For interesting low cost multihull plans

Discussion in 'Multihulls' started by metin_mehel, Jul 5, 2015.

  1. metin_mehel
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    metin_mehel mech.eng.

  2. gonzo
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    gonzo Senior Member

    I looked at the ones with oars and can't see how it is possible to row with the geometry you drew.
     
  3. metin_mehel
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    metin_mehel mech.eng.

    Could you please put a screenshoot of it? Iis pleasure me to talk about.
     
  4. TANSL
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    TANSL Senior Member

    I think that I agree with Gonzo in that, it seems to me, you have not given much thought to how people on board will manage those boats, to what extent they are habitable or comfortable or manageable.
     
  5. metin_mehel
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    metin_mehel mech.eng.

    I think I knew which model are you talking about... It is like a canoe with an outrigger? I will put more photos of this. It will have a gaff rig option.
    Thanks for the feedbacks.
     
  6. CT249
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    CT249 Senior Member

    I'm not sure if I was looking at the same boat as Gonzo, but it seemed that you couldn't row the gaff rigged cat, unless you were perhaps a much better than average rower and the water was dead flat.

    I also have to say that the reason for a low-peaked gaff escapes me completely and I wonder how the rudder would stay on.
     
  7. metin_mehel
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    metin_mehel mech.eng.

  8. metin_mehel
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    metin_mehel mech.eng.

    I think I did not understand well. Could you tell the problems that you saw in my design step by step?
     
  9. TANSL
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    TANSL Senior Member

    metin_mehel, with all my respect, your designs have very big defects. If you are not able to see them, you had better dedicate yourself to something else.
    You are a creative person and therefore deserve my respect, but from here to design boats, there is a large distance.
    Cheers
     
  10. metin_mehel
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    metin_mehel mech.eng.

    Tansl what are those problems that you mentioned? Stability? Drag? Could you explain your thoughts by solid engineering definitions?

    Thank you very much for all the answers that improve me.
    .
     
  11. goodwilltoall
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    goodwilltoall Senior Member

    How do you look at your designs without facebook?
     
  12. TANSL
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    TANSL Senior Member

    Metin_mehel,
    what I said is this: ".... to what extent they are habitable or comfortable or manageable."
    I can not talk about stability, drag, or ship theory because I have no data. You should not challenge me to speak from an engineering point of view ("solid engineering definitions", as you say) because, so far, all I see are sketches of objects that, surely, are decorative but not yet demonstrate that they are boats.
    If you have made any calculation of naval architecture on them, rest assured that I'll be happy to review them with you and give you some technical advice, if I think you need it.
    I do not doubt that you need a lot of advice before you get to design a boat and, of course, I can and I'd like to help.
    Cheers
     
  13. CT249
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    CT249 Senior Member

    OK, for one - to row well in rough water, you have to be able to drop your hands low, or else the other end of the oar will not lift out of the water far enough on the backstroke, and it will hit the waves. That makes rowing very hard and slow. The rowlocks therefore have to be well above the rower's seat - some people say about 20cm. In your cat, they look almost level with the seat.

    Having such low oars also puts a lot of strain on the arms - the handle of the oar should be higher up so you are pulling more in line with the shoulders.

    Secondly, a low gaff like that would probably just allow the top of the sail to twist to leeward, robbing you of power and adding to the weight. You may also find that a really big roach like that will cause the mainsheet loads to be heavier than you want in a little funboat.

    Thirdly, how is the rudder going to resist the side loads? There's almost no vertical distance between the top and bottom pintles and therefore there will be enormous leverage acting to break the rudder box or its attachments.

    There are other issues, like the hull shape and whether such a short boat will carry enough weight and handle well.

    I've been lucky enough to be able to interview many of the world's top sailboat designers. Pretty much the number one thing they share is respect and knowledge of other people's designs. They have looked at many, many other boats with a lot of inherent respect (funnily enough, many amateurs look at boats with contempt) and spent hours analysing why they work and why they don't, and where the compromises have to be made.

    Almost all of the top designers are very good sailors - probably most of them have won national championships - and this gives them the knowledge of how boats really work. In contrast, a lot of the would-be designers aren't very successful sailors and I think that this is partly because they get obsessed how they think things SHOULD work, rather than being open-minded enough to watch how they DO work in reality.

