Input desired on 20ft row/kite boat design

Discussion in 'Boat Design' started by W9GFO, Oct 15, 2015.

  1. W9GFO
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    W9GFO Senior Member

    I don't believe I asked for a dissertation on yacht design. What I did ask was for you to select something that is "wrong" and explain why. Instead you seem to be saying that I need to hit the books and come back when I've learned something about boats.

    Yes, I understand you are saying that I know nothing about boat design, and yes I also know that being able to draw something in FreeShip does not make me a NA.

    Sure I have, but I guess that depends upon what you consider a rowboat. My little 14 ft Starcraft is a rowboat. Put a motor on it and it planes nicely.

    Understood. Let me clarify that I want my design to plane well while retaining as much rowing efficiency as practical

    And I am here on BoatDesign.net just for that reason - to get some understanding.

    Maybe I should ask this, suppose I build the boat exactly as shown in post #3 and put it in the water and start kiting with it. What do you think will happen? What will the results be of the obvious things that are wrong with it?

    Will it be half a know slower than if it were expertly designed? Do you think it will capsize, pitchpole, pound... or what? You seem to see something very wrong. I am only asking what that is and what it's effect will be.
     
  2. W9GFO
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    W9GFO Senior Member

    The Swift Solo is a gorgeous boat, but a whole lot less displacement than I need. If I were to make my hull flatter like the Swift then the transom would drag considerably while rowing.
     
  3. PAR
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    PAR Yacht Designer/Builder

    You missed the point about the Swift Solo/Laser comparison I mentioned. These two boats use different hull shapes to achieve a planing hull form. Again, the point of study. To answer some of your questions requires a lengthy explanation, but more importantly one you might understand.

    Your 14' StarCraft is a lousy rowboat, but a fairly good modest speed, full plane powerboat. Do you understand why? This goes directly toward your desire to plane off well, yet retain some rowing ability. You can row a barge if you want, though you might not want to for very long. The same is true with most full plane hull forms, you can row one, but you'll wish it had a motor after a few hundred yards. I'm not saying you know nothing, but I am saying your questions are so rudimentary in nature, to answer them is much more complex than you currently realize.
     
  4. W9GFO
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    W9GFO Senior Member

    I think perhaps my method of learning and your method of teaching do not work well together. I really wish you could pick out something specific about my design and explain why it is bad. Since there are so many obvious things wrong, I would think you could land on one and highlight it for me. I am not inclined to go off on a comparative analysis of sailing dinghies to discover what you are trying to tell me.
    Certainly I do. That's why I said "...I guess that depends upon what you consider a rowboat".

    You might be right about that, I really don't realize why it is so difficult to comment about the design. I've done my best to explain why I designed it the way I have, I've asked for feedback on how to improve it but all I seem to be getting is advice to take classes. That's not practical.

    It is becoming humorous to me that my design is so bad, but no one can say what is bad about it. I get the feeling that I have offended by daring to design something without having formal education in naval architecture. Have I insulted those that earn a living designing boats? That was not my intention. Instead of receiving advice on how to improve my design I am being told to improve the designer. I would never treat someone the way I have been treated here unless I felt contempt toward them - and in those cases I tend to just not respond at all.

    I am very open to criticism concerning the design that I have presented. If you think it is garbage - fine, but at least do me the courtesy of explaining why it is garbage. Even more helpful would be to offer some advice on how to improve it.
     
  5. PAR
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    PAR Yacht Designer/Builder

    There's a host of issues your design needs to address some of your desires. There's also several conflicts between the desires, in the current design. The entry is too fine to offer much lift initiation, the sections not well shaped for plane mode, the volume distribution not well suited to initiate a plane, though once up, given sufficient power, you might maintain it, until that bow digs in. If you trimmed her down a tough at the bow, you might be able to row her with some efficiency, but there doesn't appear enough volume forward to support the pull portion of the stroke. In plane mode she'll be skitterish, as her sections are a fairly soft and you'll be all over the rails just trying to hold her flat. The lines drawing provided is so dense with color and lines, that seeing what her sectional shapes are, is really just a guess, try to thin them down a bit and screw the color, until it's time for the production brochure to get printed.

