Kayak design parameters.

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

  1. LP
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    LP Flying Boatman

    I'm working with various kayak designs and am currently trying to select an optimum Cp for the craft. I am not familiar enough with speed expectations to select a speed/length ratio to start the analysis.

    Is there anyone out there that can assist me in a speed estimate for 1) casual touring and 2) maybe a slightly aggressive, but maintainable pace? Is hull speed a reasonable expectation without olympic exertion?

    I'm thinking a casual, but consistent pace might be in the the 3 kt range. (?)

    Thanks!
     
  2. messabout
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    messabout Senior Member

    Your estimate for casual paddling is about right for a kayak in the 14 foot range. Four plus knots is possible for athletic types. Distance paddlers with 17or 18 foot sea kayaks can average 3.5 knots in good weather and water conditions. But those people are good at what they do and they are certainly physically fit.

    I expect that the Cp has some influence on ultimate performance but I would not get choked up on such a number. I think that most yaks will fall into the 0.52 to 0.53 range unless you make it a very long and very skinny one. I think that your best aim is to design for minimum wetted surface rather than emphasize other design parameters. Skene would have us using a larger Cp number if we want to go fast. But I doubt that Skene spent much time paddling his kayak.

    There are a number of Kayak types who participate here. Perhaps Ancient Kayaker, among others, will provide more pertinent information.
     
  3. LP
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    LP Flying Boatman

    Thanks for your response, Messabout.

    I think that Cp plays a greater role when operating at hull speed and operating in displacement mode at higher S/L modes. It's good to get confirmation on validity of ones own thought processes. For bursts of speed, we would be pushing an S/L of about 1.0, so a Cp of .52-.53 aligns very nicely. To error on the high side is acceptable.

    Operating at S/L<1.0 friction (wetted surface) concerns make sense.
     
  4. DCockey
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    DCockey Senior Member

    John Winters wrote a monograph on canoe and kayak design, The Shape of the Canoe, which is available on a CD. It includes discussion of various hull parameters and their effects on performance.

    http://www.greenval.com/order.html John developed Kaper, a performance prediction method for canoes and kayaks. Kaper is one of the methods used by Sea Kayaker for its tests, and is also included in some boat design software.

    Matt Broze developed a performance prediction method based on the Taylor ship series method. It is also used by Sea Kayaker and links to it are here: http://www.marinerkayaks.com/mkhtml/downloads.htm

    If you have Kaper and/or the Broze method you can evaluate various combinations of parameters to determine what is "optimum" for your purposes to the extent you trust those methods.
     
    Last edited: Apr 23, 2011
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

  6. LP
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    LP Flying Boatman

    Thanks DCockey and that other Guy (Leo. :p).

    DC, I couldn't get your first link to come up

    Leo good reading. I like the general conclusions. My design falls roughly into general shape criteria established in the first paper. As close as an S&G "V" hull can get anyways. I plan to put a radius at the chine and midships at the keel. That's as close to convex in the sections as this one will get.

    With the radius, I think we can estimate a parabolic-ellipsoid (or is it an elliptical-parabola? :D) section.

    The waterlines are parabolic with some class breaking hollow. :rolleyes:

    And the keel-line is rectangular(flat) with some eliptical attributes.

    It's been a while since I played with Michlet. I may have to break it out, though I believe I don't have the Length/Beam ratio for Michlet to work accurately. I'm approaching six(6) so maybe I'm good.
     

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  7. Easy Rider
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    Easy Rider Senior Member

    LP,
    I like your kayak a lot. Most are too hollow in the ends like my Necky Pinta ...especially the bow. Your's looks much like one of the Eddyline boats.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Nice lines, and lots of stability. Should paddle very easily. Shortening the hull to a few inches less than 12' may eliminate the need for a second scarf joint on the sheer planks; check the plank developments. If you haven't already, take a look at the design ratios pdf document by Eric Sponberg at http://www.boatdesign.net/forums/bo...ulation-implications-30857-18.html#post353422 - there is a table of Cp vs S/L
     
  9. DCockey
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    DCockey Senior Member

  10. DCockey
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    DCockey Senior Member

    The chines turning down at the ends, particularly at the bow, doesn't seem right to me. Is that due to trying to achieve a particular profile?

    What type of construction are you planning?
     
  11. cthippo
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    cthippo Senior Member

    I like the lines on it. Looks like it might even fit my size extra fat ***.

    So are you going to post the shapes for the rest of us once you get it nailed down? :D
     
  12. LP
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    LP Flying Boatman

    Thanks, Easy.

    I had to tweak the tail a little more. I wanted to get the inboard half of the of the chine above the waterline. More in a bit.

