carbon fibre rudder stock shaft bearings

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

  1. ramminjammin
    Joined: Oct 2009
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    ramminjammin Junior Member

    Hello and happy new year !

    I have to build a new rudder for my 40 ft Harris trimaran

    It will be fiberglass, filled with foam , I would like to use a
    composite shaft and whatever bearings that will entail.

    The rudder plan ( circa 1970's) called for a 2 inch stainless shaft with a .25 wall thickness.

    Can anyone let me know what carbon fiber shaft I could use to substitute , what bearings I would need , and where I can purchase those things ?

    thanks so much
    Peter
     
  2. gonzo
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    gonzo Senior Member

    That would require a NA to do a custom design. Do you really need carbon fiber?
     
  3. michael pierzga
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    michael pierzga Senior Member

  4. CutOnce

    CutOnce Previous Member

    Second the Phil's Foils idea. Phil and Dave have done a lot of carbon updates to existing designs - and they've got the in-house experience to make it work.

    --
    CutOnce
     
  5. sean9c
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    sean9c Senior Member

    I looked at doing a CF rudderpost on my powerboat project. It seemed like a natural replacement for SS. Talked with a NA and found it fairly difficult to develop. It's easy to figure metals strength as anything you will use, if it is tracable, comes with certs where actual testing, tensile, yield and all that, was done. Not so easy with CF as most stuff is custom made and there is no testing, so you have to start looking at each laminate and orientation and throw in a whole bunch of trust, to calc strength.
    I found that it was really hard to replace SS with CF on a rudderpost with any confidence. Also have to remember that if you whang your rudder on something a SS shaft will bend a CF one won't, it'll break.
    Check out Dave Gerr's book, he has a good section on steering gear and calcing loads. It was also reprinted in Pro Boatbuilder. From what I remember Gerr has the tensile of CF lower than that of say Aquamet 22 which was a surprise as the CF tube manufacturers list the tensile of their tubes as significantly higher.
    After fooling with it for a while I decided to be safe and stick with SS.
     
  6. sean9c
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    sean9c Senior Member

    Hadn't really thought about your first post until after my reply.
    You said your design calls for a 2" X .25 wall tube. That's not much of a rudder post unless the rudder is pretty tiny or the boat slow. Being that it's a 40ft tri I'm guess neither is the case. You might want to use Gerr's formulas to calc 'post size just to double check, that rudderpost sounds too small.
     
  7. ramminjammin
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    ramminjammin Junior Member

    thanks everyone for replying to my post
    I would have thought carbon fiber technology more advanced

    i will check that steel spec as i will probably go that route
     
  8. michael pierzga
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    michael pierzga Senior Member

    You can make a rudder as complicated as you like .... call Isotop then break out your wallet if you want a high tec one. !!
     

    Attached Files:

  9. CutOnce

    CutOnce Previous Member

    I would not dismiss carbon without talking to someone in the business. Eliminating 50-100 pounds that is far from the center of the boat can make a noticable difference in pitching, as well as better profiles can make a huge difference in steering effort and response. Some boats just don't balance right, and it may be due to material compromises made at design time.

    It really depends on what your goals are: faster, easier, better balanced or it may be bling and not substance. If you are operating within a set of one design rules that is another issue.

    --
    CutOnce
     
  10. BATAAN
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    BATAAN Senior Member

    I remember some CF spade rudder setups in the 90s that had Delrin bearings that kept failing. The rudder itself was fine, we just had job security due to someone's engineering screwup. Oh dear, it broke again. Travelift charge, blocking charge, washdown charge, labor at $lots per hour charge, materials, lay days, the yard management thought those bearings were the greatest thing since beer, which they consumed in quantity in the office I think. Rudder tubes were pretty big, about 4" on a 36' boat maybe. Don't know if they finally sorted it all out but probably so.
     
  11. ramminjammin
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    ramminjammin Junior Member

    the boat is 10,000 pounds, not a lightweight for a 40 ft tri
    however , it is not slow , so it should reach 20 kts , and with any kind of sea there will be stress
    I only wanted to go with carbon stock / tube as it will bond with the foam / fiberglass of the rudder which i have to construct

    so I can use steel that will eventually corrode
     
  12. michael pierzga
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    michael pierzga Senior Member

    Ive never sailed a boat with a carbon tube rudder shaft...they have alwvays been box section shafts . Box sections are easy to make in your shop..
     
  13. Eric Sponberg
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    Eric Sponberg Senior Member

    Roger the above on carbon fiber rudders and rudder shafts, of which most of the information is quite correct. Best that you have someone who knows what they are doing when it comes to design and engineering a new rudder and stock. I vote, too, for Phil's Foils--I have worked with them in the past and they do nice work for a reasonable price.

