Catamaran composite beam design

Discussion in 'Multihulls' started by groper, Apr 29, 2012.

  1. Alik
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    Alik Senior Member

    I agree that sailing can have some social role; but I find it amazing that those sitting deep inside sailing box are claiming to be the only creative, innovative and great designers. Maybe they just need to look around to find better application for their creativity??

    NA students in Thailand designed easy-to-build simple boat to help thousands of those suffering from foods, and in my opinion this design it is much more social than all America's Cup designs together...
     
  2. Alik
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    Alik Senior Member

    I would say more: Morelli/Melvin are designing wind farming aluminium cats! And what? Those who really work do not show this 'sailing snob' attitude... they just do their best in every area of design they touch.
     
  3. Jetboy
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    Jetboy Senior Member

    Would any of the NA's be willing to comment on what they see as the design life of a typical multihull beam? I cycle a lot. It seems to me that carbon bike frames are good for one season or less of professional use. 5ish seasons of recreational use. I believe they are designed to last about 5 years for most users. The flexing of the frame degrades the plastics and the frame gets softer over time. Since bicycle frames are probably the most common carbon structure manufactured for mass retail sales, it seems likely that they are also among the most tested and it's clear that the designs are not lifetime type designs. I personally commute daily on a titanium frame that I expect to outlive me - completely different material properties.

    It's unfortunate that no-one is really testing manufactured beams in the same way other carbon structures are tested. A good friend works for a carbon arrow manufacturer. They perform destructive testing on thousands of arrows to generate data that is impossible to glean from computer modeling and FEM analysis. I don't see any way to do this economically for boat designs because so few of each model are built. In the end it seems that engineering is very important in terms of understanding where the stresses are greatest and how to shape the materials to best handle the various stresses. I don't think there is sufficient testing, at least that I know of, to actually provide good answers on exactly what layup of carbon fiber will be the lightest and yet strong enough. So it would seem that there is still a lot of guess work in terms of guessing that a perfect FRP structure will be x strong, but one that's built by a $10/hr employee can only be counted on to be Y strong, and we'll just double that and build it to Z specs. And when we get to that point, how does one determine the life span of a FRP structure that will degrade over time when the designer hasn't the faintest of ideas on how the boat will actually be used. Will it sit in a garage for 10 years? Will it cross oceans? You can't know that when you design.

    So, what do the NAs do? Do any of you plan a design life of a beam on a multi? What do you shoot for?
     
  4. Richard Woods
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    Richard Woods Woods Designs

    I hadn't mentioned the ISO standards as I assumed you knew about them already

    And I thought you were building a cruising powercat, so the sailing catamaran hull flying state is not going to be the cause of the major loadings

    So I don't think you are actually any nearer finding out how to design the bridgedeck structure from first principles and how to get the most accurate results, which is my understanding of what you asked for.

    There are several full time professional designers/NAs writing on this thread. We aren't getting paid to do so, so are doing it in our spare time and to help out. So our replies tend to be ones we can give straight away without thinking about it. That is why I said in an earlier post "it needs thinking about" or words to that effect. So it's nothing to do with any legal liability or protecting our own interests but simply because we need to earn a living, just like most people.

    And certainly no one can tell you what the loads will be like unless we have a full set of drawings. And in that respect, as the beam design is such an important part of a multihull I cannot see how you can have finalised the design, never mind considered building it until you have done so.

    Its a bit like designing a monohull with no clear idea of how to attach the keel, yet start the build hoping that something will happen

    Or like posting on a health forum "I have a bad back" and asking doctors to give you the correct diagnosis. They won't do that unless they know the whole picture. But you may get replies from non medical professionals saying "take some tablets", or "a chiropractor worked for me".

    Sorry to sound negative, but at time is seems you want this forum to design your boat for you. Pay a designer to do it, or accept your own limitations

    Sorry

    I think most cruising designs will be designed for a 30 year life. But some (home) builders will only build for a 10 year life, or even less. Racing boats maybe designed for a 6 month life. Racing cars probably for a 6 hour life. Drag racers for a 6 minute life.

    The house I write this in was built in 1985 yet is on its second roof.

    Even if you cycle "a lot" I doubt if it is more than 15 hours a week. Often the main reason a carbon structure fails is because the epoxy has been weakened by UV. I was told by one carbon mast maker that he never understood why people wanted a black mast. "Always paint them white, never varnish them" is what he told me. I thought tennis rackets were the main use for carbon fibre?

