paddleboard and kayak teardrop shape

Discussion in 'Boat Design' started by Alwaysthinking., Sep 5, 2019.

  1. Deering
    Joined: Feb 2005
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    Deering Senior Member

    With your permission I would like to use that phrase myself. The opportunity presents itself frequently...
     
  2. Eric ruttan
    Joined: Jul 2018
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    Eric ruttan Junior Member

    Is this not a solved domain?
    One can literaly get free software than can model this domain more accurately than you can measure the difference of the produced item.
    But beyond that, there are literal competitions of human powered boats that all have very simular shapes and builds, because the most optimun shapes are very well known and understood.

    Do you really believe your guesses are better answers than well known and well understood boats built and raced to world records?
     
  3. philSweet
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    philSweet Senior Member

    I knew I'd seen something on this a while back. Here's a pfd that specifically explains what is going on at the surface. Basically, there is a region near the surface about the hull, the near field surface, that creates a momentum defect though a mechanism that is different from classical linear dispersive waves. Again, the math is closely analagous to shock waves in compressible media, and also to hyperbolic type nonlinear, nondispersive water waves such as tidal bores and shallow water waves.

    Free Surface Shock Waves around Ships and Their Effects on Ship Resistance
    Hideaki Miyata*, Member; Takao Inui*, Member, Hisashi Kajitani*, Member
    *University of Tokyo

    https://www.jstage.jst.go.jp/article/jjasnaoe1968/1980/147/1980_147_1/_pdf
     
  4. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    Feel free, but I doubt if it fits you as well.
     
  5. Alwaysthinking.
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    Alwaysthinking. Junior Member

    Hey, watch your language. I have no idea what you just said, but it sounded fantastic. The obvious question is, does this mean my idea stinks or smells like roses? Probably the former. I'll take a look it when I get a chance, preferably after I grow a larger brain. As long as the keep the words short, I'll probably catch the drift (Boy, a lot of words have nautical origins!) of the article.
    You guys should probably wait till I actually come up with a rendering, before drawing (Rendering, drawing? Is that a pun?) too many conclusions. I'm sure my boat will mutate as I see what is and isn't working and new ideas come to me,
    Maybe I should come up with two boat renderings and you guys can vote on which you'd like to see me build. My original idea had more of a flatwater racing kayak look, but wider with a flatter bottom.
     
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The teardrop shape is gonna drag really bad. It'll drag on the front end of the drop and the back both. The teardrop shape is already present in many waterline sections of really horrible, wide row boats. They are only missing the round part at the back which doesn't assist the design, but makes it worse.
    [​IMG]

    What you need are foils. They might be able to be teardrop shaped, well at least in 2 dimensions.

    [​IMG]
     
  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Please, Please! This was a statement I made lots of times with general agreement from my co-workers.

    There is nothing new in human interactions. :D
     
  8. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    Obviously I know all that. I'm trying to find a loophole in the normal understanding of how water craft work at the surface. There probably isn't one and I'm beating my head against a wall, but what else do I have to do?
    I have a question about prismatic coefficient that might have to do with my solution. I'm a little rusty. I had to take a long break from this. As I remember, the prismatic coefficient has to do with the fullness of the ends. If you know the water line length and predicted speed, you can calculate the ideal prismatic coefficient. My question is this, doesn't the prismatic coefficient have as much to do with the depth at the ends and the width or roundness at the ends? Could I not achieve the ideal prismatic with a very blunt end with a lot of rocker?
    The trick, as I see it, is having a blunt bow entry while maintaining an aft center of buoyancy and the correct prismatic coefficient. I won't know if that's possible till I spend a number of hours playing around with a CAD program.
     
  9. DCockey
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    DCockey Senior Member

    There are published curves which claim to show the optimum prismatic coefficient as a function of Froude number / speed length ratio, which are proportional to the ration of the speed to the square root of the waterline length.
    Yes, but prismatic coefficient is not the only factor affecting resistance. Two hulls of different shapes but with the same prismatic coefficient, displacement and length can have very different resistances.
     
  10. philSweet
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    philSweet Senior Member

    "
    Nope, that's backwards. You can't "calculate an appropriate prismatic coefficient". Once you have designed and validated a hull and you have all the shape information, then you can calculate a prismatic coefficient. And you may find that it is quite similar the prismatic coefficient of other very similar looking and validated vessels. That's because this really is just a tautology, and nothing else.

    Repeat after me - You can't design a boat to have a particular prismatic coefficient and think that is going to be a good boat. You can design a terrible boat with a great prismatic coefficient, but you can't design a great boat with a terrible prismatic coefficient. It's just a checksum. You have to have a corpus of very similar and well regarded vessels for comparison. If your Cp is right there with them, it's at least possible that you have an ok hull.

    "Could I not achieve the ideal prismatic with a very blunt end with a lot of rocker?"

    Sure you can, and you can change a good boat into a terrible one as well.
     
    Ad Hoc and fallguy like this.
  11. Alwaysthinking.
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    Alwaysthinking. Junior Member

    The same can be said for Froude number. I believe it is possible to design two boats such that they have the same Froude number, but one significantly faster than the other. I'm not saying my idea is right. I don't know yet. I may not know without building a boat and even then, how would I know for sure why it was fast or slow? Was it the bow, or all the other design aspects? I just have a suspicion that some benefits could be derived from a rounded entry if you can maintain other desirable design features.
     
  12. DCockey
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    DCockey Senior Member

    Perhaps you should review some of the basics.

    Fn = U / Square root (LWL * g) where U is the velocity, LWL is waterline length, and g is the acceleration of gravity. Note that the quantaties need to be in consistent units. For example U in feet/sec, LWL in feet and g in feet/sec^2/

    Froude number is a non-dimensional method of describing the speed of a boat. By the definition of Froude number if two boats have the same waterline line length and are operating at the same speed they are then operating at the same Froude number.
     
  13. DCockey
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    DCockey Senior Member

    But you can look at a plot of optimum prismatic coefficient vs Froude number and use that information to decide on a target prismatic coefficent. Or you can use something else to set a target prismatic coefficient. Or you can ignore prismatic coefficient when setting targets for the design.
    Agree.
     
  14. gonzo
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    gonzo Senior Member

    That is absolutely wrong. Submarines are not hugely heavy for their size. Simply look at a photo of a submarine on surface mode with the ballast tanks blown off. Seems like you are simply making up stuff to try to justify your ideas. The shape you suggest is simply wrong and won't work. Canoes do not turn wildly on their own, unless the operator is poor and handling them. In short, square wheels may be novel, but they suck.
     

  15. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    I can tell you I'm not making up stuff on purpose. I have my faults, but that's not me. I'm sometimes wrong, but not deliberately so.
    I have seen plenty of photos of submarines and they don't sit high in the water when at the surface. The hulls are very thick, to resist pressure at depth, and the surface area to volume ratio looks small. I stand by my statement.
     
    BlueBell likes this.
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