    Now, how much fuller should the bow be? How much firmer should the bilges be? Etc., etc., etc., well it's all based on the SOR, your GA and initial weight estimates. Generally the novice designer bases their initial work on well known models, not venturing very far out on the design limb. This is the way you learn what works and what doesn't. Its also how you learn what others did for the same set of issues. If you do venture very far out on the design limb, you should have a clue about what's going to happen. Your current model will plow pretty good, unless you have lots of power and can trim her aft enough to free the forward sections, as she climbs the hump. In an ideal plane mode hull form, you'll try to avoid the hump, but this puppy will decidedly have to do some climbing. Once up on plane, you'd trim aft a touch, which will keep the bow from ruining your day and if the planing patch is sufficient, you'll scoot along nicely, if you could keep her relatively flat. In reality, I'd suspect that hull wouldn't sustain plane for long, but she'd readily surf occasionally with passing wakes and in modest chop.

    As to what you need to do to fix it, well this is your SOR and it's a bit convoluted, by the little I know. It sounds like you want a boat that will pickup her skirt and dance. If this is the case, you could move the rowing position farther forward than typical, to get the transom clear and if the bow could support the pull stroke, you'd get some upper body workout, if desired. This would sail fairly well with the sheets eased, though up wind will be tender and tend to pierce waves, bow steer or auger occasionally. A hard roll tack would be an interesting thing to experience in this. As a rule you'll work with displacement and length to a targeted Fr, which will force most of the other decisions (midship area, Cp, etc.) about the design. Then again you could build some 1/8th scale models and see which one performs best.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The more I read stuff on this forum, the more I realise you need a very clear idea of what you want a boat to do, and do best, trying to diversify into multiple 'modes', in this case a rowboat and a planing boat of sorts, is apt to produce something that does not do either well. I'd like a boat that planes so I can run the bar without too much worry, trolls well like a displacement hull, and even sails so I can cruise long distance without running low on fuel. I am still pondering it !
     
  7. PAR
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    PAR Yacht Designer/Builder

    This is typical of most engineering endeavors, if you try to make a "combo" product. Flying cars don't fly nor drive very well, usually one aspect is better than the other comparatively. Amphibious cars are the same thing and you can find countless other "things" that try to be more than what they are, doing neither especially well.

    For a sailboat to plane, several compromises need to be accepted. This limits what else it can do, especially if you'd like it to do each well. The Mac 26 is a good example. It's not a very good sailboat, but does do better than pure displacement mode sailboats, though this isn't a fair comparison. As a powerboat it's equally as poor, with a 50 HP outboard, it's in the high teens (MPH) and compared to a typical, similar weight powerboat, it's very slow. Trying to be more than it is, though it's still very popular in spite of these shortcomings.

    With some experience and the willingness to test some ideas, a boat could be made to plane, troll and sail, though you'll need to prioritize these goals, knowing something's going to get spanked, in the effort to get the other stuff.
     
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  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I do have some ideas along the lines mentioned, but they involve "variable geometry", which is a structural and operational complication that you'd rather not contend with, but I do understand I have to conform to the physics, and not the other way round ! I'll post a thread on it when I have cogitated on it a bit further.
     
  9. W9GFO
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    W9GFO Senior Member

    Thank you, this is the kind of input I was looking for.

    I understand that concern. My thought is that during times when planing will be possible all of the crew will be in the cockpit, which will move the CG well aft of the center of the waterline area. When not planing, but still moving under wind power, I wanted the finest possible entry to cope well with chop. Only when rowing, beating to windward or in light conditions would someone be up in the front.

