Kayak design parameters.

Discussion in 'Boat Design' started by LP, Apr 22, 2011.

  1. cthippo
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    cthippo Senior Member

    What are you using for finished dimensions and cockpit size?
     
  2. LP
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    LP Flying Boatman

    I don't see a parabolic waterline requiring any hollow. Parabolic is going to be convex or concave, but in the purest sense of the word, not both. I think the advantage of hollow is in wave reduction. I suppose we could get very theoretical here and discuss the most efficient means to accelerate water molecules around a hull. If we figured that one out we'd have to take out a patent and wait for the little green presidents to start rolling in. :)

    I won't claim to be in the same league as Leo, I think we could presume the existence of a discontinuity in flow and pressure at the bow of a vessel with convex waterlines at the cutwater. Even the finest entry is still going to produce some form of a discontinuity, but maybe to a lesser degree. IMHO. Which is going to be less efficient? An instantaneous but momentary acceleration or a gentle, but continous one?
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

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  4. LP
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    LP Flying Boatman

    CT, I haven't finalized anything with regard to the cockpit ring. I'm toying with breaking out the steambox and doing an oak coaming around the cockpit. I'm not familiar sprayskirt sizes and will have to some research if I go that route. I'm anticipating protected water use only so I'd like to try the oak coaming.
     
  5. cthippo
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    cthippo Senior Member

    I did the coaming on mine using thin (<1/8") strips of cedar and mahogany worked in an electric skillet of boiling water and then bent around a form and glued. Snapped a lot of them before I got the process figured out, but in the end it came out quite nicely. I didn't have to worry about size as much because I make my own spray skirts, so once the boat is done I'll just make one specific to it.
     
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  6. kengrome
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    kengrome Senior Member

    LP, I think it's a great looking boat and I would love to build one for myself some day, assuming it is designed for a person roughly the same weight as me (160 pounds at the moment). What weight are you assuming for the person who will sit in the cockpit?
     
  7. LP
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    LP Flying Boatman

    Terry, I hope you're not thinking that I was trying to inply that a hollow entry was going to be more of a performer than a convex one. I hope it didn't coming across that way. There may be one small envelope where a hollow entry might be more efficient than a full entry. I would be speculating that would be in the 1.0 to 1.2 SLR. If my math is correct, the "K" kayaks were running with 2.7 to 3.0+ SLRs. At those SLRs, you would need a higher Cp (0.6-0.7) and that would negate the use of a fine/hollow entry.

    If I was looking for performance like that, I think I would have to revisit my design and fill in the hollows. In all likelyhood, I would have to reconsider the build method too.

    Your input and concern is appreciated. Like you, I'm just having fun with it.
     
  8. LP
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    LP Flying Boatman

    Ken, I've got her drawn at 0.10 tons displacement. 220 lbs. She would be exceptionally stable at your weight and you could bring 60 lbs. of supplies. We could certainly look at a trimmer model for your size.
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No, it most definitely does not.
    The waterlines are perfectly convex. The planview is a lens shape with sharp (but not "hollowed") ends.


    Better is:

    y = 0.5*B*[1 - (2*x/L)^2]

    where B is the beam at the waterline, and L is the length.

    Here, y=0 at the ends, (i.e. at x = -L/2 and x = L/2), and y=B/2 at midships (i.e. at x=0).

    Good luck!
    Leo.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That might work locally, i.e. close to where the hollow is located, but using hollow lines often gives rise to a "shoulder" further along the hull, and that then kicks up a wave.
    What you gain in one place might be lost in another! :)

    All the best,
    Leo.
     
  11. kengrome
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    kengrome Senior Member

    Sounds good to me. I'm good with freeship (been designing for years) and I don't really need the boat to be exceptionally stable, so I can probably "slim it down" a bit myself if you wouldn't mind sending me the freeship file.

    Or maybe it makes more sense for me to build it as-is. There's no telling how much weight I may gain in the next few years, and I might appreciate the extra capacity for carrying "stuff" even if I don't gain a lot of weight.
     
  12. cthippo
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    cthippo Senior Member

    Hmmm, I weigh more than your entire design displacement, and that's before gear. (265 lbs). Oh well.
     
  13. messabout
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    messabout Senior Member

    The light finally came on in my rum soaked brain. A parabola does not imply hollowness, a sine curve does. My only excuse is that a picture of curves of areas popped into mental view. Area curves, often with hollows, are just one more thing to boggle this creaky old mind.

    Thanks for your kind and patient reply Leo.
     
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  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    You can use a sine curve, but you should use the portion from 0 to 180 degrees (and not from -90 to 90) to avoid hollows. Many Japanese researchers have used those curves for preliminary ship analysis and design. There's not much difference in the total drag between the parabola and the sine curve for very thin, slender hulls.

    Leo.
     

  15. kengrome
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    kengrome Senior Member

    This comment made me think about the design displacement a little more. If your design displacement is 220 lbs and I weigh 160, there would be 60 pounds available for the "boat plus paddle plus gear".

    How much do you think the boat will weigh? My guess is 30-45 pounds, thus leaving only 15-30 lbs for the paddle and gear. Is this correct or did I misunderstand something here?

    If this is correct I think the design displacement is already "just right" for my personal 160 pound body weight.
     
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