the case of the square/rectangular kayak

Discussion in 'Boat Design' started by john5346, Jul 16, 2010.

  1. john5346
    Joined: Jul 2010
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    john5346 Junior Member

    Hi everyone great website, if you dont mind, I have a newbie question.

    I want to make a completely rectangular kayak out of plywood. Not understand of design much, would I lose too much energy because the hull is not properly shaped?


    Would I still be able to paddle it and go somewhere ?

    I would make it the size of a sea kayak. Imagine a sea kayak then imagine right angles everywhere.

    flat bottom, flat side, flat everywhere.. is it possible?

    Thank you
     
  2. hoytedow
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    hoytedow Senior Member

    Boas-vindas, john5346.
    It may be cumbersome. I would streamline. Square edge on bow and stern will affect steering and controll. Show us some sketches of your idea.
     
  3. john5346
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    john5346 Junior Member

    Obrigado :)

    Like that thing..
    [​IMG]

    Picture of rectangle found on the web, I plan to build it the size of a sea kayak. Like 16 feet in length, 35 inches width and 35 inches height. It is an approximation, I dont know the nautical terms for it.
    I think it would work because I have used many things that are not aerodynamic (another word I am sure for water) and still work pretty good. I have seen a log/tree floating down a river and it goes pretty good. I have also seen rafts and pictures of those old shanty boats and they still work. Sure they are going down river and everything would work but I think you understand what I am tryint to say. I think a good design might make a difference in speed for an athelete paddling it but for a normal person only trying to paddle for fun, I dont think it will make a difference. I am not sure, I think it would work.


    Thank you
     

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  4. hoytedow
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    hoytedow Senior Member

    New word: hydrodynamic
     

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  5. john5346
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    john5346 Junior Member

    some old words for you: thank you

    english is not my first language
     
  6. NoEyeDeer
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    NoEyeDeer Senior Member

    Yes. It will stop you dead in your tracks. Also your proposed proportions (35 inch beam and depth) would give something too wide to paddle and too tall to be stable plus it would have tremendous windage.

    Short version: don't do it. Waste of time and materials. Use an existing simple design that works.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    With such a high boat you will not be able to paddle it while sitting in the bottom: as soon as you try to stand up it will tip over. Even if you cut the height down so you can paddle it around it won't be much fun and could be dangerous.

    Kayaks are the shape they are to glide efficiently through the water, ensure stability and provide strength and stiffness with lightness, and allow reasonably comfortable paddling.

    What you are proposing is closer to a dock than a boat. Go look at a dock, they are far more than 35" wide.
     
  8. sharpii2
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    sharpii2 Senior Member

    A kayak is the nautical equivalent to a ten speed bicycle. Like the ten speed on land, the kayak on water must move with the absolute minimum effort on the part of the paddler. In order for this to happen, it needs three main design features:

    1.) water must be able to get around it with absolute ease. So, like the skinny tires on a ten speed, the kayak must be as narrow as possible. Displacement lost through reduced Beam can be made up with increased Length. Hence touring kayaks are very long and narrow. Some times as narrow as two feet (61 cm) or less. Kayaks also must be streamlined at least fore and aft and at least on one plane. Your box kayak, for example, could have a bottom that curves up at both ends until it clears the water when loaded

    2.) there must be as little windage as possible. All the paddler's efforts should go to almost exclusively moving the boat forward. Any effort needed to keep the boat on course gets subtracted from moving it forward. The wind can be a major culprit, so the sides must be kept at an minimum height. buoyancy lost due to lower sides can be somewhat made up with an exaggerated camber (side to side curve) of the deck. Height of the sides above water should be mere inches (centimeters) and the camber of the decks should rival if not surpass them in height. It is a lot harder for a cross wind to grab a curved surface than a dead vertical one. A pitched deck (resembling a pitched roof) can be used in stead of a cambered one.

    3.) Because the sides are so low, the kayak should be as decked over as possible to prevent flooding. It is best to have at least two water tight bulkheads, one at either end, so the cockpit (where the paddler sits) can be flooded without sinking the boat. Hatches can be built for each of these compartments, so stores can be put in them.

    These are the basic three requirements of a kayak. If you satisfy them, you will have a useful, though not necessarily good design.

    The rest is frosting on the cake. But some cakes have thick frosting.
    And kayaks are a good example of this.

    Usually, the next step is to limit whetted surface (underwater hull area) to an absolute minimum. A rounded section kayak will have less friction moving through the water than a rectangular one. But the rectangular section is usually a lot less tippy, so it is not unusual to see a kayak with a somewhat rectangular section in the middle of the boat and flaired, "V" ed, or round sections toward the ends. All cut whetted area, with rounded sections being the most effective. "V" ed sections are used to lessen pounding and, especially when at the ends, 'bite' the water to help keep the boat on course.
    This way, whetted area adding keels and skegs can be avoided.

