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Thread: Marine stern bearing design and material

  1. #1
    Join Date
    Apr 2012
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    Default Marine stern bearing design and material

    Gents; meet Mollie. She is 65 feet of glorious comfort. Steel construction 27 tonne; 5mm plate with 20mm bottom sheet.

    The picture was taken by self at Pihia New Zealand. Over the previous couple of months I had had the pleasure of being crew on a trip from NZ to Fiji, glorious weeks cruising around Fiji islands and a few weeks later joining her again for the homeward leg from New Caledonia to NZ.

    Is it too bold of me to suggest that only those who have been on such an adventure can appreciate the totality of the experience. Bootcamp, team bonding, great comradely all bound into one.



    Her captain has asked me to reach out to you all to get your input to an issue he has with the stern bearing of his beloved vessel.

    Motor power is a 6 cylinder diesel motor. Transmission is a thru a Hundested box that manages thrust, clutch and the variable pitch mechanism. The tail shaft (65mm diameter and about 1.5 meters long) is hollow to allow the tension rod access to the propeller to alter the pitch.

    The tail shaft has just one bearing; that being at the rear. The front bearing is the Hundested box itself. There is a fixed coupling between the two.










    The tail shaft runs in a hollow stern tube. The stern tube is oil filled. The front of the stern tube has a gland packing like seal and (we think) the rear has a tipple lipped rubber seal.

    About 7 years ago, shortly before the current owner took possession, the rear bearing was replaced. The images are the "old bearing". Clearly this is a cutlass bearing of some kind.

    The "new bearing" did not last long and that is where we seek the input of the wisdom here. There is a vibration in the shaft that limits the available max RPM.

    My reading has it that cutlass bearings do not normally run in oil; normally they are lubricated by and cooled by the seawater.

    On this vessel the stern tube is filled with oil; there is a header tank about 2 meters above the tube with two flexible pipes running to the stern tube.

    The stern tube itself also runs thru the diesel "day tank", the idea being this immersion in the diesel provides some additional cooling.

    The oil header tank never gets warm; hence the stern tube never heats up or the pipes to/from the header tank do not create suitable convection.

    The bearing is due for replacement. Question is should the bearing type be changed and what other "minor" change can be made to improve the design.

  2. #2
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    Aug 2010
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    My experience is limited to "traditional" cutlass bearings that are water cooled as you describe. If possible, I'd get in touch with the builder and see 1) what the original configuration was and, 2) if they have since thought about improving it, and how. It sounds like the "new" bearing was someone's idea of an improvement that may or may not have been.
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

  3. #3
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    The yacht is by Denis Ganley Yacht Designs Ltd. My understanding it was one of the last ones he designed.

    http://www.ganleyyachts.co.nz/steel_boats.html

    In asking for your input the owner is trying to get ahead of the curve.

    Boat will need to go to Whangarei to be put "on the hard" for that repair and a few others. Tis presently in Auckland. All time on the hard has cost, so the more we know of what is and could be the more he will be in control of the advice and decision making processes.

    We have been talking on this between ourselves for quite some time now. Internet searches are of some help but not all is applicable.

    One of the questions/options is; could a bronze bush be used in this location?

  4. #4
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    Cutless (sic—AFAIK it's actually a trade name that has become generic) bearings are not intended to run in oil. They are indeed designed to be water-lubricated.

    As the shaft is in an oil bath, and the seals are outboard of the bearing, then yes, a bronze bearing running in the oil bath would be a better idea. But you'll need to slide the shaft out enough to check the bearing area very thoroughly, because Cutless bearings that don't have a constant flow of water through them can wear the shaft, which will not do the bronze replacement any favours.

    Oil-filled stern tubes are common in steel yachts, the idea being to keep salt water away from steel in an area that can't otherwise be protected. The outboard end seal must simultaneously keep the water out and the oil in, which asks a lot of it.

    The vibration may be a product of a loose stern bearing, but it could also be any number of other things, like a bent shaft, an out-of-balance prop, or misalignment between the shaft coupling and the gearbox output flange—the last is sadly common, especially if the adjustment is not done with the yacht rigged and in the water, i.e., when the bending forces on the hull are those in normal use. The Hundestet gear is probably indestructible, but it would pay to check that pretty thoroughly too.

    There are good engineers in Whangarei, who will no doubt put you right.

    Incidentally, it's Paihia, not Pihia. I live just up the road.

    (Edit.) Oh, and this http://www.waikatobearings.co.nz/service/vesconite-1 might be worth a think.
    Last edited by Mike Burch; 08-25-2019 at 02:00 AM.

