On the hydro dynamics of flat bottomed kayaks

Discussion in 'Hydrodynamics and Aerodynamics' started by Christian Nally, Oct 4, 2022.

  1. baeckmo
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    baeckmo Hydrodynamics

    Alan C: I have not checked the hull shapes you refer to regarding validation. Assuming they follow multihull "practice" I'd guess they have a transverse shape that is more ore less semioval or U-shape. The validation seems ok as long as the shape is not too different from the generic one. However, I suspect that you might be stretching it a bit too far when drawing conclusions about a square cornered rectangle.

    The missile example given earlier is not representative for this case; if you look at the drag coefficient a zero AoA it is quite high for both shapes. In fact the body is truncated at a full transverse section, which means that the dominant drag factor comes from the transom pressure loss. All other losses present are minor in comparison. In addition, the missile experiment was carried out at Mach 0,8 and the body was "fully submerged", which is a different condition from a low speed body semisubmerged and depending on gravity waves.
     
  2. dustman
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    dustman Senior Member

    Oh crap, you are right! Got it backwards. Been a little loopy lately, too much work and distraction.
     
  3. Alan Cattelliot
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    Alan Cattelliot Senior Member

    I fully agree with you. As precised in the post #16 :

    How far are those calculations from the reality ? According to the study I gave in the post #4, the order of magnitude is, at least, correct.

    Are these calculations able to distinguish between section's influence ? My opinion is that, unless we do not use more sophisticated methods of calculations, we cannot know.

    There is quite a gap between the kayak shape floating on the water and the missile shape flying in the air. The problem that I (we) have is to find publications of experiments or calculations representative of our case. I doubt that kayak's designers would spend a sufficient amount of budget in CFD nor in tank testing in recognize facilities, even at olympic level. The same goes for me. I have taken the opportunity to apply the analytical model that I have on kayak's hull shapes, with a faint faith that someone could have any better quality datas. However, in the scope of this discussion, I have no intention to use my CFD code, without any fundings.

    That said, I may recall that the transom drag of a fully submerged body or the transom drag of a floating body both depend on the aspect-ratio of the transom, which is rather the same for the missile of round section and the missile of the square section, depending on the definition that we give of the aspect ratio itself. So can we get, with this study, an intuition of the influence of the shape of the section on the body drag. If, in both cases, the transom drag are almost the same, then the body drags should be also almost the same. If we assume that the transom drag of a square shape is greater than the transom drag of a circle shape, due to vortices generated at the wedges of square shape, then the body drag of the square shaped missile should be less than the body drag of the circular shape missile.

    In addition, when you look at racing multihulls history, a clear "squarization" of floats tendencies is to be observed. To my knowlegde, there is two main reasons for this : First, CFD simulations & tank tests performed on racing boats ( again, bigger project, bigger budget, bigger studies ) have shown that the hydrodynamic drag of chined hull is, at worth, equal to the one of rounded hull. At best, chined hull could have less drag than rounded hull. Second, hard chines could be used as natural stiffeners, adding subdivisions to more flat surfaces, where it is easier to place core materials.

    Allow me to share with you pieces of this study, that has been realized at the ECN ( Ecole Central Nantes, Fr), whose hydrodynamic laboratory is recognize and well known, and ordered in 2007, by MULTIPLAST, a boatyard famous for their racing multihulls. At this time, the Gilles Ollier Design Team, where I was employed, was working on a new generation of hull shape (in parma). -drawings have been stretched to make difficult any use of this study-. We were testing U shapes versus elliptic shapes. CFD has been performed on bare hull.

    i. Presentation

    upload_2022-11-4_10-32-36.png


    upload_2022-11-4_10-33-9.png

    ii. Typical output


    upload_2022-11-4_10-41-38.png

    iii. Drag computation
    In the figure down below, "Cx" is the drag coefficient. The pink curve correspond to the Q6 hull drag, with squared sections. The grey curve correspond to the drag of our reference hull, namely ORANGEII, round the world record holder in 2005, in 50 days, 16 hours, 20 minutes.
    As you can see, the drag coefficients are almost the same, with a very little advantage to Q6. The wave drag generated by Q6 with U sections is lower than the wave drag generated by the reference, with rounded sections.

    upload_2022-11-4_10-33-56.png

    upload_2022-11-4_10-50-5.png

    So, in the absence of more representative studies, I find quite conservative to conclude that slender bodies with square section or rounded sections have almost the same drag coefficient. Regarding the manoeuvring capabilities of light and small monohulls, and from my experience of windsurfing, both in waves and on flat water, I can say that, indeed, chined sections are more appropriate at high Froude, allowing a better heading in straight line, and that rounded sections are far more easy to jibe or tack in medium waves.

    Nevertheless, kayak shapes are also clearly out of my league, and I would be very pleased to have a better knowledge of these wonderfull crafts. Do you have any contradictory experience or study to share ?
     