    If you haven't done much sailing, and haven't looked at a whole lot of other designs, it will be very hard for you to design a better boat. The really good designers have learned an enormous amount from other designers. Isaac Newton said, "If I have seen further, it is because I stand on the shoulders of giants", and great boat designers are the same.

    Perhaps you could start out by googling the articles and blogs by rowers and rowboat designers - that could give you a fairly simple indication of what has to be done and the compromises that have to be made, just in the one area of rowing.

    Just my two cent's worth - as I said, I'm not a designer but I've been lucky enough to talk to some of the top ones. Good luck and thank you for having a good attitude!
     
  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    This last post pretty much nails it down. Novice designers often don't have the experience necessary, across a wide range of subjects. All too often we see lots of pretty pictures, with hours of computer modeling time invested, but to a seasoned and skilled eye, full of classic errors, misconceptions and impracticable elements and/or in engineering and hydrodynamics.

    I'm at an age now where I mentor a few budding designers. I don't let them send me these elevated 3/4 color model images. The only thing these are good for, is a sales brochure presentation. These types of images, tell you very little about the hull, the appendages, the centers, the rig, etc. It's just a picture.

    One fellow I work with, sent me a typical modern hull to have a look at. It was after I told him I would only look at technical drawings of the structure and the common elevation, plan and sectional, 3 view arrangement. My first question was why is max midship section was at 50% of the LWL and his reply was, the software he was working with defaulted to 50%, when he typed in the basic parameters for the hull form. He didn't know that though the midship section would normally be in the general vicinity of 50%, ideally, it wouldn't be dead bang at 50%, particularity if he wanted to optimize the water plane shapes, for the performance he was hoping for.

    Simply put, he accepted what the software did, not understanding the realities of lines development, for the SOR he was working with. When questioned further, he didn't know if the displacement needed to be pushed forward or aft to optimize this area. He's now doing fairly well, though tends to change his mind too much. Before he started with me, he'd looked a some drawings, but had never really studied them. Why is the max midship section at 55% of the LWL, for example, instead of the default setting in the software. Why was a 19% lead employed in the rig, when the last set of drawings he was studying (also a sloop) used 15%. This guy is coming along fine and now has a data base of many dozens of designs, many of which he's taken apart, to see what choices where made and how they may have been optimized, for its particular set of design goals.

    Yacht design is about study, with some solid engineering tossed in, so the end result doesn't fall apart on launch day. You have to literally eat dozens of designs, absorb and understand the choices made during their development, before you can also make these basic decisions for a new design. It also is tremendously helpful to have experience on the vessels with similar performance envelops. It's all but impossible to design a new full plane dinghy, without considerable seat of the paints time in one. Don't get me wrong, you could study and design, without even having been on the water, but you'll likely never develop anything of note, simply because you don't have the various experiences (hydro, engineering, seat of the paints time, etc.) necessary to permit you to push the envelop and develop something special, let alone capable of nailing down the SOR goals.

    Metin, I don't know what your education or sailing experience might be, but there's no substitute for either. Study well known designs. Take them apart and find the centers, look at the way volume is distributed, the geometry, etc. and compare this with similar designs. Soon, you'll note some similarities keep cropping up, as well as some differences. Ask your self why these choices might have been made. Eventually, with a few dozen dissected designs under your belt, you will be able to do a similar design, incorporating what you've learned from the previous models, possibly making little tweaks, to improve it's abilities (this is what yacht design is really about). This coupled with actual butt in the boat time, will make you a yacht designer, possibly of some note. Anything less and you'll have lots of pretty pictures, but that's about it. This isn't meant as a series of insults to your abilities, but hopefully a guide for you to continue learning, both in front of the keyboard and with a sheet in your hand. I'm an old guy now, but I still manage to trim sheets a few times each week, often because I'm frustrated over something and need to calm down and think about something else for a while. I find a quick sail takes away most any of my major worries and I can get back to the project, whatever it might be, refreshed.
     

  15. metin_mehel
    Joined: Jul 2009
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    metin_mehel mech.eng.

    Thank you very much for all of your comments and supports!
     
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