v-shape vs flat bottom vs round shape

Discussion in 'Stability' started by yoram, May 15, 2011.

  1. yoram
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    yoram Senior Member

    hi guys

    i was wondering about the advantages and disadvantages of v-shape bottom vs flat bottom vs round shape bottom for a canoe, assuming that the length, width the material and general mass is as similar as possible (with the bottom shape differences) and general shape is the same, all from the point of view of stability and speed.

    it is probably a very frequent theme asked by novices like me and i was looking for info in the forum and i could not find a thread specific for it. it could be great if someone could explain that to me or send me to a link explaining that.

    thanks
     
  2. daiquiri
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    daiquiri Engineering and Design

    There have been many discussions about the influence of the cross-section form on the performance and stability characteristics of boats in general. You can find many of them by searching terms "round bilge" and "hard chine" in this forum.

    When it comes to the subject of kayaks and canoes, perhaps you should read the following work by Leo Lazauskas, our distinguished forum member, and E. O. Tuck:
    http://www.cyberiad.net/library/kayaks/racing/racing.htm
    http://www.cyberiad.net/library/kayaks/skmag/skmag.htm

    Also, check out these sites, which gives some good general info about kayak design:
    http://www.guillemot-kayaks.com/guillemot/information/kayak_design
    http://www.kayarchy.co.uk/index.htm

    Cheers!
     
  3. Tim Hall
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    Tim Hall Junior Member

    Yoram, I'm absolutely not an expert here, but I have been designing a canoe for myself, and studying up on as much canoe/kayak theory as I can find.

    John Winters has some articles on the Green Valley Boatworks website:

    http://www.greenval.com/jwinters.html

    These articles are expanded into a short E-Book, "The Shape of the Canoe." I've been reading through it, and it's the best treatise on canoe/kayak design one will find. My only complaint with it is while it has a lot of general theory, there isn't much actual data or any design examples for making comparisons.

    Regarding the section, this is what I've surmised from both designing and studying:

    The rounder you can get the section the more efficient the hull will be in terms of drag. Rounder sections generally make the least amount of wetted surface in relation to displacement. Wetted surface is your main source of drag for paddle craft unless you're talking about making a high-performance canoe that will be paddled hard and fast most of the time...in which case length and hull fineness (prismatic coefficient) become much more significant in overcoming wave resistance.

    There's always give and take though. Rounder sections will give more maneuverability at the cost of less directional stability and a more tippy feel (lower initial stability). V-shapes will give you the most final (higher angles of heel) stability, greater directional stability, but less maneuverability and high wetted surface area. Flat bottoms give maneuverability, very high initial stability, but but low final stability, and also are less efficient in terms of wetted surface.

    Most designs are some kind of compromise between these general shapes to accommodate the specific purpose of the canoe.
     
  4. rxcomposite
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    rxcomposite Senior Member

    Consider also length. A longer boat with the same displacement will move much faster. There is a limit however as too much length will have greater surface area defeating the length advantage.

    I use to plot this in excell but have since lost the file. It starts with a WSA (Wetted Surface Area), displacement constant, but with varying lengths. when the two lines crosses, that is the maximum length.
     
  5. Tim Hall
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    Tim Hall Junior Member

    John Winters also has a performance prediction method, called KAPER. This is what Sea Kayaker Magazine used for several years.

    "A longer boat with the same displacement will move much faster."

    I've found that's really only partly true, depending on what your design intent is. Length is definitely important, but once you get so much length it (and Cp) have very little impact on performance/resistance at typical cruising speeds. At least this is what I've found from analyzing my own designs with KAPER. Again, at cruising speeds with a reasonably long vessel wetted area is what slows you down the most.

    For example my design is 16.3' LOA, 16' LWL, which gives me a theoretical hull speed of 5.4 knots. But in the 3-4.5 knot range, wetted surface is the largest factor by far in determining paddling effort. And altering the Cp in this range has absolutely zero effect on performance.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Use a fairly narrow "U" shaped entry, which flattens out into a flat bottom (or nearly so) midship section, then transition to a "V" shaped stern. Of course entry and exit should be well shaped, follow these guidelines and the boat will "penetrate" well, offer good stiffness, particularly if you hoist a sail and the exit will be reliable. This will produce the most comfortable and predictable little craft you can ask for.
     
  7. Tim Hall
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    Tim Hall Junior Member

    This pretty much sums up the design I'm working on.
     
