Request your input on my design

Discussion in 'Boat Design' started by richardf, Jul 15, 2013.

  1. richardf
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    richardf Junior Member

    Please point me in the right direction.
    Thanks
     
  2. DCockey
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    DCockey Senior Member

    That's a significant improvement, but still a ways to go. The chine and keel shape in side view appear to be fairly reasonable, but the sides turn in too abruptly. I doubt the the resulting bottom shape can be built using a single sheet plywood. Also, I agree that the bow shape as drawn probably can not be built using a single sheet of plywood.

    Get some cardboard, make a model, and decide how you like the shape in 3D. You will also learn something about what shapes can be made with sheets of plywood/cardboard/metal.
     
  3. hambamble
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    hambamble Junior Member

    download the demo of maxsurf from here http://www.formsys.com/maxsurf/msdownloads/demo or other ship design programs. I think theres one called free ship http://sourceforge.net/projects/freeship/ that is a similar thing.

    Design your boat in that. It will give you a 3D model that you can play around with, and i know maxsurf at least will calculate displacements at different drafts. Set the surfaces to developable, that way you should be able to construct it in plywood. Just beware that i think the maxsurf demo runs out after a month or something, or you can't save or open files.... so freeship might be the way to go.
     
  4. PAR
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    PAR Yacht Designer/Builder

    As has been pointed out, you'll work your way around the "design spiral" several times, until you get the numbers and shapes you desire. Guessing, just doesn't work without huge levels of experience.

    The entry is not developed and even if it was, you'd never be able to get plywood to make that hard turn at the stem, without laminating several layers of diagonally applied 1/8" stock, ripped into narrow strips. This is an effective technique, though usually not easy, especially compared to a developed hull form.

    The body plan looks more akin to a sailing hull form, then a displacement power hull. Through these two types have some similarities, there's some differences that can be incorporated to improve the type.

    If looking for a good displacement hull, with good comfort underway, make your forward sections more U shaped, rather then V. Transition the U shape into a nearly flat midship section, which then transitions into repetitively flatish V shaped stern sections. This combination of shapes will provide stability, an easy motion under way (unlike what you've drawn, which will tend to roll around a lot), the easy entry will provide buoyancy and the V exit will allow the water flow to reassemble aft of the hull in a peaceful, efficient manner. Also don't focus so hard on a "fine" entry. A fine entry is part of a package and if it does work with the rest of the hull, it's a detriment, not an advantage. As designed, this hull will likely "root" around, especially in certain sea types. This can make steering very hard, sometimes uncontrollable.

    Currently, the plan view shows you'll drag a huge quarter wave, which will dramatically decrease efficiency. The elevation view shows a portly refined chine sweep for a displacement powerboat. With the deadrise you're carrying, she'll settle baddy and the relatively steep buttock angles in this portion of the hull will allow the bow to rise uncomfortably at S/L's over 1.25.

    The first thing you need to do (besides the obvious yacht design study) is decide what you want from this hull. This will permit you to select a set of coefficients that are compatible with your goal targets (SOR). With the "matched" coefficients, your hull shape choices become narrower and the hunt and peck method less demanding.

    Lastly, vertical sides have distinct disadvantages, structurally and in terms of capacity and efficiency. Some designers have tried this approach and with very few exceptions, most fall well outside the mainstream in regard to acceptance and efficiency. Gaining 10 extra sq, ft. of internal volume, by making the sides plumb, isn't worth the cost of a wet boat, with no plunge resistance, weaker structural inheritance, monolithic aesthetics, etc.

    Buy this book:

    [​IMG]

    You'll gain significant insight to why boats are shaped the way they are, plus some useful formulas. You'll still need a stronger grasp of yacht design after this book, but at least you'll be much better armed, for these types of conversations and designs sketches.

