil flow to whatever bearing is chosen.

    Another thought is that the brake line could be wound into a spiral just a tad larger than the 100mm inside diameter of the stern tube and this may cause it to stay in position without touching the shaft. If the line is straight we need somehow to ensure it is fixed so does not touch the shaft.

  2. #12
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    Quote Originally Posted by Norman Bain View Post
    ...Yous have got us going on our private discussion and we are thinking that if a tube (sort of brake line size) could be run along the inside the stern tube (there is a 20mm gap to the shaft) we could get positive oil flow to whatever bearing is chosen.

    Another thought is that the brake line could be wound into a spiral just a tad larger than the 100mm inside diameter of the stern tube and this may cause it to stay in position without touching the shaft. If the line is straight we need somehow to ensure it is fixed so does not touch the shaft.
    You guys really need to seek professional help.

    I am reminded of the saying:
    "Anything seems possible when you don't know what you're doing."
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  3. #13
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    Running an oil-feeding tube down the inside of the stern tube is a seriously bad idea. It will inevitably contact the rotating shaft and eventually wear through or even break off. The integrity of the whole shaft/tube area is crucial to the sea-worthiness of the boat, and I wouldn't dream of compromising that. Forget it.

    The two tubes from the header tank will be just a filling tube and a breather tube.

    The outer seal is obviously buggered (a very common occurrence), and will need to be replaced. And that will be another section of the shaft that will need close inspection for wear and/or roughness.

    The whole business of using the day tank to cool the stern tube is just plain weird. For a start, one reason for a day tank is to site it above the engine so the fuel is gravity fed. Having it in a remote area of the bilge makes no sense to me. Secondly, the stern tube simply should not need cooling.

    What are the sources of heat? Some heat might flow to the shaft from the gearbox, but that is minimal. If the inner shaft seal is of the old-fashioned stuffing-box type (which I think you said it is), that packing will rub on the shaft and create a little warmth. The only other possibly significant source would be the rear bearing, and if that is properly lubricated it will not create much heat. The stern tube is steel and is welded to the steel hull, and the hull is cooled by the sea, so the heat flow from tube to hull should be more than enough to keep the temperature of the tube to a moderate level.

    Still, since the day tank is there, you might as well use it for cooling if you must. My advice would be to get a proper day tank fitted in a proper place, disconnect the existing bilge day tank from the fuel system, and (assuming it has inspection ports so you can still get at the outside of the stern tube) extend the fresh-water cooling system of the gearbox through it. The fresh-water coolant will be corrosion-resistant, so you won't cause any problems to the stern tube. Of course, that would mean that the stern-tube would reach the same temperature as the gearbox, which may be higher than it's used to. You may also have to fit a bigger heat exchanger for the gearbox.

  4. #14
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    Quote Originally Posted by ezduzit View Post
    You guys really need to seek professional help.
    We suspect that this is the case. That said, what for?

  5. #15
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    Quote Originally Posted by Mike Burch View Post
    Running an oil-feeding tube down the inside of the stern tube is a seriously bad idea. It will inevitably contact the rotating shaft and eventually wear through or even break off. The integrity of the whole shaft/tube area is crucial to the sea-worthiness of the boat, and I wouldn't dream of compromising that. Forget it.
    We are pretty much of the same opinion re the contact with shaft ... would be tricky to maintain clearance. That said, we need to get positive flow to the bearing at rear and push the flow forwards to the front pickup and back to the header tank. If we do not do this the oil (seal good or otherwise) is always going to go milky. And we cannot flush it.

    Quote Originally Posted by Mike Burch View Post
    The whole business of using the day tank to cool the stern tube is just plain weird. For a start, one reason for a day tank is to site it above the engine so the fuel is gravity fed. Having it in a remote area of the bilge makes no sense to me. Secondly, the stern tube simply should not need cooling.

    What are the sources of heat? Some heat might flow to the shaft from the gearbox, but that is minimal. If the inner shaft seal is of the old-fashioned stuffing-box type (which I think you said it is), that packing will rub on the shaft and create a little warmth. The only other possibly significant source would be the rear bearing, and if that is properly lubricated it will not create much heat. The stern tube is steel and is welded to the steel hull, and the hull is cooled by the sea, so the heat flow from tube to hull should be more than enough to keep the temperature of the tube to a moderate level.

