carbon fibre rudder stock shaft bearings

Discussion in 'Sailboats' started by ramminjammin, Jan 12, 2011.

  1. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    In this neck of the woods there have recently been two sinkings of large expensive boats that snapped their CF rudder shafts both vessels were lost so no root casue failure analysis is possible. Snapping a shaft can be from poor fatigue related design, layup faults, collision with an animal or debris, and you cannot design for that in your FOS.

    The catch it that you use a light material but need a large diameter tube with a material that exhibits catastrophic brittle failure.
    The shaft breaks at the maximum bending moment inside the vessel and you are left with a hole the size of the shaft. The boat can be lost in seconds

    I think it should be mandatory that With brittle rudder shafts there should be some compartmentalization of the vessel in the stern.
     
  2. Karl Wittnebel
    Joined: Jun 2004
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    Karl Wittnebel Junior Member

    There are much less risky places to save weight. And unless the class has no minimum weight, it seems penny wise/pound foolish to trust such an important part to a process which fails commonly even in highly engineered applications.

    I have a carbon rudder shaft on my sailing canoe, engineered for the application. I worry all the time about a bit of sand getting into the bearing and eating the carbon away, even though I put brass bushings on it. I have also had similar shafts fail, fortunately in racing situations where help was close at hand.

    When you have seen how fast even thick carbon laminate disappears when subjected to an abrasive sanding disk on a die grinder, it seems truly foolhardy to trust anything other than stainless for a rudder, again unless every ounce truly counts.

    Good luck either way, and good sailing.
     
  3. ramminjammin
    Joined: Oct 2009
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    ramminjammin Junior Member

    update

    update

    the rudder build is underway
    I have chosen old school , and will be using stainless 316 solid two inch steel

    cost of the stock = $275
    labor to build the rudder, epoxy and pine , machine the stock and weld $800
    materials estimate : wood, fiberglass, bearings = $325

    total cost around $1400

    thanks again for the discussion
     
  4. CatBuilder

    CatBuilder Previous Member

    Could you name the sources you found for the 316SS and bearings?
     
  5. alex_sailor
    Joined: Jan 2011
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    alex_sailor Junior Member

    Eric,

    Having received advice from you in the past, and having drawn Phil into the discussion about building something for us to replace that 1970's rudder, I need to ask what the best practice is, if for example the I.D. of the shaft aperture for the rudder post is fixed, or size limited, will a C.F. post of the same O.D. do a job as adequately as the original S.S. shaft did?

    In this case, we are dealing with a 2.25" (60mm) SS post.
     
  6. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Hi Alex,

    Assuming the current shaft is 316 stainless steel, its yield strength is about 30,000 psi, and its modulus of elasticity is about 29,000,000 psi. The moment of inertia of the 2.25" dia. shaft is 1.258 in^4 and its section modulus is 1.118 in^3. The bending moment it can support, therefore, is strength x section modulus = 30,000 psi x 1.118 in^3 = 33,540 in-lbs.

    The carbon fiber shaft should have a stainless steel sleeve in way of the bearings, and you can get a 2.25" dia x 0.065" wall 316 s.s. sleeve through McMaster Carr in short lengths. Allowing for a glue line, that effectively makes the OD of the carbon fiber shaft 2.0" dia. The carbon fiber laminate, when all is said and done, will likely have a reliable strength of about 75,000 psi and a modulus of elasticity will be about 10,000,000 psi. Working backwards from the bending moment, the required section modulus is 33,540 in-lbs/75,000 psi = 0.447 in^3. For a 2.0" OD, a wall thickness of 1/4" (0.25") will have a section modulus of 0.537 in^3. The moment of inertia will have the same digits, but in different units, 0.537 in^4. So technically, a 2.0" x 0.25" wall carbon fiber tube will be the equivalent strength wise of the 2.25" dia. solid stainless steel stock.

    However, the stiffness of the carbon fiber tube will be only a fraction of that of the stainless steel. For the stainless steel stock, the total overall stiffness is E x I = 29,000,000 psi x 1.258 in^4 = 36.482 x 10^6 lbs-in^2. For the carbon tube, E x I = 10,000,000 psi x 0.537 in^4 = 5.37 x 10^6 lbs-in^2, which is only 16.2% of the stiffness of the stainless steel stock. With so much less stiffness, the rudder is going to bend appreciably more, meaning that you may need to change the current bearing to a multi-axial bearing. This does not account for whatever stiffness the stainless steel sleeve will give to the rudder stock, which could be significant, but will it be enough?

    In my opinion, if you are going to have to change the bearing for a multiaxial bearing that allows some bending of the shaft within the bearing, then you might as well consider a whole new bearing installation for a larger stock and gain back the stiffness that you need with a bigger stock section shape. I'd still advocate a stainless steel sleeve over the carbon fiber stock.

    If you really don't want to change out the bearings and get involved in a larger modification, then I would stick with the solid stainless steel rudder stock and duplicate what you already have.

    I hope that gives some insight. If you want to talk to me directly, I'd be happy to, give me a call.

    Eric
     
  7. alex_sailor
    Joined: Jan 2011
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    alex_sailor Junior Member

    Thanks Eric for the detailed analysis, wow, there is a lot to this. One of your comments at the end caught my eye and I thought it might be worth clarifying

    If you really don't want to change out the bearings and get involved in a larger modification, then I would stick with the solid stainless steel rudder stock and duplicate what you already have.

    I am not sure if the stock I have is solid or tubular, the original rudder is still intact, so if it is tubular, would that mean the carbon numbers come closer to the steel?
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    Yes, the s.s. stock would have a smaller moment of inertia and section modulus, and so yes, the carbon fiber stock, if left at 1/4" wall thickness, would be closer to it. Can you see the top of the stock? Is it solid or hollow?

    Eric
     

  9. alex_sailor
    Joined: Jan 2011
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    alex_sailor Junior Member

    I can see welding on the very top, which indicates a hollow shaft. But I will take some pictures in the next few days and have them analysed to see if it is probably hollow or full. My guess is that it is hollow, but that is unconfirmed.
     
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