carbon fibre rudder shafts

Discussion in 'Multihulls' started by jamesa, Mar 27, 2007.

  1. jamesa
    Joined: Mar 2007
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    jamesa New Member

    Im building 15m sailing cat and would like to make carbon fibre rudder shafts but have no plans. anybody got any ideas
     
  2. Frosty

    Frosty Previous Member

    I personally have not seen carbon fibre shafts. However after making quite a few for my 13.6 meter power cat I can tell you that there is a vibration that can occure unless they are very stiff.
    I assume you will be searching for lightness? I suppose you could go up on Dia to achieve this but what about as it goes down into the rudder.

    I would aslo advise you to stay clear of making bearings from nylon or plastic of any kind. For some reason they seem to expand.

    I have seen many people with stuck shafts and rudder shafts from using nylon or its equivelent.

    I have had great success with normal cutlass bearing types.

    Although here in the far east they make a very cheap equivelent they have a slight rubber sleave wich gives a lovelly tight but less harsh feel.
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Rudder shafts of any type are an engineering problem, and it is easy to underestimate the loads required, particularly for a lightweight boat like a catamaran. Couple that with carbon fiber laminate engineering, and you have a doubly difficult problem.

    Generally, each boat and its rudders should be taken case by case. What works for one boat will not necessarily work for another, primarily due to the differences in geometry of the rudders and the hulls they are attached to.

    As Jack Frost mentioned, Nylon bearings don't work because Nylon is hydroscopic--soaks in water and swells. You are better off using a Tides Marine rudder bearing which is made primarily of UHMW (Ultra High Molecular Weight) plastic if it is a conventional sort of rudder installation. The simplest installation is pintles and gudgeons mounted on the transom.

    Contact Phil's Foils in Ontario, Canada, for further guidance on multihull rudders. Phil and I have been around the block once on a rudder for a 50' Newick trimaran which turned out to be pretty good. To be safe, too, you should consult a naval architect for advice on the design and construction.

    http://www.philsfoils.com/

    Eric
     
  4. Trevlyns
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    Trevlyns Senior Citizen/Member

    On a bit of a tangent, I just had a thought… do you think a strip of carbon fibre tape used along the length of a ply/glass rudder at the core would have any significant strengthening abilities? I’m thinking in terms of bending and side forces. Surely such a rudder would be difficult to break?
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    One of the worst misconceptions of carbon fiber composites is the thought that a little bit of carbon will add a lot of strength. Not so.

    When you engineer a laminate of any sort with materials of different strengths and stiffnesses, you need to take into account two stiffness factors: E = modulus of elasticity of the material (Young's Modulus), and I = moment of inertia of the laminate cross section. The bending and stress behavior depends on the product ExI. If two materials of vastly different stiffness are bonded together in a working structure, in this case say wood and carbon fiber, then an even match and complementary bending behavior will be when the ExI of the wood equals the ExI of the carbon. E of wood can be anywhere from 500,000 psi to 2,000,000 psi, depending on the species of wood. Carbon fiber, on the other hand, can have an E anywhere from about 7,000,000 psi to 14,000,000 psi--many times greater than wood. In a wood rudder, the I of the rudder cross-section can be relatively high because the primary determinant for I is the thickness of the section, raised to the third power. With only a strip of carbon fiber within the laminate, however, the width of the carbon tape is generally not enough and the thickness of the carbon is really, really thin; and while the tape's distance away from the center of the rudder section is important and adds to its I, usually the I for just a strip of carbon is not anywhere near enough to add sufficient strength and stiffness to the section. The I is really, really small.

    So what I am trying to say is that if you are going to add carbon fiber to a rudder, you had better be prepared add a lot of carbon fiber over the whole skin in order to make a significant difference. Stress in the laminate is always going to build up faster in the stiffer material (the one with the higher E) and if you don't have enough material in there to handle the stress that it is going to attract, then you have a potential for a real failure.

    This is exactly what happened to the carbon fiber and aluminum rudder stocks for so many sailboats in the 1979 Fastnet Race disaster. Aluminum rudder stocks were shaved down thinner a bit (to save weight) and reinforced with just a small amount of carbon on the outside. Not enough carbon was there, yet the carbon tried to assume the lion's share of the load because it was stiffer than the aluminum. The carbon fiber failed when it got over-stressed, and once the carbon failed, there wasn't enough aluminum section left to handle the loads, and so the aluminum failed.

