Scaling down America Junior plans?

Discussion in 'Boat Design' started by tkk, Jul 29, 2009.

  1. tkk
    Joined: Jul 2009
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    tkk Junior Member

    I am new to the forum although I have been reading posts occasionally. For a long time I have playing with the thought of building a boat and now I contemplate scaling down aplan by William Atkin: America Junior.

    As designed it is a little too much boat for me, but it looks so good and I have had pictures of it around for more than 20 years! It is hard to think of something else, although Florence Oakland of John Atkin would be more or less the size I am looking for.

    Bolger´s Light Scooner is too small, although longer. I would like to have minimum camping accomodation under a solid top.

    What is the opinion of forumites. is scaling down to 86% of the original pushing the envelope too much?

    I know that both of the Atkins will be turning in their graves because they were very particular about not changing anything in the plans.
     
  2. uncleralph
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    uncleralph Junior Member

    What about the Florence Oakland don't you like. It looks like a scaled down America Junior. It is about the size of an 86% America Junior, is a schooner, has minimum camping accommodation - it seems to fit what you described. I am just curious what you like better about a scaled down America Junior. Personally, I would rather not scale down a plan unless I had to. Both nice looking boats by the way.

    Ralph
     
    Last edited: Jul 29, 2009
  3. tkk
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    tkk Junior Member

    I have purchased plans for both and they are nice.

    America Jr has been my dream for 20+ years and I think it is a better looking boat with nice lines. F.O. is a little chubby to my liking.

    I also don't like the external rudder, the overhang in A Jr looks soo good:)
     
  4. uncleralph
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    uncleralph Junior Member

    Because you have both plans, it might be reasonable to scale down the lines of America Junior, but us the scantlings from Florence Oakland. There is the difference in construction, however. The AJ is carvel planked and FO is ply on frame. What method are you looking to use?

    Ralph
     
  5. tkk
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    tkk Junior Member

    I am going to use traditional ply on frame or maybe slightly modify it using glass tape and epoxy on sheer, chine and garboard joints on the outside.

    I think the scantlings of Florence Oakland might be a slight overkill using modern adhesives and modern finnish plywood.

    But what I am first concerned of is the hull form and the scaling of it, whether is it going to be feasible to scale down that much.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Considering the basic nature of your questions and the lack of general concept and dynamics understanding you've shown in this thread, you aren't remotely capable of making these decisions, let alone the scantling conversion or reworking of either boats lines to suit your needs.

    This isn't a dig or intended to disrespect, but you need to comprehend considerably more before attempting such. Down sizing in particular is quite difficult and not advised except by the quite skilled and experienced.

    Select a design or have a stock design modified or opt for a full up custom. Your assumsions about scantlings and dimensions are ill advised and frankly incorrect, reconsider your options for safety sake.
     
  7. tkk
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    tkk Junior Member

    PAR: I am curious to hear what - apart from the fact that I am asking advise - gives you indication about my understanding of general concept.

    What are my assumptions about dimensions and scantlings that are so wrong?

    I think scaling up or down 10% is done quite frequently and here we are talking about 14%, only slightly more.

    I think I can handle the calculations to get the strength right if I start from something existing. After all, a boat as astructure is not that different from a house or a bridge.

    It is the hydrodynamic and aerodynamic part that I feel uncertain about. Having made model airplanes I remember that the viscosity of the substance doesn´t scale down and it forces us to alter from straight scale-downs.But do we get that kind of phenomena with such small alterations.

    Another thing that doesn´t change, of course is people. The weight of the crew relative to the total mass will get bigger, but that is probably handled by the height of the seats.
     
  8. PAR
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    PAR Yacht Designer/Builder

    This is about the most common request a designer receives. "Can I have this, but just a little smaller/larger".

    The laws of similitude and relativity come to piss you off. Most folks just don't grasp the reality of these little ******** and trust me, they're easily little ******** to deal with.

    15% enlargements have been done with success, reductions not so much, but all with a lot of luck or considerable skill on the part of the person re-drawing the design.

    In a nut shell, lets say you want to double the size of a boat, making a 10' skiff as 20' power cruiser. Yes, this is extreme, but it keeps the math simple. It's surface area increases by 4, its volume by 8 and its stability by 16. This also happens in reverse, so a 20' powerboat, reduced to 10' has 16 times reduction in stability.

    This rule applies to forces, strains and working loads too. Then relativity comes up and bites you in the butt, which is simple put as larger boats having less difficulty with large rigs, then smaller boats, which tend to be weak kneed in comparison.

