Is rigidity a religion? Amas on the move....

Discussion in 'Multihulls' started by Qmaran, Aug 31, 2021.

  1. Qmaran
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    Qmaran Junior Member

    The mantra in boat design is the stiffer the better. In advertising boats are promoted as being super stiff and light. In racing this philosophy goes so far that hulls crack, masts fall through the bottom or boats simply break in two.

    I am toying with the idea of trimaran ama arms that allow some vertical movement of the amas. This is not for a racer but a 40ft cruiser. Advantages of this in my thinking are:

    It reduces stress. A rigid connection has to be very strong to absorb al the wave impacts and levered forces of the mast. I think these forces, especially the peak max forces (when a giant wave impact coincides with a sudden gust), would be greatly reduced.

    Secondly wouldn't the overall ride be smoother? Especially when waves hit the boat diagonally. The bow of the ama that hits the wave first can rise up a little independently and slowly force the rest of the boat to follow. This should reduce jerky sudden movements.

    Thirdly wouldn't it sort of work as a safety valve for other parts of the boat. in case of a sudden wind gust the springiness of the arms would reduce the peak force acting on the rig.

    Point number four I am really unsure about. Could it be that counter to intuition it would be faster than rigid arms? If in sudden gusts the arms flex they absorb some of the force so there is less drive...but wouldn't this lost drive be given back when they "unflex/snap back". Furthermore the amas and main hull will be able to follow a more natural course through the water surface. Imagine the follow two scenario's where one of my 40ft light weight amas is connected with a long arm to a giant oil tanker:

    1 --->My ama is connected with a flexible arm. It can rise and fall with the waves finding its own merry way.
    2 --> My ama is connected with a very large diameter rigid carbon fiber arm. The ama will fly, crash into waves, smash on the water surface and even completely submerge. Basically its movement has no relation to the water surface it encounters.

    Of course there is no free lunch. Amas not rigidly connected almost force you to have a un-stayed or narrowly stayed mast. Also it may not be suitable for a racing vessel because it will be a bit harder to fly a hull. And the construction would be novel so carry some risk/unknowns.

    My thinking is a connection with multiple flat bar glass fiber leaf springs. Search "Lauf mountanbike leaf springs" to get some idea. Also in the automotive industry they are finally moving from steel leaf springs to composite leaf springs (Volvo, Corvette, Mercedes Sprinter van...) Naturally the springs should be beefy enough to not allow excessive movements and maybe there has to be some hard stop built in (maybe a slack X of anchor rope)

    I am interested to see what others think of this. Please try to give reasons. Just saying "nobody does it so it's a bad idea" or "we all know that....." is not interesting. A nice fact or theory to show why this would be a great or terrible idea would be appreciated.

    Thanks!
     
    Last edited: Sep 1, 2021
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Flexible is OK unless the transition from flexible to stiff is abrupt.
     
  3. Qmaran
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    Qmaran Junior Member

    Yes, the limitation of flex should be gradual if you don't want the structure to disintegrate. But would the flexibility be beneficial? And why?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Well, you need to address the basic philosophy of 'stiffness' and the effects.
    Structural stiffness the EI, is related to the 2nd moment of area and the material properties.

    This in turn is directly related to the deflection...

    So, in terms of how much flexibility you want relates to have much deflection you want. So, in order to answer that one, you need to define this value, is 5mm deflection acceptable...or 10mm, or 50mm, or 100mm?

    There is nothing wrong with deflection - just look outside the aircraft next time you're flying... the wing is 'flexible'.
    But like everything structure related, it depends what you want THAT structure to do..and hence the MO of the design.

    In your #1 or #2 scenario... if the ama(s) are flexibly connected, what contribution do these amas give, in terms of stability and driving force, to the main hull. None.
    Since if they are totally flexible, the main hull will behave as-if, it is a monohull only. As the hull rolls, it rolls, as there are no amas. Ergo, less statical stability and less driving force.

    Where as in #2, as soon as the hull rolls, the ama touches the water and provides the additional 2nd moment of area in terms of its WPA inertia, and gives you the stability and driving force required.
    As soon as this starts to become flexible, the GZ curve reduces accordingly - it is directly linked to how much contribution the hulls provide when heeled.

    Having the connection "slightly flexible" is a real world solution.
    Since having the connection 100% rigid simple shifts the load path into the connections, which if they are not designed for such increase in load, will fail. The high loads means additional structure = additional weight.
    Thus, in reality the 'rigid' connections you are noting are in fact slightly 'flexible', but just not as much as you think or would like them to be. Merely to 'lower' the expected high loads at the connections. The trick is, finding an acceptable amount of flex that still delvers the stability and driving force.

    If the connection fail, that is down to the poor detail design of said connections, not the flexibility or otherwise of the beams.
    If the beams fail, then that is down to the degree of stiffness of the beam in the sea state that caused the failure.
     
    Last edited: Sep 1, 2021
  5. guzzis3
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    guzzis3 Senior Member

    I hate to shoot anyone down in flames but I think you are totally wrong.

    Early trimarans had smaller floats. The thinking was similar to yours. No one does that anymore because it was found through trial and error that over 100% buoyancy floats are faster and safer.