  5. #5
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    How long did the original lignum vitae bearing last? I think you'll have to correct the vibration to use any kind of plain bearing.

    allan

  6. #6
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    That bearing looks more like tufnol than lignum vitae. The lignum vitae I have seen is yellow.
    Last edited by old mart; 08-25-2019 at 09:01 AM.

  7. #7
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    This can be a complex subject to say the least.Oil type tube bearings come in a variety of compositions.The old one you have pictured looks to be a phenolic shell with probably a Frelon or Rulon liner which appears to be scorched.Other types will be Bronze usually lined with Babbit or Bronze with wood or plastic bearing inserts.
    https://mim-hamburg.de/en/tenmat/faq

    To complicate matters,some "closed circuit" lubricated stern tubes actually use fresh water for lubricant instead of oil.All of the parts are very similar to those that require oil,the only way to tell is per the builders instructions. Phenolic bearings can be used with either water or oil,though the type oil used is critical.

    In both types usually there is a small pump in the system to insure flow through the bearing.If it's an oil type,since oil has about half the thermal capacity as water,it is possible for it to overheat and form a carbon layer inside the bearing area(maybe why the old bearing is scorched?)

    If it were me,I would spend some time trying to work out the details of the system.Does it have a circulation pump?Is it working?Are the oil passages open and free to flow?
    I just need one more tool,just one!

  8. #8
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    Mar 2013
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    Norman this brings back a lot of good memories. I cruised from Canada to new Zealand a lot of years ago in a steel yacht that I had built myself. I agree with most things said above. I would say the old cutlass stern bearing being made of rubber would swell up if run in oil. I looks like that is what happened then it got burnt and went hard. Wrong application for a cutlass bearing. I agree with using a bronze bushing in it's place. If it were me I would pull everything out and inspect it . Like previously mentioned the shaft could be worn where the cutlass bearing runs on it. This is probably what is causing the vibration providing the alignment is OK. Get it right once and you are good. A steel yacht is far too rigid for the hull to change when taken in and out of the water. I also agree that there are a lot of good engineering people in Whangarei. I spent about 1 1/2 years there when I was in New Zealand.
    By the way she is a beautiful looking yacht. You people do it right.
    Larry

  9. #9
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    Quote Originally Posted by Mike Burch View Post
    Cutless (sic—AFAIK it's actually a trade name that has become generic) bearings are not intended to run in oil. They are indeed designed to be water-lubricated.

    As the shaft is in an oil bath, and the seals are outboard of the bearing, then yes, a bronze bearing running in the oil bath would be a better idea. But you'll need to slide the shaft out enough to check the bearing area very thoroughly, because Cutless bearings that don't have a constant flow of water through them can wear the shaft, which will not do the bronze replacement any favours.

    Oil-filled stern tubes are common in steel yachts, the idea being to keep salt water away from steel in an area that can't otherwise be protected. The outboard end seal must simultaneously keep the water out and the oil in, which asks a lot of it.

    The vibration may be a product of a loose stern bearing, but it could also be any number of other things, like a bent shaft, an out-of-balance prop, or misalignment between the shaft coupling and the gearbox output flange—the last is sadly common, especially if the adjustment is not done with the yacht rigged and in the water, i.e., when the bending forces on the hull are those in normal use. The Hundestet gear is probably indestructible, but it would pay to check that pretty thoroughly too.

    There are good engineers in Whangarei, who will no doubt put you right.

    ...
    Good post.

    The proper procedure is to remove the shaft to thoroughly inspect for straightness and surface condition. Then replace any worn or inappropriate parts. Doubt the Cutless bearing is recommended. An initial alignment procedure is done during the haulout with a final alignment after launch.
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  10. #10
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    Mar 2013
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    This is a little off this subject but related to Whangarei. When I was there I was helping a friend with a rear seal problem on a small Perkins diesel marine engine. It had a two piece aluminum carrier that bolted in behind the flywheel to hold the packing that was the rear main seal. One of the halves of the carrier had some flaws in it and we needed to find a replacement. We checked with a Perkins dealer in Whangarei and he checked his sources throughout New Zealand and Australia but there wasn't any to be had. We went to a machine shop in Whangarei to see if he could make one. We thought he would have to make a whole new one then cut it in half but he said no, he could just machine the half we needed. I don't remember how much he charged but it was very reasonable and he made a perfect match for the half we needed. We were impressed.
    Larry

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