    Finally, one of the biggest follies I have seen over the years is when boat builders use narrow strips of carbon fiber tape laid up into the hulls of boats in a cage like pattern. They think that this carbon "cage" will add a lot of strength to the hull, making it very stiff, like an internal framework. Nothing could be further from the truth. Generally, there is not enough carbon in there to do anything at all, the builders are simply throwing away money. If you want to make a really strong carbon fiber hull, then you have to have whole layers of carbon fiber spread throughout the laminate, and on both sides of the core if there is a core.

    Eric
     
  6. tri - star
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    tri - star Junior Member

    Another cautionary note:

    I agree with both, Sponberg & frost.
    I seem to remember, way - back - when; a whole series of C. Fibre / Alum.
    rudder posts breaking. At the same time. Some, in the same sail boat race....

    In part, due to optimistic eng. presumptions. Optimisim is good.
    In fact required, if we are to get out of bed in the morning.
    However, when "optimisim" is included into your calculations...

    ' Regards.
     
  7. tri - star
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    tri - star Junior Member

    That's right,
    it was the '79 Fastnet I was referring to...just found it in the files.

    The worst I've heard, is guys painting a black line of " carbon " down
    the middle of their skate / surfboard.
    To look cool....and to save on the cost of real C. Fibre.

    'Regards.
     
  8. rob denney
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    rob denney Senior Member

    G'day,

    I have built a fair number of carbon shafts and rudders for cats and harryproas. None have failed yet. First you need to engineer the laminate. Your designer should supply this, but otherwise Eric will give you a price, or I could ask our engineer to do so.

    The actual shaft building is quite easy. I use a simple mould and 6 atms (90 psi)pressure and get a superb laminate. For a couple of hundred bucks I will tell you how best to do it, once you have the engineering.

    regards,

    Rob Denney
    www.harryproa
     
  9. tspeer
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    tspeer Senior Member

    You can buy carbon fiber tubes. For example, a quick search turns up the Graphite Store selling carbon tubes to more than two inches in diameter, and I'm sure there are many, many more suppliers.
     
  10. rob denney
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    rob denney Senior Member


    G'day,

    These tubes are all pultruded or wrapped with most of the fibres longitudinal.

    They are a bad idea for the following reasons:
    A rectangular section is much stiffer and stronger than a round one of the same width/diameter. It is also easier to attach the sides of the rudder to.
    Rudder shafts see max load at the lower bearing, they can be tapered from there in both size and laminate.
    Most importantly, they need a large proportion of the fibres at +/-45 degrees and some at 90 degrees.

    regards,

    Rob
     
  11. dougfrolich
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    dougfrolich Senior Member

    I'll second Rob on that. further design elegance can be achieved by incorporating the foil skins as 2 sides of the rectangle. saving weight and increasing strength/stiffeness.
     
  12. tspeer
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    tspeer Senior Member

    Good point about the need for torsional orientation of the fibers.
     
  13. markdrela
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    markdrela Senior Member

    An effective method to impart torsional stiffness and strength is to wrap the unidirectional tube in a carbon "sock". This is a woven tube without seams, like a Chinese finger puzzle. It comes in many diameters, is available from many suppliers.

    You put the cleaned uni-tube inside, pull on the ends of the sock to cinch it down on the tube, wet out with epoxy, and wrap or vacuum bag for compaction. If necessary, more than one layer of sock can be applied in the single layup. The sock also imparts a lot of splitting resistance and toughness to the whole tube.

    If making your own wrapped tubes, then combining a mix of 0 and +/-45 uni fibers is probably a better method.
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    Phil Locker at Phil's Foils in Ontario, Canada, tried to use carbon socks on a 50' trimaran rudder at it did not work worth beans. The first sock goes on OK if the surface is perfectly smooth, but any subsequent socks pull against the fibers of the previous socks and all the weave gets pulled out of shape. They are much more trouble than they are worth--can't get clean running fibers. It's a nice idea, but they don't work out as elegantly as anticipated.

    The easiest way we found to lay up the stocks was to simply wrap unidirectional material around the basic form, whether it be oriented at 0 deg, +/- 45 deg, or 90 deg. This got the most amount of fiber on and in place in the quickest, easiest, most assured way. It goes on fine in wet layup or prepreg. Once a basic system of layup is set up, it goes quickly.

    Eric
     

  15. dougfrolich
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    dougfrolich Senior Member

    Nice one Eric- That is what we did for our back stay pump handles "Wands". We used PVC pipe to test the concept last season, then used it again as our from. now with carbon they are lighter, stiffer, etc...
     

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