Concave hull cross-section for paddle board stability?

Discussion in 'Boat Design' started by Paddlelite, Feb 3, 2013.

  1. Paddlelite
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    Paddlelite Junior Member

    First post here, and I want to say what a great and impressive resource this forum is.

    I’m trying to complete a flat-water stand-up paddle-board design of 12.5 ft. LOA, designed for “fast” touring in the 3.5 – 4.5 kt. range.

    The problem with stand-up boards is that a wide, stable mid-body is required, which I believe results in excessive wave-making resistance. So I’m trying to push the limits with a sleek, narrow design just under 26” wide, but gain stability by using a single concave nearly ½” deep to put more volume at the edges under foot. (I really think of it as two convex and parallel humps, about 2 ft. apart from one another, running under the edges of the board over about half of the hull length.) The picture shows the board outline and slices. The bow starts with oval cross-sections, the mid-section is concave, and stern is flat-bottomed and rockered up meet the waterline.

    Some specs, if relevant:
    Waterline length: 12.5 ft
    Waterline width: 25.8” at 62% of length aft.
    Width at stern: 15”
    Draft at 180 lbs total: about 1.8”
    COB: 5” aft of center
    CP: 0.6534, likely high because of extended mid-section of paddleboards
    Bow entry half-angle: 18 degrees
    Cross–sections: oval at bow, single concave in center, flat at stern
    Volume of the convex “humps” compared to flat hull: 6.6 liters total, putting about 7.2 pounds more flotation under each foot.

    Question: Other than the obvious small wetted-surface penalty compared to a flat bottom, is there a drawback or limitation, in terms of overall resistance, to using a concave for the purpose of stability on a displacement hull? How far might I take this concept before adverse effects creep in, such as interference between the two “humps”?

    I’ve also taken pains to adjust the slice areas to create a smooth volume progression through the board, as shown on the attached graph. But in doing so, my hull outline and waterline outline somewhat exceed a perfect quadratic formula or parabolic curve, mostly because the bow outline is enlarged in the area of its oval cross-sections and rocker. For the same reason, my bow entry half-angle is about 18 degrees, which I thought was perhaps large for crafts approaching 0.4 Froude numbers. (I just learned all this stuff, so I could be way off.)

    Question: Which parameter should most closely fit a mathematically defined curved – is it the volume progression, as I assumed, or should I have focused more on a perfectly described waterline, bow entry angle, or something else?

    Thanks.
     

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  2. messabout
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    messabout Senior Member

    Since noone else has made any comments, I will post a few. I'll label the comments O for opinion and F for fact.

    O; the hollow center section is a questionable idea. Transition from oval to hollow will cause some funny flow that translates to increased drag. Running the lumps full length might be a more practical idea. But of course that would increase wetted surface a bit.

    F: Increasing the beam by two inches will improve righting moment a lot more than the hollow center method that you describe.

    O: increasing the width just two inches will not make enough difference in paddling effort to notice.

    F; the transom end of the the board will drag noticeably with the slightest heeling angle.You can diminish this problem in three ways. One way is to raise the after end well above the waterline. That is not such a good idea on a boat of this type. Second option is to transition the after bottom sections into a vee. That'll diminish the initial stability a little but it will make a better exit and keep the corners of the transom from dragging. The other option is to make it pointy at the aft end.

    O; you will need to paddle very energeticly to get a 12.5 foot boat boat to move at 4 MPH. ( I do know that those boards are faster than the arithmetic would seem to allow)

    Suggestion; Decide whether this is to be a straight line boat or a highly maneuverable one. That decision will imply how much rocker to give the bottom. Rocker will affect your section area distribution and could influence the location of the center of bouyancy.

    O; 62% is pretty far aft for max beam. Look at your curve of areas. The curve need not fall off too suddenly in the back half. Too much bottom area back there will make the boat squirrely in a following sea or on the front side of a power boat wake.

    Keep on thinking!
     
  3. Paddlelite
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    Paddlelite Junior Member

    Thank you. That is a very astute analysis, bow to stern, and provides a lot to think about. Admittedly, this design is a collection of experiments, maybe too many of them to isolate any one effect. By the way, if it makes sense, the wide stern is to maximize stability, reduce the curve behind the rearward max beam, and reduce sinking in the wave trough.

    Regarding paddleboard speeds, I too was utterly amazed at how fast they go, perhaps because of the ergonomics of the paddle stroke. I plotted the speed of the black 14 foot board being paddled in this video http://www.youtube.com/watch?v=e3uxyS-art8
    and you can see from my attached graph that the average speed is 5.7 mph! There is a lot of resistance though, as demontrated by the speed dropping about 1.8 mph in less than a second during the glide portion of the stroke cycle.
     

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  4. kvsgkvng
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    kvsgkvng Senior Member

    I think that what you are trying to achieve is miniscule compared to other parameters. For example, the padler's center of gravity height is at the padler's wasteline. If the same person would kneel, the height would be halfed. That alone would affect stability far beyond what you are trying to do. Basically what you are doing is building a catamaran -- yup it would help, but the help would be very very marginal. It is like trying to achieve a significant leap forward in gasoline consumption of already efficient car by inflating tires with extra 5 psi tire pressure. Will it help? Yes. Will it help a lot -- I don't think so.
     