    Wharram built flexible hull connections on his cats. They don't work either.

    Boats break because they are poorly engineered, marginally engineered to win races at all costs, built wrong or encounter extraordinary conditions.

    If you are considering a 40' trimaran cruiser have a look at Ed Horstman's website. Unbelievable accommodations. TFJ discussed a Horstman tri surviving a hurricane in one of his books. They are as safe and seaworthy as anything you will find.

    The problem with flexible structures is fatigue and variable difficult to predict response in different circumstances. It would be like replacing the springs in your car with flexible wishbones. It is true we use space age materials and aircraft like engineering to build stiff light boats but the technology is mature and the means both accessible and affordable. Assuming you are planning a cruiser all the parameters are well understood. Buy plans from a credible designer or design it yourself within a standard envelope, build it PROPERLY and you will have a fine safe boat.

    I didn't address your automotive example. I am a mechanical engineer. If you look at any vehicle's suspension there is suspension and control elements. The control elements like wishbones are always well controlled. Manufacturers used to use leaf springs because they are cheap and the expectation for handling was low. Only commercial vehicles do that now. Cart springs, as they are sometimes called, date back to horse and buggy. If you use springy beams to connect your floats they don't just flex vertically, they flop around all over the place. So now you build a secondary structure to control that. Now you have heavy complex expensive beams and adequate rigid beams would have been cheaper and easier to build.

    Not being able to attach rigging is a trivial nuisance easily overcome.
     
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Flexible structures work best when free at one end.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Like a palm tree !
     
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  8. redreuben
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    redreuben redreuben

    Stiffness is a religion because going to windward is.

    Make of that what you will.
    Im not a designer or engineer but to me flex = fatigue which eventually means failure, sooner rather than later.
    If you want to continue down this path I believe one of the Gougeon brothers trimarans (was it the F40 one ?) had a wooden leaf spring on the rear float mount and a pivot on the front one.
    Trimaran Cool: The Adrenalin is pumping again https://trimarancool.blogspot.com/2009/10/adrenalin-is-pumping-again.html
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    Everything flexes.
    How much is up to the design if it fails through fatigue is up to the material properties and has the flexing (cycles to failure) been taken into account via detailed design.
    If it hasn't.. then it will fail.
     
  10. Qmaran
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    Qmaran Junior Member

    I don't mind going down in flames if my idea deserves it! I don't want to build something based on false assumptions!

    I am not an engineer but see where you are coming from. Regarding the material fatigue I am not that worried. In vehicles the springs have to cope with hitting potholes at high speeds, on a boat the movements would be way less frequent and abrupt. And the springs could also periodically be replaced if fatigue is a real concern.

    It is true that on trucks the springs have control structures that prevent movement at other axis than the intended one. But if you look at the Lauf mountainbike leaf springs they have to cope with all possible axis of movement between a bike and its rider and the wheels in rough terrain....and those leaf springs are tiny and wafer thin.
     
  11. Qmaran
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    Qmaran Junior Member

    Thanks for the link. Gives some hope my idea is not that crazy. Funny they banned the concept because the boat was winning too much.
     
  12. Qmaran
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    Qmaran Junior Member

    I see from the video in your link how it pivots. Maybe I should revise my idea slightly to have a very stiff spring centrally and more flexible ones fore and aft.
     
  13. garydierking
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    garydierking Senior Member

    [​IMG]
    Adrenalin's very successful ama attachment.
     
  14. Robert Biegler
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    Robert Biegler Junior Member

    Here is a powerboat doing just that, likely for the same reasons:


    A small waterplane area would do the same thing in heave without the mechanical complication. You could make your amas very narrow and tall. The drawback would be added weight and greater wetted surface.

    Secondly wouldn't the overall ride be smoother? Especially when waves hit the boat diagonally. The bow of the ama that hits the wave first can rise up a little independently and slowly force the rest of the boat to follow. This should reduce jerky sudden movements.

    I think these two are different. The snapping back sounds like you expect something equivalent to pumping.

    The bit about the mass of the boat something forcing the ama through the waves is about drag. In addition, when the added buoyancy of going through a wave does force the whole boat up, that energy has to come from somewhere, and may not be recovered when crashing down again. On the contrary, the crashing down again increases submerged volume and drag.

    Suspension might hold the rig being steadier, and so give you steadier flow.

    Not if you put the suspension at the and of the crossbeams, instead of into the beams. Think of the amas of the Gougeon Formula 40 trimaran Adrenaline, attached to be free to pitch (within some limits). If you could insert your suspension system between ama and beam, you can stay the rig as usual.

    You may want to adjust how stiff the suspension is while you sail, to avoid having to choose between dragging the windward ama through the water or large angle of heel.

    I think you would get the advantages you expect, and possibly some more. Whether they are enough to be worth the weight and development cost is another matter.

    Lifting foils may give you the same advantages by taking the hulls' volume out of the water, and not following the water surface quite so closely, but the suspension does not need as high a power to weight ratio.
     

  15. Qmaran
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    Qmaran Junior Member

    Thanks! I wouldn't have figured out the drag strut and tab without this sketch.
     
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