paddleboard and kayak teardrop shape

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

  1. Alwaysthinking.
    Joined: Sep 2019
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    Location: Colorado, USA

    Alwaysthinking. Junior Member

    Hey! Hi!
    I'm going to retire next spring. I was thinking a good retirement project for me would be to make a sit-on-top kayak that could also be used as a stand-up paddleboard. I'd like to do the design myself. I think an argument could be made for playing it safe, but my feeling is that there's no reason to build something that you can buy. Combining a paddleboard and kayak might be challenge enough, but I'd like to try tackling what I see as a long standing problem in boat design. No one else might see it as a problem, but it's been annoying me.
    It goes like this. The most efficient shape for an object moving through a fluid (air or water) is a teardrop shape. The fuselage of a subsonic passenger jet and the hull of a submarine are very similar in shape. A round, bulbous nose tapering to a fine tail/stern. Two exceptions are supersonic aircraft and water craft that travel on the surface. Why supersonic jets have a pointed nose is a question for another forum. I'm concerned with surface craft.
    The reason surface craft aren't teardrop shaped is a thing called wave making resistance. Or that's my understanding. To combat this, we've arrived at a hull shape that is far removed from a teardrop. Almost the opposite or a teardrop in reverse. This bothers me. A few feet above the surface and a teardrop is a more efficient shape. A few feet below the surface and a teardrop is a more efficient shape. But on the surface, the physics all change. I don't buy it.
    I'd like to design a kayak with a more rounded nose in such a way that it is actually more efficient, not less. If there is a solution, I think it has to do with the side profile. Center of buoyancy isn't just about width, it's also about depth.
     
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Welcome to the forum.

    First a disclaimer. I know just enough on this subject to get me into trouble. But maybe, its enough to explain without too much technical jargon.

    Super sonic aircraft have a point to disrupt and dispel the sound waves collected. It reduces the sonic boom implications on the aircraft.

    The tear drop shape you describe is very easy to push thru water at slow speeds. Enough time is allowed for the displaced water to move out of the vessel's path.

    If the vessel is traveling faster than the water can move then the vessel rises out of the water and only a small portion of the tear drop shape ingages the water. It takes much more effort or power but you arrive sooner.
     
  3. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Congrats on your retirement. Nothing like idle time on your hands to get yourself in trouble...like building a boat.

    I’ll leave the hydrodynamic lessons to more knowledgeable folks on this forum, but I would point out that even if a teardrop shape is the most efficient theoretically, that likely conflicts with the constraints imposed by your utilization. A standup paddle board needs good fore/aft stability for balance, and a teardrop is going to be very asymmetric in that regard.

    I like your idea of building a hybrid boat. That will be challenge enough. I’d suggest looking at existing paddle boards and kayaks and copy their basic hull forms.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The boat may end in a tear-drop, but the project itself, in tears. :( Re-inventing the wheel isn't always a waste of time, just most of the time. :)
     
  5. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    I can totally relate to the OPs “long standing problem” with SUP boards, my back is killing me after a half hour !
    Don’t know if there was any pun intended there.
     
  6. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    Well, yes, I know why a supersonic jet has a pointed nose. That wasn't a question.
    I'm thinking that the blunt nose rising is exactly what I need. Bow very shallow, stern deep. Might work, might not. I don't know yet.
     
  7. Alwaysthinking.
    Joined: Sep 2019
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    Location: Colorado, USA

    Alwaysthinking. Junior Member

    Yeah, I was thinking about the stability problem. A flattish bottom is what is needed for that which would conflict with my other considerations.
     
  8. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    I realize that hundreds of far smarter people than me have been working on the problem for a very long time. Sometimes it takes someone too stupid to know it can't be done with no preconceived notions. I may be just stupid enough to do it. Or I may end up in tears, but not too many. It's not like I'm building a yacht or anything.
     
  9. tspeer
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    tspeer Senior Member

  10. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Well if you design the boat so that the bulbous bow is under water in normal operations you might end up with something like those bulbous bows on large freight ships. But those need to be carefully engineered for specific speeds using fluid dynamic simulation. Here is a video about it.

    So maybe you actually could design a teardrop shaped kajak where you sit below the water surface, carefully trimmed so that only the opening you sit in shows out of the water. Like a submarine or a swath ship but with your *** being the ballast on the bottom. Might work with a large "turret" that is again sharp in front and peddling to power a propeller.

    It probably won't be efficient because the space you need for your legs increases the volume which means you would need a lot of ballast and have a lot of surface and skin friction.

    I know it's a stupid idea that doesn't even fit what you want but I'd love to see a personal "submajak" in action :D
     
    Deering likes this.
  11. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Well. you should buy it, because that's exactly how it works. Don't ever underestimate how good random humans are at perfecting stuff given a couple thousand years of experience and several millions of builds that have competed against one another. A better use of your time and money would be to try to develop and validate your understanding of the design requirements of a boat and how those affect its resistance. When you can design a boat using first principles and it turns out looking exactly like a real boat, you are probably on to something.

    Hull resistance is the result of the boat meeting other requirements. How the various components of hull resistance vary with sea conditions and speed; and how the cost of construction and operation vary when you manipulate these resistance components, is what you need to get a handle on.

    By the way, since you mentioned both supersonic aircraft and free surface waves and wakes on a gravitational free surface, you might be interested to know that they are mathematically very similar. The wake drag of a supersonic aircraft can be modeled in a shallow water sluice. The equations that to relate the sluice conditions to the supersonic conditions were worked out in the 1950s.

    https://ir.library.oregonstate.edu › downloads
     
  12. gonzo
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    gonzo Senior Member

    Both airplanes and submarines operate completely submerged in a fluid. Boats operate at the boundary between two very dissimilar fluids. That creates waves and very complex turbulent interactions. The teardrop shape is really bad for a boat's mode of operation.
     
    Manfred.pech likes this.
  13. Alwaysthinking.
    Joined: Sep 2019
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    Alwaysthinking. Junior Member

    I think I need to make it clear that my idea isn't to make a kayak that looks like a submarine.
    I also think I need to make it clear that I knew all about the 1851 America's Cup and I'm well versed in theory on surface craft and wave making etc. I have several books on naval architecture and have been reading articles related to such for about the last 40 years. I have no formal training, but a good layman's understanding. I should also point out that I've owned numerous kayaks and canoes and still do, and have a good feel for how their shape effects everything from stability to moving through chop to tracking. I should also point out that fish form kayaks actually have less resistance at very low speeds than swede form.
    My last computer had all my boat design software on it. I never got it installed on this computer. When I get it installed I'll play around with designs for a while and when I have something close to what I'm thinking, I'll post it. I think that will make my idea a lot clearer. My main concern is making a blunter underwater shape (not nearly as blunt as a submarine) while still keeping the center of buoyancy aft of center. It would look surprisingly like a normal kayak. The reason the British boat got trounced in 1851 was mostly because it's center of buoyancy was too far forward and it's a sailboat. I'm not saying I could design a sailboat with the features I'm thinking of. Sails complicate things.
    And, yes, I accept that my ideas could be totally stupid. I don't have a lot of ego. I'm not terribly afraid of looking stupid. But it wouldn't be the first time I thought convention was wrong and was later proven right.
     
  14. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Anything you'd like to tell about ?
     

  15. Alwaysthinking.
    Joined: Sep 2019
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    Location: Colorado, USA

    Alwaysthinking. Junior Member

    Nothing I can prove, so I'd rather not go there. Well, OK, I knew AC current would win out of DC;).
     
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