    As mentioned above, the properties of carbon fiber, as a fiber, are very well known. But as you process the fiber into yarns and tows, and then into fabrics, the mechanical properties diminish. Then, when they are incorporated into laminates, laminate plies and orientation and fiber-to-resin content all play a part in determining the final mechanical properties, which are diminished yet again. These values are all dependent on the skill of the designer/engineer and the builder, and that is why testing of sample laminates in a laboratory is so important. No two laminates of anything are exactly the same, and so their final properties are never the same. That is why testing is really so important.

    The other thing about carbon fiber rudders is that you have opportunity to mold non-round rudder stock sections which generally are stronger and stiffer than round sections simply due to their geometry. You then must have much larger bearings for these types of rudder stocks simply because the geometry of the stocks is so much greater. I have also had good luck with Jefa bearings (Denmark) in my designs. Another source is JP3 bearings (France).

    It is extremely rare that anyone would use an off-the-shelf carbon fiber tube as a replacement for a stainless steel rudder stock--in fact, I have never heard of anyone actually doing that. That is probably because carbon fiber tubes may not be strong enough (not enough wall thickness or proper laminate lay-up with the right fiber orientations), and almost certainly not stiff enough (most carbon laminates are only about a third to, at most, half the stiffness of stainless steel).

    If you do not want to go to the expense of also replacing the bearings, then sticking with the original metal design makes sense. If the new rudder is significantly larger or has more turning power, then the strength of the stock may have to be increased by increasing the wall thickness for the same outside diameter so that the new stock will fit in the same bearings. Frequently, on rudders that have a solid stock, we can replace regular stainless steel stocks (usually 304 or 316 alloy) with 17-4 alloy such as Aquamet or Aqualloy. I like to use Aquamet 22 which has the best strength and stiffness, and the best corrosion resistance.

    I hope that gives some additional insight.

    Eric
     
  14. mark775

    mark775 Guest

    Eric, Having held both CF and stainless in my hands and noting how much stiffer CF is for it's weight, I needed to go look up the Young's modulus for the two materials after seeing your post. Carbon of course weighs far less and is only two thirds as stiff along the grain as stainless. Other things come into play like fiber orientation, resin content and the amount of "kinking" between strands of the weave, so you mean for a given size "most carbon laminates are only about a third to, at most, half the stiffness of stainless steel"? It seems a little unfair to leave it like that when CF can easily be engineered to be stiffer by weight than SS.
    Also, the comment about not having to be round - I agree and would like to point out advantages, IMO, of a rectangular or trapezoidal shaft, namely that the shaft, having some chord length, can help support the rudder torsion and allows a narrower foil section. It also gives you some bolting area for attaching a removable rudder (handier than digging a hole to remove a one piece rudder/post!). I'd like to see this rudder post done in CF - I don't have the knowledge to orient the fibers to best effect so I concur with the notion of hiring someone who does. I looked everywhere for a company I know that was doing a great job of this but can't find them so I suppose the one everyone else is recommending will do.
     

  15. Eric Sponberg
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    Eric Sponberg Senior Member

    Mark, you are absolutely right--the specific stiffness of carbon fiber (stiffness per unit weight) is greater than stainless steel. However, for pure modulus, regardless of weight, of carbon fiber unidirectional laminates (no off-axis fiber) is typically around 11-15,000,000 psi (76-103 GPa) for standard modulus carbon fiber, and that is the point I wanted to make. Too many people, in my experience, think that if you look up the mechanical properties of pure carbon fiber that that is what you are going to get in the final laminate, which is just plainly not so. And then I wanted to make sure that it was understood that all those other processing factors continue to reduce mechanical properties.

    But, to increase the overall stiffness of a carbon fiber shaft over a stainless steel shaft, we can put geometry to work and make the section bigger and thicker, and it will still be lighter than a stainless steel shaft. Stiffness, of course, is directly proportional to ExI (E = modulus, I = moment of inertia of the cross section), so in the end, we can make the carbon fiber rudder stronger, stiffer, and lighter than a stainless steel pipe or round bar shaft because we have the ability to mold the shape to suit our needs. The bigger geometry of the stock helps us do that, both for I (stiffness) and Section Modulus (strength).

    I do a fair bit of CF rudder design, and the last one I did is told on my website here: http://sponbergyachtdesign.com/CopernicusRud.htm. Copernicus is a Spencer 42--tradition full-keel, CCA type of design, owned by my clients in British Columbia. The owner removed the old rudder attached to the back of the keel, faired in a new deadwood piece where the rudder was, and then installed a new spade CF rudder that I designed for him. This rudder was built by Composite Solutions LLC in Hingham, MA (links are in the article) and it is equipped with Jefa bearings. Other shops that I have used in the past are GMT Composites in Bristol, RI, and New England Boatworks in Portsmouth, RI. All these companies are capable of superior construction provided they have a design to work to. I am sure that others on this thread will be able to recommend other builders in Europe, OZ, and NZ.

    Eric
     
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