    For a "Cliff's Notes" of yacht structures, buy this book:

    [​IMG]

    This too, isn't the end all, but you'll get close enough you can expect a reasonable level of success.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Google "developable surfaces" and you will get the idea. And with ply you have the added complication of a limit to bending radius for a given thickness of material, which is more restrictive than for, e.g. metal sheet. It is possible to draw up a hard chine hull and sheet it with ply without going through the procedure of creating a developable surface, but the only points of contact at any given station may be at the keel/forefoot and the chine, and you have to 'fill-in" the hollows created when the material takes up its "natural" curve, standing clear of the straight section as you bend it. Not ideal.
     
  6. richardf
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    richardf Junior Member

    Thank you, but I don't want to use a computer design program to design this boat. I have plenty of time on my hands and would rather learn how to draw up developable panels without a computer program. Surely someone can tell me how to do that. Been looking on the internet for help, but no luck so far . I have in my minds eye what I want my boat to look like and what I want it to be able to do. With help here I am hoping to work out the technical end.
    Thanks,
    Richard
     
  7. richardf
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    richardf Junior Member

    Paul,
    You are really scaring me away from any design effort! Somehow I don't think that designing a useful small boat has to be that hard, but I guess it does based on the comments I am receiving. In any event, I am going to proceed further by making the bottom buildable with plywood panels. One step at a time.
    Richard
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    At the risk of sounding "new-age", designing a boat is a "holistic" experience, a whole lot of things have to be weighed up and considered at the one time, like a juggler having multiple objects in the air at the one time. Mastering the art of multi-conic development won't mean you can design the boat properly. There was some guy called S.S Rabl who popularized a method of drawing up developable surfaces on boats, you could Google his name.
     
  9. DCockey
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    DCockey Senior Member

    Cardboard. Take a piece of thin cardboard and experiment with it. See what shapes are possible and what won't work. Next try building boat shaped objects by cutting up cardboard and bending the pieces together. Then find drawings for several boats with hull shapes similar to what you want and build models of them.
     
  10. DCockey
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    DCockey Senior Member

    Arbitrary developable surfaces are not easy to design on paper. Several folks have written about methods to do so. Mr E's suggestion of Sam Rabl is good. But building a cardboard model may be easier and quicker.

    Good computer software can considerably simplify designing developable surfaces but you need to spend time on the basics of boat design before jumping on to a computer.
     
  11. hambamble
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    hambamble Junior Member

    I appreciate that you don't want to use software, but the software was created for a reason. Drawing a lines plan by hand takes time. It is particularly difficult if you want to make the surfaces perfectly developable. You will then have to loft the hull full size before you start building if you want to make it properly. Computer programs mean that you can quickly alter each design, and the surfaces stay developable, and that you can print off a table of offsets or even full size frame templates and just start work, rather than having to loft the full size hull.

    I am a naval architect, I could be persuaded to hand draw the lines for a round bilge hull if someone had their heart set on it. I am not capable of drawing a hard chine developable hull by hand. I am sure with enough time, or enough practice I may be able to learn... but its definitely not something that is easy.

    I think if you really want to do it by hand, the easiest way of doing it may be to to build a frame at the keel, the gunwhale, and the chine. Fill in the gaps with plywood, which will take on a naturally developable shape. then frame it from the inside once the ply is in place. Not ideal, but it might work
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Key to this exercise ( building a boat for family outings ) are safety and comfort. No-one in their right mind would be prepared to sacrifice either, and particularly the former, for the "satisfaction" of creating and building something "original". Women and young children go right off boating very quickly if comfort is not adequate, so whatever is chosen has to be carefully matched to the area of intended usage, and picking the right times. And I firmly believe anyone who creates such a one-off as proposed in this thread should test it rigorously single-handed before asking anyone else to be part of the experiment. Safety first.
     
  13. HakimKlunker
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    HakimKlunker Andreas der Juengere

    It is very brave (and honest) of you to make this statement.
    Think: Would you have been a naval architect 50 years ago?
    I agree with DCockey for different reasons:
    'you need to spend time on the basics ... before jumping on to the computer'
    That does not mean that I doubt your overall qualification, of course.

    I believe that computers help a lot. But one cannot make a boat with a mouse click; it requires dirty hands...
     