    So, to design a really good kayak is a sophisticated process requiring a lot of experience and thought.

    Though, IMHO, a usable, crude one is well within reach.
     
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  9. john5346
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    john5346 Junior Member

    Thanks everyone for your very informative posts.

    I will still make the rectangular kayak but with leftover materials and expecting it to fail. Imagine if it works just fine and we dont have to worry about design anymore? :)

    I will post back results with me paddling it.
     
  10. daiquiri
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    daiquiri Engineering and Design

    John, don't waste your time. Listen to the advices above.

    If you're doing it just for the funny feeling of floating inside a box, than it is ok. That is something many of us have done as little kids - one finds a big box (easy to find a box big enough when you're a kid), throws it in the water and then tries to get into it without capsizing. Those who have tried know how difficult task it is. If you heel to much, it will capsize with no possibility of recovery. No way you'll be able to climb into it from the water, provided it still floats.

    If you manage to get into it, the next task will be trying to paddle around. You will immediately discover that in spite of all your efforts, you will move at a speed of a snail. Plus it will be very difficult to keep it on track.

    An adult person can do better than that. For example, you could make a very simple flat-bottomed canoe or kayak, something like this:
    http://chestofbooks.com/home-improvement/woodworking/Beginners/Skiff-Or-Flat-Bottomed-Canoe.html
    or this:
    http://www.glen-l.com/designs/canu-row/kayak.html
    which is ugly to see and inefficient respect to a true slender round-bottom kayak, but still enormously better than your boxy idea. And it will take just little more effort to build.

    Cheers.
     
  11. alan white
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    alan white Senior Member

    I say do it. Build it and if it's successful, you might think about building other types of boats exactly the same way. Rowing shells, whitewater canoes, dories, skiffs, virtually any boat now being designed using overly-complicated curves and problematic angles.
    Imagine working up to building a beautiful Whitehall rowboat in the same way. People would say, "Hey, I like that Whitehall, but what have you done to modify it? Is she fast?"
    And you can just chuckle and say, "You've got a good eye. However, I'd rather keep my design secret under wraps for a while yet."
     
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  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    Alan: I appreciate your sense of humor, but it might be a legally wise thing to delete your post.

    John: one last request from me: wear a top-of-the-line PFD (personal flotation device better known as life jacket) designed for kayak use when you try this. Do not close the top of the "kayak" in so much that you have difficultly wriggling out: the kayak style life jacket is designed to give you maximum freedom when you are paddling and may help when you are in desperate straits. Have other people standing by to help, and try to stay within your depth.

    Stay clear of anything you can bang your head on like a dock or rock. If it sounds like I am overstating the danger, I am not. A person can drown in inches of water and you may be amazed at the amount of force a simple capsize can generate. Been there, done that, was lucky.
     
  13. john5346
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    john5346 Junior Member

    Thanks everyone for the humour, plans and safety advice. :)

    Maybe doing the rectangular kayak and failing it, I will appreciate design even more and then I will think of better designs to boats.

    I will make it with cheap materials and try it in calm water with all safety possible.

    What about building a small model and trying it? I heard that is how they do with airplanes in those wind tunnels before making big airplanes. It will be easier to analyze how the boat goes in the water. What do you guys think? Maybe use some software to analyze hull lines.
     
  14. hoytedow
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    hoytedow Senior Member

    Wear a helmet in case you hit your head on a rock when it capsizes.
    Use um capacete no caso de você bater a cabeça em uma pedra quando ele vira.
     

  15. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Now that's something you could do without spending too much. You could make two very simple models and see how they behave: one would be the box of your dreams, and the other one a flat-bottomed canoe, for example.

    A leisure paddling will be at low speeds (S/L ratios), so the frictional resistance will command here. Therefore, your models should have a comparable wetted surface. They also need to have nearly the same displacements, and similar static stability characteristics, in order to simulate their intended use.

    I'm enclosing here the data of two hulls with dimensions adjusted in such way to satisfy these requirements. If you need Freeship files, I will post them too.

    If you want to understand the main differences without spending your money, time and without getting wet, you can make these models (you can scale them to dimensions which you find handy - but use the same scaling factor for both hulls). Without going for some sophisticated test methods and equipments, just put them in a nearby pond, and give'em a gentle push. See which one slows down more rapidly and by how much. Or, see which one goes farther away on the same push force. Also take a note about which one keeps the course better.

    It is a very gross way of doing things, and the models are shaped into very elementary forms. But it can give you an idea of main differences you can expect to see between a full-size box vs. a more common hull shape. It will surely help you understand why boats are made the way they are made.
    Cheers.
     

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    Last edited: Jul 20, 2010
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