Would that catamaran ever sail?

Discussion in 'Boat Design' started by john5346, Apr 23, 2013.

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  1. Edwardn
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    Edwardn Junior Member

    I agree, among everything else he was given a lot of good advice and help... what he chooses to do with that is his choice now, some people have to learn by trial and error, I wish him well.

    Edd
     
  2. troy2000
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    troy2000 Senior Member

    And I showed him actual construction photo's, to address the same thing.
     
  3. Emerson White
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    Emerson White Junior Member


    What makes you think that this hasn't been tested?It was tested thousands of years ago, and failed. It has been tested repetedly by people trying to figure out how to calculate water movements, and continually tests as a very very poor design. It's also been tested by many a fool over the years who thought that they knew better than experts.

    If you build it, you will waste more money on extra material than if you built an expert approved free design. If you save a little time and effort in the build process that time and effort will be eaten up by the poor performance that you will see the first time that you sail. Every time that you sail after that you will be losing more time and effort to poor performance, but your time saved is a one time bargain. Each time you sail it it becomes worse and worse a deal.

    Everyone here would love to see you build it, wasting your time and money, and then post for us how terribly it performs. Unfortunately you will probably be too ashamed to do so.

    As to a Junk rig, I'm not sure it's such a good job on such a catamaran. The Junk needs an unstayed mast because it has those spars swinging around in front of/beside/ behind the mast. An unstayed mast on a catamaran is a much more involved process than on a monohull, since the catamaran mast is planted on a piece of dimensional lumber.

    One more thing about boat design. We build houses square, because they only have to hold up air and themselves and the occasional bit of snow if you live in the frigid north like me. A boat has to hold out the water, which is much heavier. A square concentrates all of that force right on the edges, and they break. If you want to build square you need to put a lot more effort and material into it than if you want to build with some curves to spread the stress.

    If you go to a museum and look at the dugout canoes and rafts that ancient people built, they are all round, they knew better than to try and build square boats, so don't use the primitive state of their technology to defend your ideas. It disgraces the memory of my ancestors to credit them with such a short sighted design with such a glaringly obvious flaw. They took their cues from nature, and you never find a square seal.
     
  4. john5346
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    john5346 Junior Member

    What? When did I insult anybody?

    It is because tree trunks are round! The square shape was actually something really difficult to achieve in the past.

    Take a look at "wave piercing hull design" pictures on the web.

    "A hull with a blunt bow has to push the water away very quickly to pass through, and this high acceleration requires large amounts of energy. By using a fine bow, with a sharper angle that pushes the water out of the way more gradually, the amount of energy required to displace the water will be less, even though the same total amount of water will be displaced. A modern variation is the wave-piercing design."

    Look at the pictures of it, it resembles more a square and fine triangular shape than anything round.

    Thank you Edd for your kind words and understanding my side of the story.

    Below is a wave piercing hull. :)
    Now what?

    [​IMG]
     
  5. Emerson White
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    Emerson White Junior Member

    No, No, No!

    A) No, a square really isn't that hard to make
    B) No, When you make a dugout canoe from several logs you have to square them first, then make them round so they work in the water. It was much more work for them to make things round than it would be for you, they found it worthwhile.
    C) No, there really are hydrodynamic reasons for round. Why don't you find square seals, otters, eels, salmon, tuna? The only square fish that you find are the highly toxic boxfish and cowfish that move at a snails pace. The high speed high efficiency fish are all round, or very nearly round, in cross section. Orgle's second rule, Evolution is smarter than you are. Nature has been making animals for a billion years, and none of them follow the principles that you are trying to sell us on.
     
  6. john5346
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    john5346 Junior Member

    I dont know how dugout canoes are built, I will read about it.
    Nature? ok, that is the fastest fish on the sea with the name of Sailfish.
    [​IMG]

    Lets go back to "wave piercing hull", that is a very fast boat.
    Now what?
    [​IMG]
     
  7. Emerson White
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    Emerson White Junior Member

    Pointy at the ends, round at the parts that move along the water.

    Some dugout canoes are built with one log. Others are built by squaring logs and fitting them together side by side. None are square.
     
  8. troy2000
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    troy2000 Senior Member

    What's your point? That wave-piercing hull is a very sophisticated collection of curves, with no square corners at all. It doesn't look a bit like a nailed-together box with a nailed-together blunt triangle on one end, which is what you were talking about building.

    And why did you show the picture of the sailfish? It doesn't have flat sides and sharp corners, either.
     