  8. yoram
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    yoram Senior Member

    thanks

    thanks you very much, guys for your answers. since i am looking to find also a very simple way to build a canoe with a few students, i thought flat bottom would be the easiest to build (also since i built one already) and probably i will built another one like that with the students but i wanted to be able to answer, of course in a very superficial way, on a hobby/layman level, about the differences between the hull shapes. i have been reading and will continue to read more about it and the principles involved. i think that for beginners paddling on a canoe flat bottom will give more stability and i also add so kind of amas (like bicycle for kids) just because the water here is really cold...

    in a canoe like the one in the photo, how big, if it is possible to measure, would be the difference in the speed between different hulls? is it possible to estimate roughly?
     

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  9. Tim Hall
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    Tim Hall Junior Member

    Well, the speed has a lot to do with the engine (paddlers) of course. But there's a kind of maximum speed for a canoe, kayak, or any other displacement-mode vessel called it's 'hull speed.' This is directly related to it's length. This is the speed at which wave resistance starts to shoot up exponentially. You'll have to do a little conversion here:

    hull speed in knots = 1.34 X (length @ waterline in feet)^.5

    But again this is a sort of 'maximum' speed. Average paddlers typically don't push their boats to this point, or at least not for very long.

    Below hull speed, various factors in the hull shape, and predominantly wetted surface area determine how much effort is required to obtain a particular speed. This is a much more complex calculation, and is only an estimation of performance.

    There are a few programs/spreadsheets you might be able to hunt down: Michlet, KAPER, Broze/Taylor method.
     
  10. Tim Hall
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    Tim Hall Junior Member

    For working with students a 'stitch-and-glue' method of construction with plywood is probably the most practical. But this will give you either a flat bottom or v-shape hull by necessity.

    For a truly round or arched hull you'll have to employ a much more labor-intensive construction method like strip planking.

    It is probably easier to estimate wetted surface area of hull with a stitch-and-glue design though, because your geometry is more or less linear vs curved. In other words you can easily compare your wetted surface area-to-displacement ratio between various flat-bottom and v-shapes.

    Wetted surface area-to-displacement = surface area in square inches/(displacement in cubic inches^2/3)

    -8.0 is low, +9.5 is high. The lower the number, the more efficient the hull displaces water.
     
  11. yoram
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    yoram Senior Member

    thank you all very much. i will read more about the subject. your answers shed more light on the subject for me and showed me how little i know and i am in this phase where every detail seems very exiting.

    thanks again.
     
  12. vignesh
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    vignesh Junior Member

    v-shaped hull-helps in moving faster in water[for a narrower beam],,most important point is it prevents damage due to slamming.

    u-shaped hull ,if the beam is wider above the waterline it provides stability.
    but flat bottom hull are susceptible to slamming damage.
     
  13. gonzo
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    gonzo Senior Member

    Kayaks with Vee bottoms can have higher top speed with the same length and beam. Uffa Fox designed a planing single rowing shell, I believe in the Thirties, that was almost as fast overall as the conventional type. If you have the power to keep up, a hull that will lift will have a higher speed.
     
  14. Mountain man
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    Mountain man Junior Member

    Hello Tim, I would like to use your formula "Wetted surface area-to-displacement = surface area in square inches/(displacement in cubic inches^2/3)" but math class was along time ago, how do I find (displacement in cubic inches^2/3)?
     

  15. messabout
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    messabout Senior Member

    Yoram; The flat bottom canoe is probably best for student building. Check out the Six Hour Canoe for example. That one has a small booklet that describes every detail of the building process. It is a little less than 16 feet long and uses only two sheets of ply. It is not a stitch and glue type so it is much faster to build. Yes you could probably build it in six hours if you are an experienced craftsman. This boat has been used for many school wood shop projects. Plan booklet from Wooden Boat Magazine It is a pretty, but not particular efficent, hull form. With kids, who needs efficient? What we need is fun.

    As mentioned above the wetted surface is one of the main determinents for a paddle powered boat. If you lay out some pencil sketches of a typical canoe like boat you can use some simple arithmetic to analyze the differences in sections. Just look at the center section of the boat. Anticipate all up weight, say with one paddler, at about 240 pounds. We are merely doing ball park examinations here. Let us say that the length of the waterline will be about 150 inches long. The center section of the boat will need something in the region of 85 square inches of immersed area. The flat bottomed boat..say 28 inch wide at the chine would need 3 inches draft. Just add up the wetted surface. 28+3+3=34 inches. Now consider a vee bottom with the same immersed area. With 28 inch chine beam the depth of the vee would be 6 inches. The wet part would be about 30.5 inches. You can tinker with the round chined cross section and arrive at a wet surface of less than 29 inches.. If you are teaching kids, then the math exercises are useful too.

    Wet surface is not the only consideration. Initial stability, righting moment, eddy making, tracking ability, and a lot more factors are involved including considerations for where and by whom the boat is to be used.
     
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