Controlled Flex

Discussion in 'Boat Design' started by Stefan H, Dec 31, 2009.

  1. Stefan H
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    Stefan H Junior Member

    Experiments with viking ship replikas has shown that the vikings used to look at flex as something positive in boatdesign. http://vikingeskibsmuseet.dk/index.php?id=1094&L=1&tx_ttnews[year]=2008&tx_ttnews[month]=08&tx_ttnews[tt_news]=1866&tx_ttnews[backPid]=1093&cHash=3a1262bc95

    My Questions:
    In theory and in controlled conditions (calm water/constant wind) a stiffer boat or foil is always faster True/false?
    Carbon flex respons is faster, therefore carbon should be used in areas of controlled flex. True/false?
    Wood should be used since a tree naturally flex (?)
    What wood is best suited for a flexible construction?
    How would I determine how much flex is practical and safe? I know basic calculations but what is a reasonably margin before breaking for carbon, plywood, and for solid wood?
     
  2. marshmat
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    marshmat Senior Member

    The big downside of highly flexible structures is that it is hard to accurately predict the shape they will take while in use. A stiffer boat or foil is not necessarily faster than a flexible one simply because it is stiffer, but it is easier to analyze, more predictable, and thus easier to design such that the actual in-use properties are what the designer intended.
    Flex response being faster..... I suspect you're talking about stiffness (modulus)? (The actual speed of a response- the time between when a load is applied and a corresponding stress develops in the material- is incredibly short, and generally irrelevant.) Carbon can be used in flexible systems, but is not easy to design with (it is anisotropic with complex fatigue issues and low breaking strain).
    Many woods have excellent flexure and fatigue properties (look at a traditional archer's bow, for instance). The ideal species, though, will be different for each application and depends on cost and local availability as well as purely technical concerns.
    Are you asking for a way of calculating deflection limits for a given structure? Or are you looking for breaking strain data for various materials?
     
  3. Stefan H
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    Stefan H Junior Member

    I have been windsurfing many years and I know that the reason why a carbon mast makes you go faster than a glassfiber mast is because of a quicker response since the mast flex control the sail twist. Eg Neil Pryde compensate a lower carbon content mast by adding stiffness.

    I have a Neil Pryde 60% carbon mast that I have been using for about 15 years, there is no sign of fatigue issues.

    Since I'n not a proffesional boatbuilder, I only know what you call "breaking strain data" in combination with practical tests. Basically I just wanted to know how I can much a piece of wood or carbon/epoxi can be repeatedly bent without risk of fatigue or breaking. Maybe as a % of its breaking point.

    I am modifying a small dingy that I believe will be a unique concept, I will post some pictures here soon...

    I would be grateful for any links about building flexible structures, hulls, foils etc.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    You probably already know that Herreshoffs cat was designed to flex and worked very well. John Ilett,the originator of the bi-foil Moth hydrofoil used a unique hinge on early foils: the hinge material was laminated into the foil as it was laid up . The hinge material was kevlar but the resin used over the flex part of the hinge was a very flexible epoxy compatible with the laminating resin. The result was a flexible and extremely strong hinge for the foil flaps.
    Looking forward to hear about your boat. Good Luck!
     
  5. wet feet
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    wet feet Senior Member

    Two things come to mind with respect to the original post:the Vikings did the best job they could and used the boats well.Accounts of them reaching Vinland-as they named the large country to the West-and the Mediterranean illustrate that.The earlier post relating to the better performance of a sailboard with a stiffer rig accords with the experience of dinghy sailors,who regard a stiff boat as a better option.What might the Vikings have achieved if they had epoxy and carbon?
     
  6. portacruise
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    portacruise Senior Member

    My experience with small "flexible" boat is with medium pressure inflatables and the Portabote
    http://www.porta-bote.com/ . The portabote uses very flexible polypropylene braced with wood. Both types are slower than stiff boats of similar dimensions under your described "controlled " conditions, AFAIK. They appear to absorb and dissipate water displacement propulsion energy causing losses. Their advantages are in "niche" conditions like shallow water where solid objects will be struck or river rapids and in small survival rafts where only inflatable rafts can survive. The fasteners are the weak link in the portabote as these fail under the flexing forces, whereas punctures or leaks are more likely with inflatables.

    If a way could be found to make the flexing constructive with displacement forces, they might surpass stiff hulls in speed. Marine life could be studied to see how they make use of flexible exterior muscle layers when in displacement mode propulsion.

    "Reasonable margin before breaking" might also consider the progression and suddeness of failure. For example when a steel fails, it doesn't give much warning and it is often catastrophic. That's why airplane structures have to be checked by magnetic/x-ray etc. methods. Plastics and some other materials may weaken before they break completely, in so doing- giving a warning.

