paddleboard and kayak teardrop shape

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

  1. portacruise
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    portacruise Senior Member

    What is the best result you might get compared to the best off-the-shelf units already available? Would it be something along the lines of jumping up to a foil for efficiency? Seems like it would be hardly worth the effort if there's only a only a 5 or 10% difference. But go for it- you have the time, especially if this is a passion or the type of thing you love to do!

    BTW, within these forums (and HPB) archives there is record of my personal experience where conventional thinking was proven wrong, so that is another reason for you to go for it.
     
  2. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    Why stick to my guns?
    One of my other interests is traditional archery. I shoot recurves and long bows. I was on a traditional archery forum and saw a lot of questions and suggestions, from bow hunters, on how to make their arrows heavier. Apparently it was a commonly held belief that heavier arrows penetrated better. I'd always heard that velocity trumped mass, so I wrote a post stating that I thought making their arrows heavier was counter productive. You would not believe how mad that made people. Not a single person spoke up to agree with me. A doctor, that has written technical articles for the forum and contributed to books on archery, explained to me why I was wrong and even had a physics formula. I had one too.
    So, I bought three identical arrows, except for shaft stiffness (keep it clean guys) and tuned the arrows to work with my bow by changing the weight of the tips. The stiffer the shaft, the heavier the point you need.
    I then shot the arrows into a pristine foam block and measured the depth.
    The lightest arrow went deepest. The heaviest arrow went least deep. I know that wasn't a fluke because I found a video on line where some other guy did they same kind of test and came up with the same result.
    So, everyone, including this doctor, was wrong and I was right.
    Well, sort of. Turns out that if you are shooting a powerful compound bow (with the pulleys) a heavy arrow will actually go deeper because it can shoot the heavy arrow nearly as fast as the light arrow. There is also some evidence that, if you are shooting at something like plywood or bone, a heavy arrow may penetrate better.
    So, you could say we were both right.
    What should my take away from this be?
     
  3. Alwaysthinking.
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    Alwaysthinking. Junior Member

    Building any boat, even something like this, is a big, unpleasant project. If, after playing around with the CAD program, I'm not satisfied that I've come up with anything really new, I probably will just buy a boat.
    But there is a real satisfaction in paddling something you've designed and built.
     
  4. portacruise
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    portacruise Senior Member

    Yes, especially if it turns out that it can do things that no other craft can do, as it turned out for me.
     
  5. upchurchmr
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    upchurchmr Senior Member

    Portacruise,

    Please give us a link - I'd like to see your proof.
     
  6. portacruise
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    portacruise Senior Member

    I'm on the road right now enjoying my portable watercraft and away from my main computer which has a few of the the bookmark links that I have saved. I will post some links when I return to home base.

    Probably most of the readers on this blog wouldn't be interested in what I have done, because it fills a particular niche according to my particular interests and disabilities. Mostly used weekly at a local River and when I travel by airline to places of interest in remote areas like Alpine Lakes and small rivers. The craft is a 10 lb inflatable fly fisherman's Float Tube modified to be used for my particular disabilities. It is powered by a very small collapsible 10 lb electric motor system which I developed over 20 years ago, and have continued to refine. Two of tubes can be packed in an Airline carry on case, it easily runs in 6 in of water and can spin around hands-free almost in its own 6 foot length, plus has some other unique adaptations. So I haven't seen anything else that comes close to those particular characteristics which are of interest to me. It does draw curious onlookers should I be around people, but mostly it is used away from crowds to reach remote fishing areas.
     
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  7. upchurchmr
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    upchurchmr Senior Member

    Still interested.
     
  8. Barry
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    Barry Senior Member

    The assumption that a tear drop shape is the MOST efficient shape is incorrect. It appears that you agree with your second sentence?
    If you are wanting stability, you might consider a 32-34 inch wide catamaran which can offer you much more stability than a paddle board or kayak shape. As one hull sinks due to weight shifts, the hull offers more stability
     
  9. upchurchmr
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    upchurchmr Senior Member

    When you go to 32" wide catamaran kayak, then look in to hull wave interference as in a recent thread.
    It will be bad
     
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member


    supcatboards
    Google Catamaran paddle boards, images, or catamaran paddle board videos
    Evidently these work quite well.
    To limit wave interference if it would be significant at such low speeds 3 knots, maybe, I would consider not using a symmetrical board but rather half of the symmetrical shape.
    IE split a kayak in essence in two so that the facing hulls, ie between the pontoons are straight, lessening the wave making profile on the inside
    What is most important to note is the stability of this design as noted in the you tube video reference above
     
  11. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    I think my biggest problem isn't
    I actually came upon that same solution by accident, while using the Shape 3D can program. I was having difficulty getting the program to make an arched (rounded) bottom, but instead ended up with a split down the middle. I also played with this concept a few years back. Kind of a tunnel hull design.
    I'd prefer the mono-hull solution for a few different reasons, but I'm open to other ideas.
     
  12. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    I may fail in my attempt at proving my theory, not because the theory is wrong and I need to learn the basics of NA, as someone suggested, but because I need to learn the basics of CAD. I'm gradually working toward a shape that is in line with what I had imagined, but then I have to figure out how to get it balanced fore and aft and the waterline where I want it. If the bow is down and the tail up, it isn't going to perform as I'd like. Or, if the actual waterline is above or below where I wanted it, that would certainly mess things up.
    I'd very much like to have my boat design milled out of a foam blank by one of the companies that makes surfboard blanks. This would make the build a heck of a lot simpler and I'd end up with a very strong, unsinkable kayak. I think this process would go smoother if I could get the Shape 3D program to do what I want, but so far I'm really struggling with it.
    I'll probably show a rough draft from another CAD program in the next week or two.
     
  13. portacruise
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    portacruise Senior Member

    Here is a link to my motor (1st used on 5-18-95) within most of the prop shaft systems thread, but especially posts #47 (3rd paragraph), #53 (video) #54 (video) #57 (2nd paragraph). It took me a while to convince Rick Willoughby, a well versed engineer, to believe in and try the unsupported shaft which I had been using for quite awhile at the time.
    Prop Shaft Systems. https://www.boatdesign.net/threads/prop-shaft-systems.24636/page-4

    Unfortunately, the float tube I am using is no longer made but my adaptations have me in a dry (calm water), self bailing seat, with custom foot rests both above the water line. Mine is similar to the one pictured is the short video clip under "Trailer" in the link below, different brand.

    TapamTheMovie http://www.tapamthemovie.com/trailer.html

    It is a surprisingly stable design for something so small, as I have run at least up to class II rapids, but some other designs are built with rating to class IV. It has an open wet storage compartment behind the seat which floods/ drains slowly thus adding to overall stability, and also feet can dropped down in deeper water to act as a keel and lower the COG. Easy entry/exit/ stop at shorelines by sitting down or standing up. Calm wind and water range about 8 miles with 3# of Li batteries at about 2.5 knots top speed.

    Hope this helps.
    Apologies for those not interested, please ignore.
     
    Last edited: Sep 17, 2019
  14. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    Something like this …, to offer both stand up or sit paddling ?
     

    Attached Files:

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  15. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member


    I like your design. Could a flatter thourt ship bottom add to stability?
     
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