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    Last edited: Nov 4, 2022
  4. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Here is the update of the compared influence of the section on the drag of no-rocker kayaks . I've roughly made models of the hull shapes, and corrected the wetted surface calculation for the triangulare shape and the Rec_smooth shape, by direct measure of the surfaces on the 3d models. Again, with the evidence accumulated and a conservative reasonning based on my experience, I find that Rec and Tri have almost the same drag, and that the variable chine of the smoothed-rectangular hull shape give it a small advantage, compared with the two other shapes.

    upload_2022-11-17_11-41-7.png



    upload_2022-11-17_11-42-4.png
     
  5. rob denney
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    rob denney Senior Member

    A couple of years ago I was asked to teach a group of Marshallese Islanders how to build a simple, easily sailed boat for transporting copra, waste and groceries across lagoons and for fishing.
    The proa we built has a flat, rockerless bottom and decks and vertical sides. The long hull is 3 sheets of ply long, the middle is paralell sides, the ends curve to a point. The windward hull is 2 sheets long, without the middle parallel section. The rig is a 30+ year old crab claw, the steering is by paddle and it has a leeboard. With 5 people on board it has been gps clocked at 13 knots. This was good enough for me to build a 24m/80' version which will soon be launched in Fiji for testing and improvements prior to production.

    Not sure how this will relate to a human powered version, but it is far quicker and easier to build than any other boat type.
    I'd say go for it, and if you want to play with different hull shapes, use cheap foam glued to the bottom and hand shape and fibreglass it.
    More on the build at Mini Cargo Ferry Prototype – Harryproa http://harryproa.com/?p=3155
     
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  6. Alan Cattelliot
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    Alan Cattelliot Senior Member

    I just couldn't stop at seeing your video. Nice ride. Is it harder to steer when the boat speed increase ? The 24m version, with his overall simplicity, is very nice. So refreshing compared to what is sold around here.

    Thanks for the share. (and if you could post picture or videos of the 24m version, it will be wonderfull)
     
  7. rob denney
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    rob denney Senior Member

    Thanks. The faster you go, the more load on the paddle, but the quicker the response. Not a big deal. Easiest way to steer is by moving back and forth. In the video, the paddle is not in the water much as I was moving to steer.
    Pics and build blog on the 24m are at Cargo Ferry Prototype – Harryproa http://harryproa.com/?p=3788. Updates happen periodically, next one should be when it is launched, although there are several issues outside my control to fix first. Email me at harryproa@gmail.com and I will add you to the update list.
     
  8. rwatson
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    rwatson Senior Member

    Sailing is a totally different job to paddling.
    The power as wind rises exponentially, while the power of muscle reduces linearly over time.

    The acid test is 2 hours of paddling against the wind. Every little bit of hull drag becomes a challenge.
     
  9. rob denney
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    rob denney Senior Member

    For sure.
    As well as the smaller cargo proas we are running classes for Women in Fisheries Fiji. The big problem they have is lack of boats, which if they exist are old, leaky and heavy. We have designed a simple to build, lightweight (~15 kgs) rot proof boat which they can build in less than a week. Classes start next year, I will let you know what they are like to paddle when the first one is launched. I built a prototype which is hard to paddle due to the topsides height and the poor length to width ratio as I was seeing what was required for stability, and get the paddler weight low. The build method was a success, despite using several different thicknesses of foam. It weighed 9.5 kgs (21 lbs). The kids and students here love it.
    The class boat will be longer, thinner and have an outrigger.
     

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  10. Paul Scott
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    Paul Scott Senior Member

    NW Native American racing canoes (really long, really narrow) have had flat bottoms and nearly vertical sides, as have a lot of other canoes in the area. Minimal rocker.

    Moth low rider hulls, before foiling, had flat bottoms, vertical and near vertical sides.

    FWIW, Vacanti software shows, for shallow rocker, flat bottoms, & vertical sides, there is much less wave drag than other hull cross section shapes, esp at higher S/L ratios, where wave drag can actually start to diminish. (Modeling assumptions at work?) Friction drag tends to be dwarfed by wave drag fairly quickly anyway, & at fairly low Froud numbers.

    Initial stability can be a bit of a trickster though….
     
  11. rob denney
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    rob denney Senior Member

    Thanks.
    Rocker has always seemed counterintuitive to me. Looked at from the side it is a very low aspect ratio foil shape, with a centre of thrust about 20% aft of the bow at low angles of incidence. This can only serve to drive the bows deeper. Rockerless, with the bow inclined upwards, gives the opposite effect.

    Fortunately, stability is not an issue with outriggers and proas.
     
  12. rwatson
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    rwatson Senior Member

    You need to study hydrodynamics of low speed vessels. Rocker never gets close to having any kind of foil effect at paddled speeds.
    It does however make a really big improvement to efficient travel through water, as the entry and exit of the water encounter less resistance.
     
  13. rwatson
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    rwatson Senior Member

    You are mixing Apples and Oranges.
    Sail Power versus Paddle power.

    Using a primitive engineering cohort, that lives in areas where trees were straight, is no recommendation.

    Edit: Oh, and I am forgetting the biggest reason for a bit of rocker - water isn't always dead flat.
    [​IMG]

    Getting a perfectly straight hull to travel efficiently through waves is very energy intensive.
     
    Last edited: Nov 25, 2022
  14. Paul Scott
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    Paul Scott Senior Member

    So… Moths are Apples or Oranges??
     

  15. Paul Scott
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    Paul Scott Senior Member

    Here’s a neat quick build method

    Cajun Pirogue Wooden Boat Kit and plans https://www.unclejohns.com/boat/

    Pull the top of the sides apart, set with a stick the max beam location you’d like, and create different rockers & beams, plane/sand the chine logs, and attach bottom.

    you can build models quickly out of model ply or balsa, tape the ends together, and yoke test (a la Bethwaite) different rockers, flare, etc.. up to 4’ is easy, since you can buy, for example, 3’ balsa sheets from hobby supply houses, so you don’t even need to do much cutting. Increase the number of cross sticks to experiment with prismatic.
     
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