    Porta

     
  7. Stefan H
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    Stefan H Junior Member

    This is interesting! Although I don´t understand "flexing constructive with displacement forces". Maybe a naive question but..what are the displacement forces?

    Yes, I think metall should be avoided. Although airplanes are now being built of plastics and that makes it more difficult to visually spot failures. Before it was easy to spot when eg rivets breaks etc. How can you see if plastics is weakened unless there is a crack or a delamination?
     
  8. portacruise
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    portacruise Senior Member

    When you power a portabote thru water the sides and bottom move in and out to various degrees depending on speed, wave action and what I think are displacement forces. I speculate that this flexing of the hull is in response to pushing water aside (displacement) as the boat moves forward creating turbulence and a form of wave drag. In any case this flexing consumes a considerable amount of energy from what I have observed. But, I am only a scientist and not an engineer so I don't know the proper name and if this flexing reaches an oscillation frequency which could be harnessed.

    There are still metal parts on planes though not so much in the skin- engines wheel struts? and perhaps some special alloys, titanium or aluminum structural parts in various areas. So these are specially tested at periodic intervals since surface visual inspection cannot tell what is going on inside. I don't know if plastics need to be testing for internal integrity in their own ways. Seems like defects in fiber and glue would be the initial concern and if everything is OK there, fatigue limits would be much higher if the plastic parts have been designed correctly.

    Hope this helps.

    Porta
     
  9. marshmat
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    marshmat Senior Member

    Non-destructive testing of composite laminates is a field in itself, and a rather complex one at that. For the moment, suffice it to say that a good ultrasound technician with appropriate equipment can often find bubbles and pockets of delamination; checking the same structure with the same equipment after a period of time gives some idea of how delamination and fatigue are connected.

    It's quite possible to design flexible structures in aluminum or other metals. Look at the mast of a Nonsuch, or the wing of a Boeing 767, both of which bend significantly under load.

    The key is to know and understand the properties of the material you're using. A B767's wing can bend like it does because the design was so thoroughly analyzed, and the flex so accurately predicted, that it can be designed to have the proper shape in its deflected state. The wing doesn't fall off because the engineers took the time to figure out how it would be affected by fatigue, and how to reinforce it so that these issues were accommodated.

    I can't think of more than a handful of marine design houses that would have access to the armies of engineers necessary to get a good understanding of flex and fatigue in complex structures. The guys who design those fast-ferry cats, perhaps. Big freighters flex, and their designers have been allowing for this for many years, but they're structurally much simpler. The high failure rate of high-tech race boats is evidence enough that this level of structural analysis is pushing the limits of what even these well-funded design houses can do.
     
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  10. Stefan H
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    Stefan H Junior Member

    Thanks alot Porta and Matt for some really good formulated answers! It will help me alot.

    Stefan
     
  11. Stefan H
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    Stefan H Junior Member

    I bought one of the first shipment (in Sweden) of the VPLP designed Multi23 tri (Hull No. 11) and all four of these center hull where claimed due to cracks where the front crossbeam is bolted. On my boat it was also a crack in front of the centerboardbox. I´m not sure if VPLP or Multisailing did the calculations wrong or/and it was a production mistake but it partly proves your point though.
     
  12. portacruise
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    portacruise Senior Member

    Metals have a repeating lattice structure arrangement in their atoms and thus are more likely to be vulnerable to flex stress. Maybe you can overdesign or use more springy alloys in the predicted flexing locations to compensate. Plastics are polymers which are long intertwined chains of molecules which have more spring to them at the molecular level and are less vulnerable in theory, anyway. Also, you can custom tailor plastics to a flex or structural load with fiber construction and orientation http://dragonplate.com/ecart/product.asp?pID=4107&cID=79
    I have even pondered using carbon fiber for drive shafts.

    Porta
     
  13. jonr
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    jonr Senior Member

    Use of inflatables structures in the areas of a boat that are not in contact with the water (ie, where flexibility doesn't create inefficiency) seems to be an effective design. Filling a void with pressurized air makes it stronger.
     
  14. apex1

    apex1 Guest

    What a utter nonsense...........

    ...have you ever seen a boat?:?:
     

  15. PAR
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    PAR Yacht Designer/Builder

    I let this thread go far too long without mentioning Lindsay Lord's strip plank method. I would have suspected it mentioned by now. I've modified he's techniques and have a good understanding of how it works. His book "Naval Architecture of Planning Hulls" details it well and a very short description can be found in Dave Geer's book "Elements of Boat Strength". Many substantial craft have been built of this method including 80' naval patrol and it's awell documented method, receiving considerable testing.

    Most notably are two things, very light weight and a flexible hull shell. It's a cored composite build, but Dynel or Vectra are employed instead of conventional 'glass fabrics. The basic idea is the fabric elongation is much closer to that of the resin system used and as a result slightly flexible structure can tolerate as much energy as a heavier, thicker, stiffer one can.
     
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