Flexible Hulls.

Discussion in 'Hydrodynamics and Aerodynamics' started by SURFFOILS, Dec 26, 2011.

  1. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I think the concept has some merit. ON an auto suspension, the tires, springs and dampeners (shocks) act like a flexible contact point, it will actually improve handling and control over a car without a suspension. The faster the impacts, the stiffer the suspension has to be (hence the reason sports cars and race cars have much stiffer suspensions). I have helped design and install suspensions that have gone into race cars. The principle should be the same for a boat in water.

    And of course if the suspension on a car is too soft, it can also reduce control. Too little stiffness and damping or too much stiffness and damping will both be less desirable than when the stiffness of the suspension and the damping match the conditions for which it is designed. I think with a soft hull section you run the risk of it flexing in an uncontrolled fashion and might be too soft, so it is a matter of matching the amount of flex to the conditions you intended to encounter.

    I also know that a wood skin-on-frame kayak with a bit of flex is much easier to control in rough water than a factory hard shell kayak (I even know of a fiberglass hull made from a skin-on-frame biadarka hull and the rigid one was almost uncontrollable in rough water). On a larger hull that goes much faster over the waves it would be a matter of matching the flex and the damping.

    I would think what would work better is than a soft section is to have large sections of the hull made from a heavy fabric with perhaps some spring loaded stringers behind it. I would use a rigid keel line with a fairly sharp prow, and than have large flexible panels of heavy fabric on either side of the keel. With spring loaded stringers you can adjust the amount of flex in the panels. My thinking is that a sharp keel line is not what is causing the pounding so much as the large field of rigid hull on either side of the keel line, sudden contact of wave tops over large areas of hull will feel like a hard impact. Make those large panels flexible or with some give in should reduce the impact.

    Another alternative is to have large sections of rigid panels that are hinged with perhaps an inflatable bladder behind it, that allow panel to give. I would envision heavy fabric hinges on either side of the keel, with some kind of heavy rubber fabric around the edges of the hinged rigid panels to allow them to flex without leave large gaps to allow water into them.

    This would allow the flexing to occur in a more controlled way, reduce the impacts and if down properly would actually improve control in rough water I would suspect.
     
  2. SURFFOILS
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    SURFFOILS Junior Member

    Thanks for the various ideas Petros, The basic PVC fabric is OK for now on the test craft but I will have to look into other options down the track. Thanks again.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    You have to think an inflatable is a fair test of these ideas, and travelling at speed in the non-rigid variety is not comfortable. In addition, sprung or dampened seats in fast boats get mixed reviews at best, otherwise we'd all be going like scalded cats and not be worrying too much about how much deadrise works best.
     
  4. tom28571
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    tom28571 Senior Member

    The original suggestion concerned inflatable hulls with flexing ability. Some comments seem to be about comfort by reducing shock loads by spreading the G force over time. Some are about improved control and some are about greater speed by reducing drag. These look like quite different goals with different solutions. Soft skins have been experimented with in an effort to duplicate the supposed greater efficiency of sea creatures with inconclusive results. At least inconclusive in a practical sense.

    Mr efficiency's comment about dampened seating in fast sea boats may ignore the fact that all very fast ocean race boats do adopt very complex cocoons of damping to protect the crew and they still are often injured by the slamming of the boat. The deep V hull is an effort to reduce the peak G forces by spreading or damping the slamming loads over time. Some boats have been built with sprung suspension systems for the same purpose. The rules of sprung to unsprung weight and shock absorption used in autos are just as valid but would be much more difficult to apply successfully.
     
  5. upchurchmr
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    upchurchmr Senior Member

    Any one have a powerboat? How about making a test article and actually finding out? Talk, Talk, Talk
     
  6. tunnels

    tunnels Previous Member

    If you going to make a flexable boat you have to first think what is the flex you are looking for ! flex for a softer ride ? flex to be able to hit waves at speed or just flex for the hell of it . how do you want to achieve that flex and how much flex do you need . bottom flex you need stringers that flex Ridged frames will cause hard spots so have to think long and hard about how to go about getting flex without distruction !!
    What sort of glass or what ever you going to use to achiveve the desired amount of flex ?? what sort of resin ??? :D
     
  7. upchurchmr
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    upchurchmr Senior Member

    Talks cheap, pointing out problems, or imagined problems is lame. Who is going to do something?

    Advance a theory and prove it.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    So, when are you going to start the flexible hull build Upchruchmr?
     
  9. tunnels

    tunnels Previous Member

    And how do you intend to go about it ?? its a whole hull bottom excise including frames ,stringers not just a part of half hearted effort !!:eek:
    And if there is twist involved then the topside, deck and whatever else needs to be taken into consideration as well !:confused:.
     
  10. upchurchmr
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    upchurchmr Senior Member

    PAR,

    Good point, except that I don't have a dog in this fight. I doubt that any design will have a value over a wide range of conditions. If you were doing a power boat run at 30 mph for 50 miles over a similar course weekly, you might be able to design to those specific conditions and make a gain. The variable conditions a sailboat sees would probably result in a gain in one condition, and a loss in the next one - hopefully resulting in a net zero benefit instead of a loss.

    Except that my imagined structure would end up being heavier and I assume the average would be a degraded performance overall. I'll just skip talking about additional cost.

    I am interested in seeing anyone else's results and am willing to pay appropriate tribute to any who succeed.

    Marc

    PS: my father told me stories about riding a crash rescue boat in San Franciso bay just after Korea during heavy storms, skipping from wavetop to wavetop in a PT boat doing 50Kts. Sounded just like some of the descriptions earlier, but that was in a plywood boat long ago. I'm sure it was somewhat flexible but I expect the rigidity was what allowed it to survive.
     
  11. tunnels

    tunnels Previous Member

    It really comes back to design at the end of the day but an almost impossible task designing a hull that will cope with everything and for all and every condition !
    Flex would only be there to make sure the hull holds together and dosent break up and fall to bits . Is better to flex and release excessive pressure than to be ridged and unforgiving . The sea will always find that one little weak spot and keep working at it over and over all the time . then snap its broken and you swim for shore :eek:.
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Which is precisely what I told my ex-wife . . .
     
  13. tunnels

    tunnels Previous Member

    Did it work ??:confused:
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Anyone who has ridden in a flexible glass hull powerboat, that works and oilcans, and a cored boat to the same design and weight, knows which rides better. It's a no-brainer.
     

  15. beachcraft
    Joined: Nov 2011
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    beachcraft Junior Member

    I am looking at RIBs and inflatibles. DuxBoats have an option of "a Fiberglass, flotation foam filled Planing Pad, called "Dux Feet" attached at the rear bottom of each Sponson, for fast planing and low friction." That got me thinking what the flexibility of RIB tubes in the water does compared with boats which ride only an aluminum or fiberglass hull in the water at speed. That model is also available with or without the fiberglass rigid "dux feet".
     
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