Flexible Hulls.

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

  1. SURFFOILS
    Joined: Dec 2011
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    SURFFOILS Junior Member

    Whats your opinion of using a flexible, possibly inflated bow section to soften the impact and reduce form drag for planing craft.
    im thinking that the pliabitlity of the fabric will 'accept' the impact and reduce the shudder from hitting thousands of surface peaks and ripples.
    Thanks/.
     
  2. tunnels

    tunnels Previous Member

    You dont need a soft tube for the bow just make the hull flexable and twistable !! tubes will slow you down and toss the front high in the air !!:p
    reduce the shudder from hitting thousands of surface peaks and ripples.
    this is where design comes into play so your boat will stay straight and true to where you point it .do you try to go over the rough or through it ??
     
  3. SURFFOILS
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    SURFFOILS Junior Member

    Please excuse my inelegant English, I was thinking of having a solid deck with a flexible hull.
    Generalising the height of ripples the inflated section could be only 2 or 3 inches thick /deep like an inflated veneer under the bow.
    I hope this makes the idea clearer.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Actually, this idea isn't as odd as you'd think and has been addressed, over a half a century ago, with Lindsay Lord's strip plank method. His idea was to use relatively cheap and lower grade core materials, such as pine, cheap plywood, cedars, but embalm them in a high elongation laminate such as Xynole. The resulting hulls, some of which were quite substantial, high speed patrol craft (25 m), flexed under load, returning to shape after impacts or release from whatever strains that exisit. These are probably the lightest forms of wooden cored composite construction you can have. It works because the light weight shell and structure, with it's high elongation sheathings, can deform and flex without concern, unlike a true FRP laminate, absorbing shock, impact, bend and twisting loads without damaging the fibers within the laminate.

    I don't think an inflatable structure, would become better or a more efficient planing form, when deforming under impact or wracking load, but it certanily could soften the ride a wee bit. I suspect the cost of this comfort would be planing efficiency, but if the craft worked in a rugged environment mostly, possably a design option. I've never been especially impressed with RIB's, but the Lord method has had my interest for many years. From a cost view, the Lord method has more going for it, both in materials and build ease. He documented the process and concepts in "The Naval Architecture of planing hulls" and I'm surprised it didn't take off more then it has. Working with Dynel and Xynole can be troublesome compared to other fabrics, but you can get a handle on it with some practice, plus with modern techniques, it's easier.
     
  5. SURFFOILS
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    SURFFOILS Junior Member

    Thanks PAR, thats what I was talking about, but couldnt articulate. I have read Lords book and my idea came from his theories.
    Your xplanation is clear as to how 'bow-flex' could reduce resistance.
    Consider a standard 12ft Al "tinny" with a recessed bow section for the front 5 foot.
    That section has a Hypalon or PVC skin that covers this section and is moderately inflated so it has bouyancy but also rebound and can accept millions of impacts with offering the lowest resistance.
     
  6. PAR
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    PAR Yacht Designer/Builder

    I don't think you'd find any "gains" with a "soft" entry, in fact, I'd suspect just the opposite to occur. Any substantial deformation will increase drag and eddie making. Getting pressures just right for changing conditions, might be an insurmountable issue as well. Picture a flat calm, where your soft bow would prefer to be stiffer for better penetration and preformance, but then you run into some chop where the "cushion" pressure is just right, softening the ride, but then more chop comes up and now the cushion is being mashed against the underlying hull. I guess you'd have a computer controlled set of sensors and a pump?

    In short, I think you can take this way farther then necessary, adding considerable complication and complexity to a fairly simple set of variables. In other words, unless you're physically changing the entry's shape, to enhance preformance under a specific set of sea state variables, then an inflatable cushion, just doesn't seem as reasonable as more conventional approaches (inflated or not).