  5. Paddlelite
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    Paddlelite Junior Member

    Yes, a cat-like effect is a better description. I shouldn't have said "concave" since that implies an impression or depressioin in the hull. I do expect only a small, but noticeable, effect from the increased volume under the edges, but hopefully it will mitigate the instability of a narrower hull. Paddler COG is definitely critical, so on the topside of the board I'm using a sunken standing area with drain holes which will put me an inch above the water. Most paddle boards have you standing 3 inches or so above the water. These are all small differences, but I'm searching for any way possible to use higher L/W ratios and increase the crusing speed of these things.
     
  6. 805gregg
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    805gregg Junior Member

    Fast touring on a SUP? You had better hope there is no head wind. I was peddleing my Hobie Outback with the mirage drive next to a SUP guy in about 10 knots of head wind, he had to get on his knees as I passed him going twice as fast. SUPs are a joke, paddle on one side and turn paddle on the other and turn the other way. A great work out but not a practical means of transport.
     
  7. Petros
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    Petros Senior Member

    There reasons hulls are not made that way.

    the stand up paddleboards have always been a mystery to me, why stand up on a wide slow board when you can sit and relax in a low and slim kayak? Other than exercise and short distance site seeing, I do not understand their attraction. I would rather go further and see more, with the ability to relax and rest at anytime, in my kayak.
     
  8. messabout
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    messabout Senior Member

    I agree Petros. The only advantage I see for a SUP is that scanning vision is better. There are some adventurous dudes in Florida who paddle their boards among the alligators and water moccasins. I do reckon that you can get on and off the board faster than in a kayak. Sometimes you can get off faster than intended. A board might be practical around a boat livery but then speed would not be a prime design objective.

    I'll still take the kayak and yes I have paddled my kayak among gators. At olympic level speeds I might add.
     
  9. Paddlelite
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    Paddlelite Junior Member

    There is almost a hostility towards SUPs, perhaps because they can put a lot of unskilled, unsafe people on the water.

    They are obviously not without drawbacks, but they do actually have some advantages over kayaks:

    Very lightweight, typically 20-30 lbs., so easy to load and carry
    Easy getting on and off the board at shore; easier at difficult launch sites
    Easy rescue; just swim back onto the board. A leash tethers your ankle to the board.
    Easier to stay cool in warm weather
    Fun on waves and in surf
    Can move around board, kneel, sit, lay down
    Variable standing position allows adjusting trim, COB/COG
    Great perspective and visibility, especially for seeing below water surface
    Less equipment needed because they can't fill up with water - no skirts, pumps, floats
    Very safe because they have a foam core and are a life raft even if broken in half
    Inflatable SUPs available that function nearly as well as a hard board
    Uses whole body and helps develops balance skills
    Perhaps best of all, while serious kayaking women wear rubber berkas called sprayskirts, serious SUP women are tan fit, fit, and wear bikinis.
    http://www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Sexy-girl-SUP-pics/?page=1
    And while paddling against the wind is the biggest drawback, check out what happens with the wind in your favor.
    http://www.youtube.com/watch?v=UKk7RGqn9Lk
     
  10. messabout
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    messabout Senior Member

    Paddelite; we are not ragging on your favorite boat. It is a young persons boat and not too practical for creaky old guys. Actually I think a SUP would be great fun if you have the physical capacity to make it so.

    Keep on working on the "Paddelite" boat. We'll cooperate with design observations and comments if you agree not to make geriatrics paddle the finished product.
     
  11. Paddlelite
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    Paddlelite Junior Member

    Very kind of you. Thanks. I'm actually pretty creaky myself. It was a hip injury that forced me out of kayaks and onto my feet. With more injuries after that, and more time off the water, I turned my interest toward design.

    Off topic, but I was going to download either Freeship or Delftship, but can't figure out whether they're the same program, or if not, which might be better for designing and analyzing paddle boards and other small craft. Any advice appreciated.
     
  12. NoEyeDeer
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    NoEyeDeer Senior Member

    Hallelujah brother! I have seen the light! :D

    Srsly, it comes down to personal preference (whether you're referring to SUP's or wenches).

    FreeShip and DelftShip are much the same AFAIK. The latter is a development of the former, but is of limited functionality. Some more advanced functions are only in the paid version. The free version of Delftship is still a very powerful and useful application. You could always try both, and see which one you like best.
     
    Last edited: Feb 7, 2013

  13. Petros
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    Petros Senior Member

    I am not hostile towards them, I just do not understand the attraction and why they are so popular. Using them around a tropical resort in the shallows I can see as a rental. But why would you want to own them for general use.

    My skin-on-frame sea kayaks weight in at 16 to 25 lbs, including the paddle. My 110 lb wife can carry the one I made for her with one hand, and it does not even need a roof rack it is so light.

    All this is true of the Sit-on-top kayaks, which are better in warmer climates.

    This is also true of sit-on-top kayaks. Plus they have way more cargo capacity of you want it.

    Also true of kayaks, espcially if very narrow, which I like better since it paddles easier and is faster (unlike the paddle boards).

    also true of sit-on-top kayaks. Of course, there are some tourists bodies I just as soon see covered in a berka anyhow.
     
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