    DC,

    The downturned chine is visual only, especially in the bow. If you look at the forward most section line, you'll see a continuous "V" section from keel to sheer. By turning the "chine line" down, I can pull the keel line deeper for a finer entry.

    The exit has similar properties, but because of the fullness aft it doesn't work quite the same. By raising the chine aft, it raises the turn of the buttocks above the LWL for smoother departure flow. The turn of the aft immersed buttocks is not so great and a radius there will keep the flow going there. Again, by dropping the very aft chine line, I can pull the keel a bit lower and farther aft for directional properties.

    I have to admit that the chine line is partly esthetic, too. Artistic license you might call it. To some degree I'm trying to emulate a bow and stern wave at hull speed. Exaggerated, I hope.

    The easy build method will be S&G with marine ply. I've been following txriverrat's strip build with a lot of interest. I could go that route if I wanted.

    Terry, I was looking forward to hearing from you. I figured I'd just scarf a couple of 8' sheets and be done with it. There are enough other random pieces, I'm sure I can chew through a big chunk of the remaining ply. Of course, I should just make her a 15.5'er and use the full length. :cool: I'm actually working my way through that post already. It came up on a "Cp" search. Thanks though.


    CT we'll see what shakes loose from the tree.
     

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  13. LP
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    LP Flying Boatman

    For casual cruising (3 kts) my S/L ratio is 0.8-0.9. If I kick her in the skirts and burst out at 6 kts, I've got a S/L ratio of 1.7. For that small speed variation, I span several different S/L ratio vs. Cp ranges.

    The book I have on hand is Cyrus Hamlin's "Preliminary Design of Boats and Ships." This chart is expressed.

    SLR < 1.0 -------- Cp = 0.60-0.80
    SLR = 1.0 -1.2 --- Cp = 0.53-0.58
    SLR > 1.2 -------- Cp = 0.60-0.70

    I find this all very interesting because there is such a variation in optimum Cp depending on my energy expenditure. Considering that Cp's greater than 0.60 are used in ships and semi-displacement modes, I'll limit my choices to below that value. An SLR of 1.0 gives me 3.5 kts and an SLR 1.2 gives me 4.2 kts. With this in mind, I think that it boils down to the fact that it doesn't matter as long as I have a Cp between 0.53 and 0.59. The current design sitting a 0.55. I think that I could push it to the upper limit and maximize performance in the lower speed range and the upper speed ranges, but I suspect that most operations will occur in the mid range where the Cp is sitting right now.

    This analysis of Cp might be excessive for the project at hand. In the papers that Leo recommended, K1, K2 and K4 olympic kayaks were being optimized. Average speeds of over 11 kts (5+ m/s) were maintained on courses of 500 and 1000 meters. Very impressive and certainly beyond any of my own expectations. Wave making (Cp) only represented 20% of total resistance while skin friction represents 80%. I suspect that in the speed ranges I'll be operating in, wave resistance could be half or even less of that value. As mentioned earlier in the thread, reduced wetted surface will play a major role developing an efficient kayak.

    Unfortunately, to minimize wetted surface, I would actually have to fill out the ends of the kayak to maximize volume, thereby reducing draft and wetted surface. I'm not overly eager to go this direction as it would change the design considerably.
     
  14. LP
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    LP Flying Boatman

    Plates

    Here's a teaser CT.

    A preliminary nesting layout for the major panels. This would be cut from 3/16" ply. I think that I can get the curvature needed for the forward deck from that thickness of marine ply. I have never tried it yet though. In my earlier prototypes, I used low-grade 1/8" lauan to get the curvature needed.

    This nesting diagram does't leave enough excess to make the seat, seatback and frame. I would have liked to keep it at a 2-sheet boat, but may have to push it to 2 1/2 sheets.

    There is still some tweaking to do to get a tighter nest configuration. I'd like to get the developed sheer line straighter to straight. That will draw the sheerstrake ends up (+Z) and work the keel and chine lines to tighten that nest, too. All of that will allow the deck panels to slide in farther and release a little more material. Hopefully enough to for the remaining pieces.
     

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  15. messabout
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    messabout Senior Member

    Leo; It pains me to reveal my ignorance in a public forum such as this. I do have a question or two.

    Your papers says that parabolic waterlines appear to be optimum. That must imply some degree of hollowness in entrance and exit. (I recall reading of Herreshoffs insistence on hollow forward sections.) Hollowness has been counterintuitive when I attempt to think in terms of particle acceleration. I may be too hung up on the likes of F= Ma. Force being a function of acceleration and all. Yes, I understand that water is a viscous substance that is not directly comparable to a bath tub filled with ping pong balls. Just the same, water must be moved if the boat is to move. I'll put Newton aside, I am here to learn. Please comment.

    If I write an equation, to describe a waterline, will it take the general form of y= x^2-x+1.................?? :(
     
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