    The first job I did when I started working for James Wharram in 1976 was to help replace the cross beams on his Tehini. The boat was 8 years old and on its third set of beams.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  5. CatBuilder

    CatBuilder Previous Member

    As usual, I have to agree with Richard here.

    Groper, you may want to step back a moment to finish the design before you do any more building. If I recall, you already started the shoe/keel area?

    You should really see the project from start to finish in your mind, every detail (such as these in this thread) before you just start building.

    You may very well paint yourself into a corner.

    Just a thought...

    My basic understanding on cycling is that the larger the amplitude of the cycling, the shorter the useful service life of the sandwich glass boat, correct? So, the stiffer the boat on day 1, given all other parameters the same, the longer it will last, correct?
     
  6. Jetboy
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    Jetboy Senior Member

    The real problem with bikes is that weight goals push the materials to their extreme. An ultra-light carbon frame weighs around 650-700 grams painted. For comparison the ultra-light wheels will weight about 1500 grams for a pair - without tires or tubes. To get a frame that is that light the service life becomes very short. If you were to add, say 500 grams of material, the frame might last 10 times as long. Many are so thin that you cannot put them in regular bike clamp or it will crush the frame tubes. The bike builders know that the service life is short. And at $5-10k per frame every couple years, that's good business.

    In comparison a titanium, steel, or aluminum frame is more or less a forever frame unless it's damaged by crashing or something.

    I think the 30 year life sounds like a reasonable design goal. I was just curious as to what that goals was and how you'd really build for that without having any reliable test data to use. With bikes, it is a lot of trial and error. The stakes of the game are a bit higher in boating.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    So how many cycles will a cycle absorb before it breaks? That seems to be today's trick question

    Quite a good analogy to hull skins is to look at sails

    A carbon sail may have a very short life, measured in hours. But it will be very light, have a predictable stretch (if at all) and so hold its shape exactly as designed. But at the end of its life it rips/tears/breaks with no warning

    Whereas a dacron sail will stretch and distort from day one, but will be heavy and slow. Yet after its racing life is over it will still have years of cruising use.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    It depends upon the stress magnitude. That is the difference between maximum stress and minimum stress, which is the same as saying the difference between maximum amplitude and minimum amplitude. All amplitudes are not the same, even if the period remains so. The minium amplitude may also not be negative at times....it gets complex!
     
  9. CatBuilder

    CatBuilder Previous Member

    That is a good analogy.

    However, it seems to provide an analogy between glass sandwich and carbon sandwich... was that the intention?

    My thinking was if you take a glass/foam sandwich with core thickness X, then compare it to the same exact panel with core thickness 2X, the stiffer panel will have less amplitude in the stress cycle*, resulting in the availability of more cycles until end of life.

    The limit being a perfectly unbendable panel (say a normal boat panel subject to a mosquito landing on it repeatedly) which does not experience large enough stress to bend it at all, therefore having an infinite life.

    Would that be correct?

    I don't remember where I picked this up.

    *Thanks to Ad Hoc for pointing out the amplitude of the stress cycle as being a difference between a max and a min.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    The structural arrangement shall be stiffer comparing one with the other. But the cyclic loading ie the number of cycles remains the same. The actual deflection is only determined by the stiffness of the structure. The amplitude loading of each cycle is independent of the arrangement and remains the same.

    It is a bit like saying one boat, because it is stiffer, shall reduce the waves amplitude that it encountours. Clearly this does not occur. The loading remaining the same, the response shall be different, owing it is structural stiffness. But, as noted, it depends generally, on the stress range (max and min, and how long the min {or negative} cycle last for etc). Not to mention a multitude of other things in composites.
     
  11. CatBuilder

    CatBuilder Previous Member

    Ok, I though it was the deflection's linear amplitude that caused the loss of life that we are calling a "cycle" here. For instance, in another limit, if you bend a panel by massive force very far, it may have 6 cycles before it stops resisting the bending force.

    So to reinterpret and make sure I understand your point, are you saying that the distance the panel deflects is not a factor, but only the number of cycles (with all cycles being of the same force) controls the eventual demise of either of the two panels?

    That, given the same input force over the same number of cycles, identical panels which only differ in that one has double the core size, will both reach our theoretical end of life at the same time?
     
