Carbon fiber over alloy mast?

Discussion in 'Boat Design' started by paultuxworth@gm, Nov 10, 2012.

  1. paultuxworth@gm
    Joined: Nov 2012
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    Location: philippines

    paultuxworth@gm New Member

    HI All
    I am currently in the philippines with a dis-masted 40 foot cat. Not wanting to go to the expense of importing a mast, I have found a alloy mast of a 50 foot yacht that looks like the right height. Can I put some epoxy carbon around the outside of it to stiffen it up to suit a multi?
     
  2. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    You can, but it probably won't do you much good. Carbon fiber unidirectional laminates have about the same modulus of elasticity as aluminum, and carbon fiber fabric laminates less so. You should never put unidirectional in a laminate all by itself because the fibers can break away if not captured by an overlay of cloth of some kind, and carbon cloth is usually called for with carbon fiber UDR. A mixed carbon fiber laminate will have less modulus of elasticity than the aluminum when all is said and done. The load in the mast is always going to favor the stiffer material, so it will likely still pass through the aluminum more so than the carbon fiber. A carbon fiber overlay will only add thickness to the moment of inertia that is required, and that just does not jack up the strength and stiffness of the mast section enough to accomplish what you want to do--take an undersized monohull mast and put it on a highly loaded multihull. It just is not going to work.

    The other thing to remember is that carbon fiber is a very noble material, very high on the galvanic scale, about as noble as gold and silver. Aluminum is way at the other end of the scale, the least noble. So, in a marine environment, aluminum in contact with carbon fiber will corrode very quickly. The aluminum part of your mast will disappear in short order.

    I would advise you to keep looking for a more suitably sized mast, and you may just have to bite the bullet and import one.

    Eric
     
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  3. Steve W
    Joined: Jul 2004
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    Steve W Senior Member

    You would need to isolate the carbon from the alloy with a layer of glass first and make sure the carbon cannot some in contact. Apart from the height , how does the section compare?, if its comparable then just use it as is. What i would do is get the measuremants off the mast section,including of course the wall thickness and if any of the dimensions are smaller,contact the designer of your cat, if possible and he could tell you if its suitable, if its close but a little larger you could just use it as is. I presume you would be just using the extrusion with your existing rigging.
    Steve.
     
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  4. michael pierzga
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    michael pierzga Senior Member

    Even with sandblasting, etching and carbon isolation, its difficult to keep epoxy bonded to aluminum over the long term.

    find out what extrusion the mast is , then consult with a sparmaker. Perhaps the mast section is capable and all you need is addition standing rigging , local reinforcment or compression reducing measures like two to one or halyards locks
     
  5. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    Whilst I agree with most of what's written above about galvanic corrosion and the likelihood that adding CF to a alloy mast isn't likely to be a practical fix, I do disagree about the stiffness observations.

    I've made a lot of alloy and CF aircraft components, and even a hand layup of uni CF with epoxy will give a value of E of around 2 1/2 times that of aluminium. As noted already, using a plain uni layup wouldn't be wise, but a layup schedule that consisted of a layer of 45 degree biax glass next to the alloy section, followed by alternating layers of uni and 45 deg biax CF to the thickness required, and finished with a sacrificial layer of biax glass would probably work.

    Because the CF will be far stiffer than the alloy, and because it will have a greater Area M of I (because it's further from the centre of the section), it will take the vast majority of the load, the alloy section would, in effect, just be a former.

    The cost of making such a mast would be pretty high and involve a lot of work, so that, together with the difficulty in ensuring that it stayed true (I always layup things like this vertically, not practical on a mast of this size) it probably wouldn't make sense.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    That would make the modulus of elasticity about 25 x 10^6 psi. In my experience, I have never seen a hand lay-up of standard modulus carbon fiber and epoxy UDR laminate exceed about 11 x 10^6 psi, and that is for about 60% fiber content by weight and under a vacuum bag, laminated by a professional laminator.

    I agree with everything else, including that such a repair would not be practical or cost effective.

    Eric
     
  7. Jeremy Harris
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    Jeremy Harris Senior Member

    I accept that there is some variation in E with layup technique, but it would be a very poor layup (excessively resin rich) that reduced the stiffness down to the rather low level of aluminium. The generally used figure for aluminium and its alloys is around 69 to 70 GPa, a hand layup of CF uni (at around 50% resin/reinforcement ratio) will give between 150 and 180 GPa.

    If the CF layup was vacuum bagged, or even just wrapped in heatshrink sleeving and compressed, then the resin/reinforcement ratio could get to 40/60 and E for the uni CF layup improve to over 190 Gpa. A good prepreg layup, autoclaved to cure, could get to well over 300 GPa.

    There is a lot of design data available to confirm these general figures, including the excellent Engineering Toolbox site. My experience has been that the figures they quote for hand layups tend to be proven to be pretty close to those achieved when samples are tested.
     
  8. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    ...broken mast can be repaired easily with a sleeve....can you make one or get just the sleeve section imported.
     

  9. bertho
    Joined: Aug 2006
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    Location: asean archipelago

    bertho bertho

    mast philippines

    I don't know where you are in the philippines, but when i was there, a nice profile (proctor ) was used as flag pole at cebu mactan marina .!!. and was for sale..
    will fit a 40/45 cat
    I still have the owner contact in case..
    best rgds
    bertho
     
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