    I once toyed with the idea of a variable shaped hull, using these inflatable cushions. The idea was a sailboat that could claw upwind, like a monster, but also scoot down wind with the best of the fat butted buoy knockers. The quarters would have the cushions and the hull form would go from a narrow, well balanced upwind form to a triangle shaped downwind sled. Of course the engineering and control for varying conditions proved to be nearly imposable to predict, let alone execute.
     
  7. tunnels

    tunnels Previous Member

    I made inflateables for surf life saving in New Zealand ! the ridged glass hulls with hypolon tubes could out perform and were much faster than soft bottom boats and the young guys absolutly loved them but the older guys couldnt handle them because they were so much faster could turn 180 degress in there own length at full tit !,could point and go in any sea conditions and wave size running along a rolling wave is impossible with a softie because they via off and almost impossible to keep in a straight line . the ridged when where you pointed it . The other advantage was being able to go up the face of a breaking wave and punch out the top of the wave and land on the back side and carry on . Its impossibel to do in softies because they fold in half and flip over backwards and end of story .
    I have video of proof and it was done many times once the guys got over the shock of what they had done and tried it over and over again till they got used to it .
    Surf Rescue is all about getting to the person , recover the body alive or dead and get back to the beach as quickly as possible .
    This hull form could run up the back of a big breaking wave full out and catapult off the top of a breaking wave and land at over 45 mph plus ! a fully wrung out outoutboard just hung out the back it was going so fast .
    I worked making race boats for a few years so a bit of trick stuff went into making them !!.
    The hull was able to flex and twist but was as solid as a brick loo. Was lighter than any of the softies the company had made and exstremely strong .
    Its the glass you use and the orientering of the glass fibres thats the secret key !!
    Harbour chop up to 18 inch high was a breeze and the faster you went the better and smoother the ride .
    The 30 hp motor we swapped and tried a 50 hp and it could fly just touchng the tops of the waves .

    You want to know about flex and twist ask the vikings and there long boats ! its nothing new its just a forgotten art of making and understanding the reasoning and advantages .If its ridged it will break and has to be built heavy
    :D:p:p:)



    and so they never eventuated .
     
  8. tom28571
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    tom28571 Senior Member

    In terms of high speed planing craft, I think that flexibility of the hull contacting the water is a negative to performance. The reasoning goes this way. Any flexibility is certain to introduce some bumps and convexity (dimples) into the surface. Minimum drag results from a surface that mimics the natural streamlines. Any departure from a natural streamline introduces drag and a flexible surface introduces these variations from natural streamlines.

    When towing both hard and soft bottom tenders, it is easy to see and feel the greater drag of the soft inflatables.
     
  9. MechaNik
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    MechaNik Senior Member

    There is research proving the drag reduction benefits of flexible surfaces at high Reynolds numbers. This is through reduction in turbulence and boundary layer thickness and so doesn't benefit all scenarios which have seen increased skin friction at times.
    I don't think an inflatable surface has ever been successful to my knowledge.
     
  10. m3mm0s rib
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    m3mm0s rib Senior Member

    Here another view
     

    Attached Files:

