flexable bottomed boats ??

Discussion in 'Fiberglass and Composite Boat Building' started by tunnels, Aug 25, 2013.

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  1. tunnels

    tunnels Previous Member

    what would you do if the bottom of you boat flexes up and down ?? is this good thing or a bad thing ??
    what about suspended decks and flexible floors ?? is it a sign of weakness or is it going to fall apart ??
    Is a stiff non forgiving hull a better hull than one that has give and bends a little ?? what is best ?? I'd like to know your views on this subject !!!!!:confused:
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    The response, flexing or no flexing is a result of the structural design, which is based upon the expected loads and the loads are established from the operational profile given in the SOR.

    If the SOR states, the hull must flex – for whatever reason - , then the NA/designer accommodates this requirement, just like any other requirement in the SOR. Just as a wing in an aircraft flexes since making it so stiff not to flex adds huge amounts of weight for starters, not to mention many other factors. Ergo if it is “designed in” and expected there is no issue, if the NA knows how to do this of course too!

    However, if the hull is not designed to flex yet it actually does flex, then of course there is a serious issue to address.

    A naval architect can design to whatever a client wishes, the SOR. I designed parts of my structure so it flexes, for a very sound structural reason, not everyone does this.

    Thus you’re actually putting the cart before the horse, so to speak. Since hulls can flex and do, the questions is, have they been designed to do so! If not, then there is an issue to address! Without knowing the SOR, one cannot say for certain...
     
  3. tunnels

    tunnels Previous Member

    we are talking glass here !!

    so are panels that moves potentially going to hold together longer than ones that have very little or no movement ??
    What about internal structure inside of boats can these be made to move as well ??
    Is movement a good thing or bad ?? :confused:
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    You can only answer that if you know what it is "designed" to do, and the SOR.

    You can "design" the structure to do whatever you want. You can design it in Switz cheese if you want, so long as....you know what the response shall be from a given load(s). It is as simple as that. Is the response to flex, or not to flex...and then what limits are placed on each etc etc. It is up to the SOR/Designer to specify as such what the RESPONSE shall be. But, if it is "designed" NOT to flex, but it does, then you have serious problems. Similarly, if it is designed to flex and does flex, job done.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The problem with flex is where the flexible parts merge with the less flexible, it needs to be a progressive transition. Take a fibreglass fishing rod, you can virtually bend the tip back round to the butt, but shove it down a steel pipe halfway, and it will snap off like a carrot when you apply the same force.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    On the contrary. You need less, if it has not been accounted for since if you add more, you're making it stiff, thus a bending moment occurs, which may not have been designed into the structure to begin with. If there is bending it is no longer a simple reaction load at the ends. Hence, if you're objective is to have a simple reaction load at the ends, thus making the structure flex easily. If you add constraint at the ends, it changes the whole loading and distribution and thus, its deflection.

    Deflection is related to the slope is related to the shear force is related to the bending moment. Change one and it affects them all.

    If it is designed to flex with simple supports then its fine and is accounted for..but if the loading changes to a "built-in"...like your example above, it changes the originally designed response (as well as the span at which the laod is applied), thus the desired outcome (the design) is different!
     
  7. tunnels

    tunnels Previous Member

    so flexible panels between stiff non movable frames would be a major problem !!
    But if the frames were also able to bend but still support part of the load then this is a better thing ??
    when loads are released panels and frames would return to there rightful place and shape ready for the next onslaught of pressure and so on and so on as a boat crash's through waves !!
    What kind of glass structures could take this relentless punishment ???:confused:
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Not sure what Ad Hoc is saying, but the traditional GRP lay-up, which is inherently a more flexible material than most at a given thickness, requires a graduation in thickness to avoid flex-induced cracking at transition points like a hard chine. I don't want to be on any aeroplanes where the wings are as "bendy" ( a technical term, that ) at the wing roots as at the tips, either !
     
  9. tunnels

    tunnels Previous Member

    there some magic going on here some where !!

    so putting things in order everything needs to be able to move !! panels make frames move and frames have a chine and because of its shape real doesn't want to move because the top sides of the hull are a panel on edge so is just about impossible to bend . so frames would have to be built purpose built to absorb bending yet remain stiff as well ?? tricky stuff !!
    Ok now we have a hull that's got lots movement , what bout the internals floors and deck structure meeting the floors that cant really attach to the frames of the boat because they move !! one is ridged and the other flexing . :rolleyes:
    mmmm that's pretty complicated stuff !! so the deck has to become suspended !! but from where ???:confused:
    wow that's weird !! never heard of that before !!:idea:
     
  10. TANSL
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    TANSL Senior Member

    I almost totally agree with what he says Ad Hoc. Everything can be as you want, if you design it properly. However I believe that some areas should not be "too" flexible. For example, hull panels should not acquire bulges greater than its thickness (there may be other views). I can not think of any reason to justify a hull (monohull) is flexible, but there may be such a reason.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    No..why is that?
    Says who?

    Everything moves, but how much and is it a problem?

    Ok, it seems you’re unable to understand the basics of the theory and words, so I’ll use numbers as an example to make it simpler for you both.

    Take a panel that spans 1.0m and is say 250mm wide x 3mm (like a stringer panel on a boat). It is simply supported and the panel has a 50x3 FB shape on it, vertically, so it makes a “tee” type panel. The material doesn’t matter which really, but, is say ally, with a point load of say 1.5kN, what is the stress and what is the deflection?

    Well, if it is simple supported, the deflection is 3.8mm, and the stress is 147MPa.

    But if the same structure is now built-in, i.e. fixed ends, the deflection changes down to 0.9mm with a stress of 73MPa.

    Nothing has changed except the end/fixity conditions of the panel, yet the RESPONSE is different, everything else is exactly the same.

    If you consider that you need 3.8mm of deflection, for whatever reason, you have a problem because the stress is too high being 147MPa, it’ll fail. But if you have it built-in, the stress is lower and passes, but, the deflection is more than 4 times less, this too is a problem if you want/need the 3.8mm deflection.

    So, you design the structure to give you the stress and deflection that you want by calculating the response to the applied load. You don’t build a structure and see what happens…that is too late and simple trial and error way of design!
     
  12. tunnels

    tunnels Previous Member

    could this give a smoother ride ??

    ok so the effect of a bottom of a hull moving up and down or in and out could soften the ride over a choppy surface ???? :confused::idea:

    Could be like springs in a car and evening out the bumps sort of !! what do you think ?? is this possible or not . I do remember seeing a boat hull once with like coil spring on like skis to do just that soft ride ( poplar mechanics dating back a lots of years !! )
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I think not, makes it worse.
     
  14. TANSL
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    TANSL Senior Member

    Perhaps what you need is a boat type "SWATH", much less affected by the movement of the waves. For passenger ships or recreational, whose displacement changes little between the various loading conditions, is an ideal configuration.
     

  15. tunnels

    tunnels Previous Member

    Thinking caps everyone !!

    The moment you get into quoting numbers I loose interest and it becomes gibberish !!


    Panels that move between frames and stringers that have virtually no movement would be far more likely to fracture and or break than if the frames also had give and were able to move but also support those panels on all 4 sides !! yes or no???

    if for arguments sake just the panels deflected and changed shape then there would be a greater loss of speed than if a larger areas flexed with smaller deflection but over a bigger area !! Yes or no ??

    we want the panels to move !!,we want a softer ride .
    If panels don't deflect then the ride will be hard and that's not what we are looking for .
    We want a softer ride , that is what we are after !! but how do we get that soft ride that's the burning question ???:eek:
     
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