Buttocks

Discussion in 'Hydrodynamics and Aerodynamics' started by DogCavalry, Sep 30, 2019.

  1. DogCavalry
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    DogCavalry Junior Member

    I’ve been wondering about a particular aspect of non-tripping chine design for about 30 years, but 30 years ago, asking a community would have been impossible. Not so today.
    I’m familiar with Albert Hickman’s invention and patenting of the chine, and why his early sea sleds required them, and we see them everywhere today. Glen-L8’s Rebel is a beautiful example.
    But here’s what troubles my sleep at night. I’ve included a set of lines someone else posted in the forum a few years back. If anyone knows their origin, I’d love to know. So we see here the inner chines, at the lowest points of the boat. Note how they aren’t parallel, but instead run in towards each other from bow to stern. If one draws buttocks through all the areas including any part of the non-tripping chines, for a significant percent engage of the beam, those buttocks show an angle of inflection. Specifically, for the front part of the hull, the buttock line descends, and water flowing down this surface generates lift. Then the buttock crosses that inner chine, and suddenly the water is rising, generating negative lift, and inevitably, significant drag.
    Also, note how for much of the chine, the water flow is rising. Now my trusty copy of Marchaj’s Aerohydrodynamics of Sail, now residing in a cardboard box since it fell apart in the 90’s, would describe this as very bad in a planing hull, since it generates lift by momentum transfer, as opposed to static displacement. This of course is why all aft buttocks show parallel lines in monohedral hulls.
    I’m prepared to accept that at certain Reynolds numbers, the inward turned bow waves formed in the inverted v establish a net inward vector, such that the bottom chines are running parallel to water flow. But Above that Reynolds number they wouldn’t be. And the constant rise in the non tripping chine surface is simply bad at all Reynolds numbers.

    Anyway, I hope I haven’t bored anyone.

    John out
     

    Attached Files:

  2. Ad Hoc
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    Ad Hoc Naval Architect

    The US Patent is noted here.

    He's calling 'tripping" what many today call chine-walking and dynamic instability of a monohull at high speed. To overcome the capsize "tripping" the boat at high speed.
    This topic has been researched over the years by many, notably Ikdea & Katayama and Blount & Codega and Werenskiold to name a few.

    Essentially the roll restoring moment is heavily influenced by the trim (pitching moment) at high speed and is a function of the GZ. Some guidelines have been provided over the years by many:

    upload_2019-10-1_7-0-57.png

    The famous Bailey series of experiments at NPL, also indicates this as well as the influence of the B/D (beam/draft) ratio.
     
  3. DogCavalry
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    DogCavalry Junior Member

    The attached patent document is extremely interesting. None of the rest of it relates in any way to the question. Still, thanks for the input. You’re the first. I’ve read a few dozen threads. I’m thinking this may be a tough one.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Possibly because i'm not entirely sure what it is you are asking/seeking...other than reference the the 'tripping' of the chines by AH. Can you elaborate further?
     
  5. DogCavalry
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    DogCavalry Junior Member

    Sure. Why do the bottom chines curve in towards each other, rather than running parallel? That’s just going to cause drag. At some Reynolds numbers there might even be flow separation.
    And why do the non-tripping chines rise from station 5 to 10? More drag. Without flow separation past 5, they just lift water. That makes about 20%of the bottom causing negative lift. There’s a corresponding issue in aerodynamics where the tail of an aircraft creates negative lift around 12% of the total. It’s a significant loss of speed and efficiency. The reason there is mostly “cause that looks normal”. I’m hoping there’s a compelling reason in empirical hydrodynamics. Otherwise, I’ll build the bottom chines parallel and the non tripping chine providing lift its whole length, and find out.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    He is talking about "tripping" as in, the boat's slab slide catching in a hard turn. But the concern seems to be that the boats narrows as the stern is approached, which would seem to be another "anti-tripping" device. This narrowing is also bringing what he calls the "bottom chine" closer to the centreline, creating a slight rocker, or potential negative angle of attack, on that outside chamfer. But inside of the "bottom chines", the lines have a strong positive angle of attack. The inner would have greater effect than the outside, on lift, being somewhat wider. It isn't the end of the world, if some portion of the aft surfaces have negative angle of attack, at some trim angles, you'd have to water test to see how it all worked together.
     
