Fiberglass Rudder Shoe

Discussion in 'Boat Design' started by CharlieDanger99, Aug 4, 2022.

  1. CharlieDanger99
    Joined: Jun 2021
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    CharlieDanger99 Junior Member

    Hello, I'm looking for some opinions on my thoughts for replacing the corroded rudder shoe from my 1960 Rhodes Swiftsure project boat. As you can see in the attached picture, at some point the boat was left floating without zincs and the rudder shoe ended up being the next best anode. I've been toying with the idea of setting up a basic forge and casting a bronze replacement, but it occurred to me that if I'm going to create a mold from the old shoe it would be significantly easier and cheaper to simply lay up fiberglass. I know that a 1708 cloth can conform to the necessary radii, and this would essentially future proof the new shoe from ever being subject to electrolysis again. I expect this would be plenty strong enough to handle the loads from the rudder at the same thickness, but being that I haven't seen any similar examples I wanted to hear from others. The only real modification I would have to make over the original, aside from material, is the addition of a replaceable bushing to prevent friction damage from the rudder post rotating in the socket. Feel free to make any suggestions.

     
  2. gonzo
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    gonzo Senior Member

    I think this is the rudder gudgeon that goes at the bottom aft corner of the keel. Fiberglassing is not a solution for electrolysis, since there will still be metal in contact with the water. It would be easier to fabricate it from bronze stock. Bronze can be welded or brazed.
     
  3. CharlieDanger99
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    CharlieDanger99 Junior Member

    I'm talking about replacing the entire bronze shoe with one made exclusively of composite (aside from fasteners), not glassing over the original. Yes you are correct this attaches to the aft lower end of the keel.
     
  4. gonzo
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    gonzo Senior Member

    The tensile strength of silicon bronze is 108,000PSI and fiberglass/epoxy at most 18,000 ( with a perfect vacuumed laminate). Bronze is also stiffer and harder. The new gudgeon would have to be much larger and also have some wear surface for the pintle.
     
  5. CharlieDanger99
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    CharlieDanger99 Junior Member

    Well even in a worst case scenario with a tensile strength of say 10 ksi, the shoe is at least a couple square inches in any given shear plane and the rudder post is supported on top as well, so by my estimation the rudder would have to generate something like 40,000 lbs of force to cause a failure. That said I wouldn't have a problem with adding some thickness or perhaps putting some carbon fiber in the laminate. I just have a hard time justifying the materials cost alone to make a new one out of bronze for something that will never be visible.
     
  6. rnlock
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    rnlock Senior Member

    10,000 psi seems awfully pessimistic unless we're talking about properties out of plane. Particularly if done with perpendicular layers of unidirectional glass or, better, s-glass. OTOH, if this part can be bashed or dragged on the bottom, bronze might have a more graceful failure mode, by bending instead of breaking. And I suspect bronze is more wear resistant. If it's cast bronze, though, done at home, I expect it might not reliably be all that much stronger than a fiberglass layup. Machined from some kind of bar stock might be a more controlled situation.

    I'll admit I have a hard time visualizing the real shape of the part from looking at that picture.

    Carbon fiber is slightly conductive. I don't know what that does to the corrosion situation. I do know, however, what it can look like if it's unfortunate enough to short out high voltage wires. Also what it smells like. And then there's the little mushroom cloud.. ;-)

    I'm not quite sure how one works a bushing into a fiberglass layup, but I imagine some people know.
     
  7. wet feet
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    wet feet Senior Member

    The picture doesn't really tell us very much and needs a bit more context.I would be a bit hesitant to mix Carbon and glass as the properties are a little way apart and there is the minor ,but real, matter of carbon being a bit electrolytically active.I also have a little difficulty visualising quite how one would laminate such a fitting.Simply pouring a resin and fibre slurry into a mould gives no control over the fibre orientation and I can't see an obvious way to laminate in a female mould.Building up layers of material over a male mockup of the heel of the keel would ensure that the item would fit it's location,but then there is the challenge of adding the projection for the rudder bearing.If I was unable to avoid the job,I think i might use a section of thick 10G40 Tufnol (the American equivalent would be G10) and wrap some woven roving and unidirectional glass over it,followed by some cloth to allow a decent surface finish after the inevitable sanding.It could be done,but with the proviso that an amount of filing and sanding to the finished shape will be needed.
    Adding a bush is no different to adding a bush in any other situation-you need a sufficiently large counterbore to accept the bush in question.Conceptually,it isn't that different to a rudder tube through the bottom of the GRP hull,the proportions and location being the principle difference.Creating the counterbore would be interesting.
     
  8. rnlock
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    rnlock Senior Member

    If the part is mostly flat, machining a piece of G10 sounds like a good idea. Assuming it doesn't cost an arm and a leg.
     
  9. Tops
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    Tops Senior Member

    Bronze rudder shoe - Taiwanese boats - Page 2 - Cruisers & Sailing Forums https://www.cruisersforum.com/forums/f55/bronze-rudder-shoe-taiwanese-boats-258160-2.html#post3647620

    The OP on the CF thread just went through this process. Found the right 'guy', drew and 3D printed a mock-up, and machined from stainless. They could have also taken the mocked-up data, scaled it for shrink, and printed foundry patterns for bronze. Getting a pattern to be drafted, filleted, and smooth can be an undertaking all by itself.

    [​IMG]
     
  10. gonzo
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    gonzo Senior Member

    Some people find the most difficult way of doing things. That gudgeon is standard and typical of full keel or skeg/rudder boats. All you need is flat bar and a pipe of the right diameter. Weld them together, make holes are correct location and install it.
     
  11. Tops
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    Tops Senior Member

    Gonzo is onto something that it could be made as a weldment (machined collar, 2 plates for the sides, one plate for the bottom) instead of a cast or machining project.

    Northern Yacht Restoration | Waanderlust, a Rhodes Swiftsure https://lackeysailing.com/archived/waanderlust/waanderlust.html < link is a restoration log for a Swiftsure

    cd99_swiftsure_shoe_lackery.jpg
     
  12. CharlieDanger99
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    CharlieDanger99 Junior Member


    Here's some rough sketches to give an idea of the original shape
     
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  13. Tops
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    Tops Senior Member

    Thanks for the images. Patterning that for casting would not be so bad, especially as a lost foam or lost PLA pattern. It would be neat in bronze...
     
  14. CharlieDanger99
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    CharlieDanger99 Junior Member


    Yes 10kpsi is substantially lower than I would hope to achieve with a laminate. G10 has a tensile strength of 40 kpsi. I'm struggling to find numbers on what percent of this value can be achieved with a hand layup of equal thickness, but my point still stands that I don't foresee any situation in which the rudder can apply this amount of load before it breaks itself in half or snaps the tiller, except possibly in a hard grounding. Even in such a case the leading edge of the keel would make contact first absorbing most of the shock load. I suspect the reason these shoes were cast bronze is more to do with the labor cost of what will likely be a fairly meticulous layup vs the existing tooling for the various other bronze components on board.
     

  15. gonzo
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    gonzo Senior Member

    I think that the limiting factor in using a fiberglass gudgeon is stiffness. To achieve the same stiffness as the original, it will be several times thicker.
     
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