    That makes sense to me for a rowing shell but for a half tonne boat like this - do you really think the pull stroke would have that much of an effect? There's a ton of reserve buoyancy in the bow. Maybe I'm misunderstanding something here...

    It will definitely need a skeg to keep the pointy end going forward and of course it will have a centerboard. The bilge sharpens considerably in the aft portion, do you feel it would need that sharpness extended further forward? Also, my understanding is that a rounded bottom makes for the least wetted surface and likely to roll less abruptly as something with a squarer section.

    Agreed. I have attached cleaner images.

    Length and weight are pretty well fixed. I want to keep it as narrow as possible for efficiency's sake. The pull of the kite is extremely low, making for little to no heeling moment - in some cases negative heeling.

    Piercing waves is fine with me, bow steer though is not appealing. Would making the forefoot rounded make a meaningful difference? Something like an IMOCA 60?

    I have made several models of earlier designs. Having a large laser cutter makes building models quite easy. Testing models is not something I have experience with. Playing, yes - but not testing. I'm not sure how I would go about testing them in a way that would generate useable data.
     

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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    W9GFO, your boat just looks way too fine up front. If a couple of people moved forward, it would be quite unstable. It may well turn into a sub running downhill in a chop, there is not enough meat up there to stop it happening. As I said earlier, this is not something I'd take anywhere near ruffled water.
     
  11. W9GFO
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    W9GFO Senior Member

    The front half will be decked over, only 30" of "headroom". One person could go forward to lie down, when conditions allow it.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Anything that pushes the bow down will render it a liability, and with your bouyant stern, that will happen running with a chop, not to mention you will need to get some substantial lateral plane aft to control broaching tendencies. Otherwise the stern will just skate around, especially if you apply a little helm at the wrong time.
     
  13. HJS
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    HJS Member

    Back to basic

    Please, gentlemen

    Go back and start over again.

    This is a boat that will be powered by a kite or oars. This means that there is no motorboat or sailboat.
    How much power is available and which direction has this force. A kite has a lifting force which must be taken into account.
    Moreover, this force can be placed so that there will be no heeling moment all.
    From this, then an optimized hull can be designed.

    js
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    He anticipates enough power to plane it, I see no advantage in the super slim bows, but potential for trouble from it.
     

  15. messabout
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    messabout Senior Member

    W9, none of your respondents are deliberately trying to rain on your parade. The fact is that you have designed a boat that is way out of the realm of practicality.

    You have stated above that you are striving for the finest entry angle. That is all very well but there are limitations to what is serviceable. Entry is worth consideration but you must not ignore exit characteristics either. In displacement mode, (rowing status) the waterlines work best with configurations somewhat like a parabola.

    Look at the pictures of the boat by HJS in post number six. Make your boat look more like that one and it will be far more suitable in actual operation. I hope that you have not been misguided by the popular notion that you can "cheat the wind", or in this case, cheat the water with a wedge shaped body. That is not going to happen so we'd be better advised to go with what actually works, not what the general public has been led to believe by Madison Avenue advertising hustlers.

    If the boat is to plane then it must have some characteristics that make planing feasible. The boat bottom must have a positive angle of incidence, with respect to the waters surface, in order to initiate planing. The dynamic lift is not concentrated at the transom, it is distributed closer to the point of impact or leading edge of the lifting plate. The most aft parts of the lifting surface probably does contribute to the total lift but to a lesser degree. It is easy to be misled about this if you observe a racing hydroplane dancing on its transom. It should be obvious that a boat will need adequate forward bearing if the bow is to lift in such a way as to induce the required incidence angle of the bottom surface.

    I submit that a simple flat bottomed skiff, if properly designed, will be quicker to plane than some of the more complex shapes that you might envision. Such a skiff can be made to row decently but it will almost surely have more wetted surface than a more complex bottom shape might have. But the more complex shape will not be so easily persuaded to plane. You can't have it all.
     
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