  7. DogCavalry
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    DogCavalry Junior Member

    Fantastic, Mr Efficiency! That is exactly my concern. Is it critically necessary?
     
  8. DogCavalry
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    DogCavalry Junior Member

    Actually, those are the lines of a Hickman Sea Sled. Around 6000 were built between about 1924 and 1932 or so. Many before and many after, so we know they were very successful despite some odd hydrodynamics. And maybe the rocker eliminates self exciting roll. Aka chine walk.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    This falls into the naval architecture realm of art and science.

    1) Science.
    AH felt that doing so would create a 'cushion' of trapped air and would reduce the pounding, as he saw it. Stating he felt the vessel would be riding on compressed air.
    He also felt it would eliminate 'tripping' - or chine walking. and thus become an "automatic" stabilising effect.
    And that the hull form allowed a greater carrying capacity for its length - a high planing weight as he calls it.

    2) Art.
    Whatever one claims or states, whether it is supported by independent evidence or research, naval architecture still has the "art" side to it, the creativity. As such modifications like that proposed by AH, whether one believes the claims or not, and whether the claims are supported by independent research or not, if the naval architect "thinks" or "feels" or "believes" the design would benefit or be better, then so be it. In essence it is a "fashion" feature. In today's world, this is known as marketing.

    A vessel is greater than the sum of its individual parts.

    As such in a crowed field of many, one is always looking to get noticed or be 'different' or to be seen at the trail blazer. Whatever the reasons, you will always find features on boats the often defy logic or common sense. It is the designers artistic flare to create something they can call 'theirs'.

    Thus on can construct an argument for and against, in the manner which you are attempting. But, to AH, he's convinced. Whether you are... is another matter.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously it wasn't a vexing problem, if thousands were built to those lines. I'd say the slight convergence of the "bottom chines", also acts to further "compress" the flow down the tunnel, increasing lift back there. The thing about these boats I had doubts about is the ride, and slamming. Morphing into cathedral hulls certainly did not solve that.
     
  11. DogCavalry
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    DogCavalry Junior Member

    Actually they were, and are, famously easy riding. Morphing them into cathedral hills was a cynical move to defeat the patents. Made fantastic boats into dogshit.

    Marcus Lee up in Sitka built at least a dozen out of welded aluminum, up until he retired a few years back. One for sale recently, called shuttle. Lots of great pics online. Wish I had the skill to weld aluminum.

    J
     
  12. DogCavalry
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    DogCavalry Junior Member

    A
    And I was wondering about that aspect of convergence as well. But AH might just as well had a feeling about sufficient width of non-tripping chine, but didn’t want to add any beam, so he surrendered tunnel width instead. As AdHoc points out, AH was an artist.

    J
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    There are many small boats out there with the same or similar claims.
    But the science part - to me - suggests that with a ratio of density of only 1 to 800, how much "lift" can an open system provide, since the transom is "open". The 'compressed air' can vent to atmospheric pressure aft without trouble.
    The 'feels' like a nice ride, this is all subjective, in the absence of hard data of with and without.....which also leads into the Art side of the design. Art is always subjective.

    Thus, to me, unless there is 100% verifiable independent evidence, it remains a fashion piece and a nice marketing feature and like with any design, you either like it or you don't.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, given that patents have presumably expired, I don't see any great uptake of the idea today. Why ?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    I give very little "value" to such patents because this is just the author/originator of the"invention" tries to assert their claim and prevent others copying it - they have bought the cool aid, and they assume so will you too. It presumes from the outset there there is "something" magical. All it does, is stifle open creativity and research and as such leaves the subject/topic stationary.

    The market often dictates whether an 'idea' is worthy or not. One could invent the best device - whatever it may be - in the world and solves all. But if the market does not exist or unwilling to take it up....it stays where it is...collecting dust. A patent does not automatically equal brilliant invention. Conversely a brilliant invention does not automatically mean it will be a success.
     
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