  14. PAR
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    PAR Yacht Designer/Builder

    I learned how to design the old fashion way, by hand and I can tell you unequivocally, software makes these tasks so much easier. The conceptual phase and the drawing time, actually takes a little longer with a machine, but and it's a big but (like my exwife's), when it comes time to make adjustments, calculations, alterations, volume redistribution, etc., this is where machine generated drawings and hydro's, really stand well above hand drawn.

    Using an eraser shield and reworking a set of lines, can be extremely tedious and time consuming. This plus the math and regeneration of drawings, is where the machine really shines.

    Even with excellent mechanical drawing skills, you're still in need of the basics, in yacht design, assuming you want efficiency, an easy, comfortable motion underway and something that will tolerate the conditions you might encounter. There is (unfortunately) no short cut to this understanding, though you also don't need a full up, naval architecture degree either. Basic yacht design principles and some basic engineering skills, are what you need. Read both books I've shown. You'll get much of the "feel" for things with these. There are other books that will go into much more detail and offer a precision in understanding that can help, but after Geer's books, you'll know one way or the other if this is what you want to do.

    Doing developed surfaces by hand is a painful, tedious process, even with simple shapes. Just wait until you do a true weight and center of masses calculation study by hand. These things alone will make you regret, not using the computer or having to redo these very tedious calculations, for each adjustment you make in the lines. I recently finished a set of lines for a skiff like hull. I redrew the hull several times, before settling on it's current state. I need more volume forward, for the Cp target, I redistributed the volume fore and aft to "sweeten" up the lines a bit, for better flow, I moved the chines outboard a touch, I flattened out the buttocks aft for a better run, plus a number of other things. Each one of the changes where required for some reason and the lines redrawn and calculations realigned, to reflect these changes. 30 years ago, I would have had to redo them by hand, with the eraser shield and new pencil lines. I would also need to recalculated the new immersed volumes (Simpsons rule), even with a pocket calculator a tedious process by hand.

    Though a computer has taken much of the tedium from us, we still need to have the understanding of why we use certain engineering approaches and certain shapes. Think of it as trading in your T square and drawing table, for a computer screen and keyboard. The tools have changed, but the skill sets are still required.

    I suspect you can do it, but you do have a bit of study ahead, before real fruitful drawing can produce something viable. This is especially true of hand drawn stuff, because you don't want to have to redo it too many times, so the yacht design stuff is usually ironed down solid, before the pen hits the paper.
     

  15. hambamble
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    hambamble Junior Member

    Thanks HakimKlunker. I found being honest is the best approach.

    I totally agree that it's a lost art, and 50 years ago I would have been proficient in hand drawing developable surfaces or i would be out of a job. The fact is, today computers have taken over, and the time they save translates into $. No one wants a hand drawn lines plan anymore, they want 3D drawings that look pretty and anyone can understand. To be honest, I would be surprised if any recently trained (within say the last 10 to 15 years, probably longer) could draw it either. Even if they did, the time it saves drawing a computer model to do the stability analysis makes computers essential. days of the planimeter are gone.

    I too agree with DCockey, you need to know the basics. I always start by sketching a hull on paper, some rough estimates of displacement (usually based on other designs), rough L:B ratios, speed requirements, cargo capacity... the works really. Then you get into the details. I think computers are great, because if you need to change something, its easy. That being said, I have so much admiration for the draftsman who hand draw.

    Criticism of computers though is that software is expensive, requires training, can't always do what you want it to, and is easy to make a hull that is not completely fair (particularly for the inexperienced).

    Anyway, if you want to hand draw the lines, go for it! If you do your research right i am sure it will work out for you.

    On a side note, if people are recommending books, I think the best resource for designing a boat is "principals of Yacht design" by Larsson and Eliason. It is primarily for yachts, but much of the theory is there for powerboats, including propulsion, resistance, seakeeping, hull design, strength, stability... just ignore the sail and rig design. It can get a bit technical for the inexperienced, but all the steps in boat design are included chapter by chapter.
     
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