Aluminium rudder stock - bearings choice

Discussion in 'Boat Design' started by Ugo, Dec 2, 2012.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Better for what??

    You're now going from a heat-treatable 6000 series to a non-heat treatable 5000 series.

    Until you know what the "better" bit you're referring to is, how do you know whether the change is better or worse or no different?
     
  2. Ugo
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    Ugo Junior Member

    Thanks for your advice.
    As regards welding strenght, I'm going to meet ISO reccomendations:

    A.1.1 General
    Welding generally reduces corrosion resistance, mechanical properties and impairs heat treating. Therefore:
    • in way of significant welding, one shall use the as welded properties given in Table A1 (design stres for 6082: 78 N/mm2) ;
    • here shall be no "end to end" joint rudder stocks using welding within 0,15 hb from the point of maximal mechanical stress, unless specifically engineered.


    all the parts (solid bar + spokes) will be machined at my shop; all the weldings will be performed by a pro welding shop using MIG.
    No welding is supposed to come in contact with water or spray, since only the spokes inside the foil are welded and they are supposed to be protected by the GRP and the core...unless it fails letting water entering in.

    as regards fatigue I actually need to go further in detail, I'm assuming it is considered in ISO scantling rules but I'll double check.

    and...no, I'm not sure I want to go for 6082, I'm triyng to figure out if it will work or not! ;)

    thanks again
    Ugo
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly...never assume :p
    Secondly...not it is not 'considered'. You have to do that separately.
     
  4. tazmann
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    tazmann Senior Member

    I thought we were talking about rudder stock ?
    If I was to build a rudder like the OP here or even a simple transom-skeg mount and I wanted to use aluminum to save weight
    What grade aluminum would you recommend for the pins/shaft ?
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    I wouldn't.
    I would use stainless steel, 316L for the welded part, elsewhere 316.

    In a high fatigue and sea water environment, aluminium is the last material to choose. It isn't about strength it is about fatigue. This is what design is all about, not just numbers and sales brochures with pretty pictures of “this is where the product is used successfully”. It is about understanding the environment the final product, beit boat, rudder, mast etc, and establishing what the environmental effects shall be and the anticipated lifetime loadings and service life, maintenance etc etc.

    The difference in weight is nothing compared to the problems of not doing it this way, such that the difference in weight is in reality is minor, may be about 5-10kgs at most, on a boat this size. Unless you’re going for world records or in competitive racing on a weekly bases, a few kgs for peace of mind is well worth it.
     
  6. Spartan
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    Spartan Junior Member

    I'm chumming in here with two hands tied behind my back.....

    I have used UMHW on everything from working table tops to slide bearings. Dirt cheap. Available everywhere, can be worked on with woodworking equipment and I'm going to guess that it is not affected by salt water. Note the word "guess".

    Also, nylon stock would be another option.

    As to accuracy, I don't know how much is needed. If the housing design has a cap that can be compressed then one can V a set and compress the slack out of it.

    For a low stress area, it could be an easy solution.

    Just a thought...............don't shoot me. :cool:
     
  7. sean9c
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    sean9c Senior Member

    You're going to anneal 6061 by welding it. Are you going to re-heat treat after welding? If not you better be using 6061-T0 for your strength calcs.
    If you do plan on heat treating you might want to check the costs before going forward. Heat treating 6061 is a 24+hour cycle, so it's expensive.
     
  8. tazmann
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    tazmann Senior Member

    Thanks, At least now I know your take on the subject
    The rudder I have now is built from 12 gage steel with 1-1/2" pipe in the forward 1/3 running up the length of it , 5/8" stainless pins welded directly on the front side . pretty simple setup but heavy and I also want to at self steer off the backside, add to the fact an outboard with bracket and a solar panel arch all at the transom is going to add up a lot of weight so going aluminum I think would be a better option, Just need to figure out the aluminum pin material
     
  9. JSL
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    JSL Senior Member

    Not had time to read all the posts in detail but:
    Suggest you follow a classification society (ABS, DNV, Lloyd's, BV, etc) or industry (ABYC,etc) guidelines.
    A scenario based on some first hand experience (some pleasant... others not!)