  11. tunnels

    tunnels Previous Member

    Flex And Twist

    i THINK when you put these words together you have the idea that its going to behave like a soft bottom boat !!,well it dosent !!.Its behaves like a little of both . its hard enough to be related to semi flexable but flexable enough to have just a little of the soft bottom . Flex panels under normal conditions are stiff enough to be a ridged but add speed and riding waves then panels move, the bigger the waves the more they move and when jumping and landing they bend even more ! they absorb the shock loading and immediatly return to there proper shape . Everything like stringers and the keel also need to be able to flex and bend and it comes with years of inderstanding your product that you are able to understand the strengths required to get close to the amount of flex that gives a softer safer ride . Its also important to understand that decks on smaller boats are suspended from around the very outside of the hull only and does not touch any part of the actual hull any where just round the very outside .
    Now lets get into twist ! this is a tricky animal!. The height differance between the hull and the deck is critical ,to high theres no twist to low and the deck will touch the hull when it flexes ,again lots of exsperimantaion .
    To understand twist take a big plasitc container without a lid it will flex quite easy , now clip on the lid and twist vanishes but flex still remains !! Take a shallow container same size and do the same . the shallow container you will notice you now get twist , if you take a hot air gun and heat the lid and press it closer to the bottom and let it cool then you can get even more twist .This same principle applies to the hull and deck, finding the comprimise and changing deck heights takes time and exsperimantation . Was a very interesting excise and when you get to where you want to be the boat is magic to ride in and drive specially at speed over rough water and in the surf its even better !! The other very interesting thing is after a long week spending hours in the surf thrashing and crashing and pounding and jumping waves that would have distroyed an ordinary ridged boat there wasnt one single stress mark any where on the hull or deck . Only took a couple of months of exsperimanting to reach that point starting from a bare tube only and building everything from scratch. Interesting also only used standard poly resin and two differant glassed through out the whole build !!. What materials would you have use ?? :confused::D:p:p
     
  12. SURFFOILS
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    SURFFOILS Junior Member

    As a similar concept, Imagine covering the whole of a solid hull of a powerboat in a 1/2 in thick soft closed cell rubber gym mat... I believe with the rubber lining it would soften a lot of the physical effect of chop and noise from the chatter, but it would also provide a smoother ride as the rubber would instantly yield to the water surface peaks instead of colliding with them.
    Even if it only nullifies the small chop it could turn the top representation of a choppy surface into the smooth blue line below.

    [​IMG]




    Heres a pic of the inflation idea.

    [​IMG]

    Its positioned where the bow contacts the water when its planing and the depth of the inflated part is 1~2in max. Theres still a hard bow under the inflated section. Its not the whole hull just a veneer of air-cushion under the bow.
     
  13. tunnels

    tunnels Previous Member

    i personally dont think iadding foam will make all that much differance if any, even having 2in thick !!.its shape you need to change in the bow to make the big differance where what i call water entry area fron the intersect point of the chine down the bow to the forefoot area and keel . When looking at a boat mentally divide from the chine at the bow to the stern into 1/3s first 1/3 is for water entry and the other 2/3s is load carrying . So what is the best shape for water entry ??what is the best shape for carrying weight ? marry the best of both together !!:D
     
  14. PAR
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    PAR Yacht Designer/Builder

    Your drawing of what would happen with a soft impact area is incorrect. It is correct in that it would tend to bridge peaks, but it would bridge them all. It would adopt a shape associated with the various pressures waves and impact points, which would of course be random and widely dispersed. You'd have multiple pockets, dimples and possably wrinkles and folds, not the nicely smoothed bridging you've drawn.

    Something like this, which is better, but still not nearly as good as the "forced compliance" of a rigid form in terms of efficiency.
     

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  15. SURFFOILS
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    SURFFOILS Junior Member

    The foam idea is just a conceptual example of how a flexible hull, even a slightly flexible one would react.
    Ive been making small 4 foot versions to tow behind a 'tinny' to see how they react.

    Low inflation double pontoon nose entry.
    [​IMG]

    PVC material attached to vac-bagged epoxy/EPS/ carbonfibre

    [​IMG]

    [​IMG]

    Par, I thought that would be correct too, that " It would adopt a shape associated with the various pressures waves and impact points, which would of course be random and widely dispersed. You'd have multiple pockets, dimples and possably wrinkles and folds, not the nicely smoothed bridging you've drawn."

    But in the physical trials where the ups and downs are many and varied across the hull, it seems to flow over the lateral average of irregularities as well.

    Take 4 different longitudinal lanes of surface contour that the boat travels over

    [​IMG]

    and the average of these removes the lowest lows and highest highs.

    [​IMG]

    But the small scale models are only a general guide of whats likely in a full scale version so Im not guaranteeing a similar result.
    Its just promising feedback atm.
     
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