  12. Ad Hoc
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    Ad Hoc Naval Architect


    Hmmm..this is where it does start to get a bit complex. In essence yes this is correct. The failure of any structural item is not related to the single loading, i.e. apply a load once then remove and that is it…but a multitude of cycles. The action of a single cycle (laoding) may be not sufficient to cause any damage at all, but cumulatively it does.

    So you have firstly the number of cycles….i.e., in your boat the hull encounters a wave of 5 seconds, the cycle is related to this encounter, in this case one wave every 5 seconds. If you experienced this same encounter for a whole day, the number of cycles would be 60 second in 1 hour x 24 hours (dived by 5)= 288 times. If you experienced this over 1 year it would 105120 times. If after one wave passing the structure returns to its datum/equilibrium position and has no detrimental effects, it is the same as a single static load application. In other words the stress is within the yield/proof stress of the material….so all is well.

    The boat has not changed, the structure remains the same after 1 wave passing the hull compared to after 1 day 288 times or 1 year 105120 times. Thus the loading cycle is the how many times a load is applied to the structure. Yet if the structure or rather material being used, has a fatigue limit, shown on an SN curve, of failure after say 50,000 cycles, the boat shall failure after just 6 months roughly. Since the boat experiences more than 50,000 cycles in one year.

    If you sailed her every 6 months out of every year, then the life of the boat has increased by a factor of 2. So she shall fail in her second year not her first.

    Note, I am no where mentioning deflections or panels nor actual stresses. It is this, the number of cycles, that causes the damage in fatigue. The more cycles the more prone to fatigue failure.

    However, the whole picture as such is rather complex and has many other elements that can be brought in to affect fatigue life. But, it is all about the number of cycles to begin with.

    If during one of the cycles, if a larger than expected load is applied (during only one cycle), this does start to affect the behaviour since when did it occur, at the beginning of the 1 year loading or near the end, since how much of the structural has already begun to “degrade” owing to the cyclic loading..and on it goes…..which is why we try to use base values of SN data of cycles not hey…lets whack it with a bigger load just once. All these aspects come into play for the NA to decide what are the appropriate methods to determine its fatigue life. As I mentioned, it does get very complex.
     
  13. CatBuilder

    CatBuilder Previous Member

    Very complex, but very interesting. Thank you for the discussion. It was enjoyable.

    I need to read up some more on this stuff at some point. I still have this gnawing feeling that the stiffer (by increasing core thickness and maybe density) the panel, the more cycles it can take before degrading ( as compared to a thinner panel undergoing the same physical conditions).

    It's just a feeling, rather than anything I've derived from first principles, or have first hand knowledge of, so it's something to read up on.

    Great thread.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Material properties certainly play their part…which is the designers prerogative, which to choose, for the life of the vessel.

    A simple example, as it is an easily understood isotropic material, (i.e. no bond or resin failure of interlaminar strength to understand and establish nor fibre direction etc), so, lets take steel and ally.

    If you designed an I-beam that ends up with say a stress of 100MPa, in steel and in ally, great, go for the ally, it is lighter. Ok, well ally has no fatigue limit, thus you need to allow for this, by making the stress the member experiences much less (in a simplistic manner). So if you made the ally I-beam experience a stress of just say 20MPa (5 times less) great…you could safely say it should be ok, under certain conditions. But hey weight a minute, that ally I-beam is now not lighter than steel equivalent, but heavier…what gives?...well if you look at the fatigue limit of steel, the 100MPa is inside the fatigue limit. So no matter how many cycles it should not fail. Thus which is best to use…the steel …or the ally?

    So stiffness is one of many aspects to consider when deigning for fatigue.

    Such as, SN curves of a Tee fillet weld will be different from Butt welds, as an example. Yet both are ally, or steel or whatever…the way something is attached/joined/fabricated etc..all play their part, some in major ways too. And any one of them can easily render the ‘stiffer’ material unsuitable, for a multitude of reasons.

    This, as I’m sure you appreciate is a simplistic approach, but it gets the message across. The SN curves are pertinent to the material but also the correct method of application.

    So, again, it does get complex and is not as straight fwd as first meets the eye.

    Happy reading….
     

  15. redreuben
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    redreuben redreuben

    Surely the thicker the panel the more load on the skins ? To maintain the stiffness ? Just thinking aloud !
    Ex glassie !
     
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