    You have spent many hours and money in your boat--->You may want to get insurance. -->If you do, the insurance co. will want a survey,--> The surveyor will follow a classification guideline.(if they don't, they will consult someone) -->If the boat does not comply, you may not be able get insurance.
    Option 2
    You 'wing it' [with a non approved material or technique], --> are in an 'incident' with another boat , (dock, etc) --> there is an investigation, --> you are found non-compliant with industry standards, --> lawyers jump in --> they have a field day ---> guess the rest!

    moral of the story: do it right the first time.

    The same applies if you want to sell you boat.
     
  10. Ugo
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    Ugo Junior Member

    Dear JSL,
    I followed ISO CD_12215-8 scantling rules actually (mainly based on ABS scantling rules).

    My point was originally about bushing designs (if a low module material is used, will plain bushing work properly or do I need to switch to self aligning bearings? Is there a "rule of thumb" based on shaft deflection at bottom bearing neck that can be used to decide which kind of bearings shall be used?).
    Later I've been suggested to check my design against fatigue, if alu alloy is used.

    I'm working on the second matter at the moment - I'm trying to figure out the load pattern for my application (daysailing inshore sailboat)

    thanks
    Ugo
     
  11. Steve W
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    Steve W Senior Member

    Were you planning on a solid shaft or a tube? Often times a thick wall tube will work and allow you to go to a larger OD so you will have less deflection and fatigue without paying a weight penalty, you are pretty much governed by what you can fit within the blade generally requiring some tapering of the bottom of the tube. regardless i dont see any need for anything other than plain bearings on a boat this size but you generally machine the bearing to match the shaft. The Wylie 40 we have locally uses a large diameter thick wall tube, 30yrs old and counting, its always been in the great lakes though so no corrosion.

    Steve.
     
  12. Ugo
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    Ugo Junior Member

    The SS shaft I am using at the moment is a solid 48mm O.D. bar tapered at both ends.
    The new alu shaft should be 50mm O.D. solid (OD calculated by ISO scantling rule)

    I have to admit, however, that I did not considered fatigue so I'm a bit concerned about it - now I'm not that confident to go for alu (6082 T6) as initially planned

    rgds
    Ugo
     
  13. Steve W
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    Steve W Senior Member

    I wouldnt be too worried by fatigue, its a red herring, you will only get fatigue if you get a lot of flexing, the bigger the shaft OD the less flexing, hence my suggestion to use the biggest diameter you can fit inside the rudder blade while keeping the weight down by using a thick walled tube instead of solid. That said on a boat the size of yours i would use a solid extren bar or similar, its well proven on the Santa Cruz 27 and they race offshore in the races to Hawaii surfing well into the teens in the trades.If i were doing a bigger boat i would probably make my own composite tube so i could get some axial material in there and then build up a bit of sacraficial glass to machine true where the bearings would go.

    Steve.
     
  14. michael pierzga
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    michael pierzga Senior Member

    As for bearings. Self aligning or fixed.

    This is an all to common rudder with a fixed delrin ? bearing and stainless rudder sleeve.

    The delrin ? bearing swelled, gripped the stainless rudder sleeve, tore it off the composite shaft and you can see the sleeve, buried in the rudder tube, still locked into the delrin bearing.

    It will have to be cut out and rebuilt. The boat is only a couple years old .

    Machine your bearings carefully if you go with a fixed, non self aligning bearings
     

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  15. SukiSolo
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    SukiSolo Senior Member

    When machining acetals (Delrin) watch out for 'spring back'. It is very common on long holes. Measure each end Ø - bet you have a taper! You need to ream out carefully or gently redrill the hole until the correct size. this is the most common problem leading to seizure. At least that has been my experience.
    Check the exact grade for water absorbtion but I don't recall it being a problem on the common grades.

    BTW not all Nylons swell with water - try Nylon 12 and that is used in transparent type as oil inspection covers on automotive (motor cycle amongst other) applications. It is mainly 6 and 66 which are too brittle out the mould and need 24 hrs+ to absorb water to flex correctly.
     
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