    These rules also partly explain why boat costs seem to rise dramatically, not proportional to length. Whereas a boat twice as long as it's brother can cost 10 times as much.

    There are techniques and methods to work within these physical laws, but generally you end up with an all new design, not a down or up sized version of something else. An all new design, based on or "in the spirit of" requires serious design skills.

    I hope this makes sense, as it's pretty difficult to lightly describe the laws and their affects.
     
  9. wardd
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    wardd Senior Member

    i worked in boeing r&d in the 70's and boeing was interested in producing the german helicoptor bo105 i think it was, anyway boeing wanted to use inch material, the conversion got so hairy that they dropped the project
     
  10. tkk
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    tkk Junior Member

    PAR, I hope you will not get pissed off with me :D , but I want to understand this. I am the kind of stubborn donkey who doesn´t want to take a straight advise like don´t do it. I want to understand WHY it is a bad idea, that is the only way to learn something new.

    I think that what you posted confirms what I thought before. Statical things like sizes, areas and mass scale up and down together with forces and strength, but stability, sailing properties and other dynamic stuff pose the main problem.

    In a way that is good news: instead of breaking into pieces I risk "only" capsizing my boat :D
     
  11. PAR
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    PAR Yacht Designer/Builder

    I believe I've offered considerably more then "don't do it" and you can elect to accept this or one of a few options, all but one may require more research on your part. A treatise on the few necessarily related subjects, wouldn't be something I'm particularly interested in this arena.
     
  12. timothy22
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    timothy22 Junior Member

    PAR touched on the easiest part of scaling for me to understand,

    and that is that almost everything one wants from a boat seems to get a little better when the boat gets a little bigger, and everything seems to get worse when the boat gets a little smaller. One reason Florence is a little chubbier than America Jr. is that it needs to be in order to restore the stability that is lost (relative to the larger boat.) Beyond adding a couple inches to the station spacing, scaling is best left to those who know the adjustments to make in order to end up with a satisfactory boat. Not to say you can't have a little fun trying things out. I enclose a scaled comparison of the two hulls, and a composite picture of America Jr with Florence's easier run. I like easy runs, even if I have to give up the inboard motor to get one. An expert with the soul of an artist might design a perfectly wonderful little boat with pictures like these as a guide, and provide you with detailed construction drawings as well. If you decide to do all these things yourself, you will later be amazed at the time you will save by having these things done by folks who do not have to reinvent the wheel at every step and who do them all right the first time. There is also considerable resale value in the name of a designer associated with a boat, even a small one.
     

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  13. PAR
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    PAR Yacht Designer/Builder

    This is one of mine, considerably smaller, but a schooner of good proportions and traditional style.
     

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  14. tkk
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    tkk Junior Member

    PAR, that is one damn pretty schooner!
    You have confused me totally with that picture, I don´t know anymore what I want :D

    Timothy, reinventing the wheel is part of the fun for me. Like I said before, I want to understand why things work or don´t work. That is the way I have built elecrtonics, tandem bicycles, log-houses, a lot of furniture and reengined a couple of cars.

    I have the patience to learn enough to either implement what I want or to be convinced that it is beyond my reach.

    Spending a lot of my time is not a problem, neither is the resell value. If an investment was of interest I would be looking at stock exchange, not a wooden boat :)
     

  15. timothy22
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    timothy22 Junior Member

    I sympathize completely with your sense of fun/curiosity having been bitten by the same bug myself. I actually built one of my designs, used it for 4 years and sold it for more than it cost me to build. So, of course, I quit while I was ahead! Skene's "Elements of Yacht Design" and Chappelle"s "Yacht Designing and Planning" are both available from Amazon for much less than I paid for them years ago. Skene is perhaps more to the point in answering your questions, but Chappelle is more old school and less math oriented in his approach. And there is a BEEyootyful schooner inside. There is also Dave Gerr's "The Nature of Boats"
    A few links
    http://www.amazon.com/Yacht-Designing-Planning-Yachtsmen-Students/dp/0393037568

    http://www.amazon.com/Nature-Boats-Insights-Esoterica-Nautically/dp/007024233X

    If you get really hooked you will need something handier than Simpson's rule to estmate the areas of irregular shapes.
    http://www.boatdesign.net/gallery/showphoto.php/photo/1124/size/big
    (I used to use the old kind made of bent welding rod, but this is easier to use. If you let the blade trace on waxed paper it will be easier to read.)

    http://www.planimetervault.com/hatchet/hatchet.html

    I hope you really dive into it and come to realize why so many people get into yacht design even though it is not generally a high dollar career. (and why it looks SO much easier than it is.)
     
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