    Still, since the day tank is there, you might as well use it for cooling if you must. My advice would be to get a proper day tank fitted in a proper place, disconnect the existing bilge day tank from the fuel system, and (assuming it has inspection ports so you can still get at the outside of the stern tube) extend the fresh-water cooling system of the gearbox through it. The fresh-water coolant will be corrosion-resistant, so you won't cause any problems to the stern tube. Of course, that would mean that the stern-tube would reach the same temperature as the gearbox, which may be higher than it's used to. You may also have to fit a bigger heat exchanger for the gearbox.
    The day tank is in the right spot. Location is pistons height wrt to the motor. There is fuel pump and lotsa filters etc. It is correct and motor purrs.

    We have not access to the stern tube once it enters the day tank at this stage. There has been some issue with diesel bug. Flushing is tricky due lack of access below and to sides of stern tube. Owner is contemplating putting cutting a temporary access (re-welding when done) from outside and cleaning tank properly. Maybe add a sludge draw off mechanism; could even be a permanent sucker tube to bottom of tank.

    The boat is 20 years old and has not had the tank cleaned properly. There have been multiple attempts will all sorts of suckers etc etc.

  6. #16
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    Quote Originally Posted by Norman Bain View Post
    ...we need to get positive flow to the bearing at rear and push the flow forwards to the front pickup and back to the header tank. If we do not do this the oil (seal good or otherwise) is always going to go milky...The boat is 20 years old and has not had the tank cleaned properly. There have been multiple attempts will all sorts of suckers etc etc.
    Quote Originally Posted by Norman Bain View Post
    We suspect that this is the case. That said, what for?
    The above "solutions" show that the owner doesn't even have a clue. Tanks not cleaned for 20 years? Trying to suck them clean? Positive oil flow at rear bearing to solve a seal problem? Come on!

    To prevent seawater from entering past the seal requires only a good seal and that the oil in the shaft tube have positive pressure as compared to the seawater. A header tank located sufficiently above sea level easily provides this with no complicated additional cooling, plumbing or pumps--just pathetic considerations.

    Fuel tanks need manholes to provide access for periodic cleaning and inspection. If they don't have these, put them in and clean the tanks. Period.
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  7. #17
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    Norman, a flow of oil to the aft bearing has absolutely nothing to do with the incursion of water past the seal. As ezduzit says above, an oil bath with sufficient pressure coming from a header tank situated well above sea level is ample protection—IF the seal is good, which the present one clearly is not.

    Your skipper seems to be obsessed with having a flow of oil up the tube. It is not at all necessary, and merely introduces another complication to go wrong.

    Of course there's a fuel lift pump! In your set-up, it sucks fuel from the day tank and feeds the injector pump. Sucking fuel is BAD! It means that any tiny leak will allow air into the system, which will stop the engine. By utilising a gravity feed, the possibility of an air intrusion is minimised, and when the lift pump packs a sad there will still be fuel getting to the injector pump. The filters are, of course, vital but irrelevant to the discussion.

    I'm surprised the day tank is at piston height if it also surrounds the stern tube—clearly the bottom of the day tank is below the stern tube.

    And where does the fuel in the day tank dissipate the alleged heat to? The flow rate in the tank is far too slow for the fuel passing through it to carry the heat into the engine, so presumably that heat gets carried away either by conduction from the tank into the hull and thence to the sea, by radiation to the surroundings, or by convection of the surrounding air, any of which are a bit ad hoc to put it kindly. But as I've said in a previous post, there should not be any significant heat generated in the stern tube anyway!

    I'm sure you will find a sensible engineer in Whangarei, who will politely disabuse your skipper of his wilder ideas and sort something sensible out for you.

  8. #18
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    Just a suggestion, you could email Keith Fenner, the You Tube guy who has worked marine machine shop stuff all his life.
    Turnwright Machine Works.

  9. #19
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    May 2006
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    wow cool looking boat and does sound like an adventure, id be more into some kind of solo float (much smaller craft of course) though because thinking about being stuck on a boat with other people makes me sea sick...

    what about UHMW for the material? self lubricating and durable as all hell, and yes first on the list is take care of the imbalance whatever you do...

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