  9. Petros
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    Petros Senior Member

    shape your hulls exactly like the sailfish and it should perform well. It operates in a speed range similar to most boats.

    the power boat operates at a Reynold's number and dynamic that is WAY beyond the speed of your proposed boat so copying a shape like that is not the best you could do. Have you notices that sail boats have very different shaped hulls than power boats? They do. The operate at different speed ranges. And as pointed out, there are no flat sides on it, but rather long curves. the most important part you can not see, below the water line.

    Having a boat with smooth graceful lines is always nice, it makes it a joy to use and will perform better. You can go ahead and build your box catamaran, but for the same amount of work, cost and trouble, you can build a much better shaped hull. That is all the people here have been trying to convince you of doing.

    If you do not care about performance why not just build a barge type hull? It will be more useful and much less work to build. You can even get plans for free of the Puddle Duck.

    Good luck.
     
  10. john5346
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    john5346 Junior Member

    I think you are being helpful. Lets think we are not arguing, only not agreeing on something. It makes it easier to talk about it, like in a university when talking about a study.


    I think we have all preconceived notions of what a boat should be. That is why you could extend that notion that curved is better for hydrodynamics even without testing it because you have seen many boats.
    Is the hull below really curved? If not, why would the US Navy build it in 2013?
    Is that curved?

    [​IMG]

    [​IMG]
     
  11. JRD
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    JRD Senior Member

    John, Thats an interesting example, though I think you will find that that shape is driven by a stronger desire to minimise radar detection. If you look carefully at the underwater sections, they do appear to have faired in the curvature at the waterline. Though its hard to see for sure from the angle of the photo. There are a few tricks in shaping curved underwater sections that can make the corner go away.

    Im not sure that many of the best spending decisions in history have been made by the military..... of any country :rolleyes:
     
  12. Emerson White
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    Emerson White Junior Member

    Every part that touches the water is curved. Like the sailfish it doesn't really make your point, it makes the opposite of your point.
     
  13. john5346
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    john5346 Junior Member

    That is a better way to look at the hull. I dont know if that is what you mean by faired or a curve but is that enough to say that is a curved hull?
    If that is curved then I need beer now. :)

    [​IMG]

    [​IMG]
     
  14. Emerson White
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    Emerson White Junior Member

    Do you see how the keel is basically straight, but the waterline gets wider as it goes back? Yes that is a curve. Stop looking at everything above the waterline, it doesn't matter nearly as much as what is below the waterline (water is about 750 times as dense as air). The most significantly curved portion is behind that building off to the side.

    Edit: Here, I've attached a picture where you can see what the profile looks like under the water. This is the exact same ship, just focusing on the parts that matter hydrodynamically.
     

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  15. Petros
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    Petros Senior Member

    I am trying to be helpful. I do not have any preconceived "notions", I have worked as a design engineer doing CFD on military aircraft to make shapes flyable that did not want to fly. We built flying prototypes and they behaved as predicted, some of the shapes are now in flying aircraft. I have built some 18 or 19 small boats, none with motors, and I have sailed or rowed or paddled well over a hundred boats designed and built by others. I am all for innovation and building usual designs, I have done many myself. I can tell you that from an engineering perspective, from a structural standpoint, and from a practical standpoint as one who has built many small boats, what you want to do will be more effort, more costly to build, and not perform well than a simple design with curved surfaces.

    You do not need to question it, I have done it, I have tried it, I have calculated it, I have felt it, I have measured it. I know, like many others here, what you proposed is not the best, not the simplest, not the cheapest and not the lightest way to build a boat.

    The military has other priorities that are higher than efficiency, survivability is far more important. And lowering their radar cross section is one of the way to make sure their ships stay on the water and their aircraft stay in the air. It is not a good example unless you want stealth capabilities (and as simple as those angles and shapes appear, they were developed first with lots of sophisticated software and much testing, you can not expect a "copy" to do the same thing). And to my eye it appears the below water line shapes do have cures to them. it does not even have hard chines below the water line.

    If you want simple, build a puddle duck. It will be easier to build, and they are actually useful boats even if they are performance limited. these were designed to be simple and low cost to build.

    [​IMG]

    you can get plans for free, just google it.

    I already posted a link to free plans for decent plywood catamaran called a Hobby Kat. It is an old design but a proven one. You can scale it up if you want a bigger one.

    [​IMG]


    if you just want to build your idea than why waste time here? Go ahead, no one will stop you. We are just trying to help you build something that you will enjoy more. boats have been made from anything and everything, from concrete to ice, they just have to float.
     
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