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Thread: Penalties for scaling up plans?

  1. #36
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    Default Re: Penalties for scaling up plans?

    He asked not what's the difference between a short boat and a long boat, he asked what happens 'when I scale this boat'.

    Chris paddle a 10ft LWL canoe and you will suffer with wave making resistance. Paddle a 20ft LWL canoe and you will suffer with wetted area. Paddle a 16-17ft LWL canoe and you will be at optimum balance: not having excess of either.

    Your arguments that extending a 10ft boat hold true, but only until reaching 16-17ft waterline. After this your over the singlehanded peak without introducing increased power output, from crew, beam or ballast: you enter the ballasted dayboat category.

    The boat, an unballasted singlehanded planing dinghy was already at 17 LWL, so will be made worse. Scaling is relative: it depends when your moving from as to what you'll end up with: Scamp was scaled in every dimension from the initial design to 12ft and is a great success.

  2. #37
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    Default Re: Penalties for scaling up plans?

    And I stand by the gist of my answer, which comes from talking to current designers, reading designers like Ben Lexcen, Uffa Fox and looking at the proportions of their boats, and experience in sailing (for example) an 11 footer designed along the lines of a 20 footer. Adding length normally makes a boat more stable even if the beam stays the same, and it normally reduces form drag, DLR and hull speed which both have beneficial, not negative, effects on speed. As Ian Colgin also noted "In general, simply stretching a boat has the effect of improving form stability."

    Given that the original Gartside could be seen as an example of the way extra length increases stability (as demonstrated by the fact that it carries far more sail than a14ft singlehanded dinghy) there seems to be no reason to believe that it is on the edge where adding length reduces speed in such a boat, if indeed that ever occurs in a dinghy.

  3. #38
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    Default Re: Penalties for scaling up plans?

    At low speed the principle resistance is wetted area not as you assume “form resistance, dlr, stability, or hull speed”.

  4. #39
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    Default Re: Penalties for scaling up plans?

    I know that WSA is the principle resistance at very low speed. It's not the principle resistance overall because it's not the main factor at medium to high speeds and when at the planing threshold. It's interesting to discuss this with guys like Phil Morrison (designer of world champion boats and some of the most successful production dinghies, including the Laser Stratos that I believe you enjoyed so much). When we were discussing the importance of length on speed I mentioned that there's one class that often (but not always) uses the hull from a class 9" shorter. Phil's response was simply that if a boat was competitive against similar hulls 9" longer, then it couldn't be a very competitive class. He didn't raise the WSA issue or anything - his response was swift and made it VERY obvious that he strongly believed that length was absolutely critical when it came to speed.

    So I'm not claiming that I'm an expert - I'm just repeating what the true experts like Julian Bethwaite, Phil Bieker, Phil Morrison and others have told me or what guys like Lexcen and Fox have written. Yes, WSA is important, but only at very low speeds. Most of the time, longer = better.
    Last edited by Chris249; 09-14-2018 at 05:10 AM.

  5. #40
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Edward Pearson View Post
    If you started by making it longer (Mk1) it will have more wetted area drag at low speed and it will be heavier.
    In my first reply the effect of increased wetted area was qualified.

  6. #41
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    Default Re: Penalties for scaling up plans?

    Barkla, Allometric Scaling of the Sailing Yacht, Proceedings of the 16th annual AIAA/SNAME Symposium on the Aero/Hydronautics of Sailing, Vol 33, 1986, pgs 96-105

    My summary:
    1. take your hull length increase factor and apply that to all rudder and rig linear dimensions.
    2. Apply the factor^0.8 to beam, draft, freeboard, and keel span, chord, and thickness.
    Note on keel: Seems like the paper focused on fin keels.

    The rest is derived. Scantlings aren't addressed, just sail and hull performance like wetted area, stability, prismatic coefficient, etc.

    Example:
    For a 10% increase in length, it's an 8% increase in height and beam (1.1^0.8). Your former 30' LOA, 10' beam cutter is now 33' LOA with 10' 10" beam. Had you *simply* scaled 10%, that beam would be a whole 2" wider!

  7. #42
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    Default Re: Penalties for scaling up plans?

    Scaling up a boat design is often a matter of using Kentucky Windage as a measurment factor.
    Jay

  8. #43
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    Default Re: Penalties for scaling up plans?

    I'm (slowly) building a wee rob, by Ian Oughtred, and the plans show three versions from 11' 7" to 13' 3" achieved by increasing the spacing between the molds. No corresponding changes to beam, freeboard, etc. That is an increase of about 16%. Maybe he designed it for scaling.

  9. #44
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Jay Greer View Post
    Scaling up a boat design is often a matter of using Kentucky Windage as a measurment factor.
    Jay
    Kentucky windage raised to the fathom per smoot sounds about right.

  10. #45
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by David W Pratt View Post
    I'm (slowly) building a wee rob, by Ian Oughtred, and the plans show three versions from 11' 7" to 13' 3" achieved by increasing the spacing between the molds. No corresponding changes to beam, freeboard, etc. That is an increase of about 16%. Maybe he designed it for scaling.
    I think that canoes and kayaks often scale pretty well. Part of the reason is the incredible variation in the weight of the users really demands that boats be made that fit by size. However it rarely happens. So even though the effect of merely increasing length does not do much to adjust for the possible weight range of paddlers, nor does it retain fundamental characteristic of the original design, it can end up making a better boat. One of the reasons one can get away with it is because the human factor is so enormous, and disconnected from design. Normally designers will balance out the factors in a design, but when a 200 pound person tries to paddle a boat that has been optimized for someone of 110 pounds, the only salvation is in the paddling. Lengthening the boat may not yield the perfect result, but it is probably somewhat better. I know of only one designer who made his solo canoes, probably targeted to men, in three sizes.

    In general though, if a person has the capacity to make sense of the changes wrought by a 10% increase in length, why can't they just optimize the design more generally?

  11. #46
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by mmd View Post
    Yeah, again. This has been a recurring topic since I started participating in the Forum in 1999/2000, usually with the same results (and often with the same participants) each time. Essentially, the consensus is that, for wee small boats (under 10 - 12 feet) an across-the-board increase of 10% - 15% ("Xerox enlargement") doesn't cause many problems. For small boats (12 - 20 feet) the mob thinks that 10% - 15% increase in length only (setting up molds farther apart than in original plans) is OK. Beyond 20 feet, you start getting into scantlings and stability issues.

    There is always a vocal contingent who don't wish to be restricted in their freedom to mess with plans and 'experiment', and as long as they don't risk too much money on a failed experiment or risk the innocent lives of their friends & family, I'm mostly OK with that.

    My issue has always been with the "but quite a few designers have routinely stretched or enlarged some of their designs" (apologies to skaraborgcraft) viewpoint, which is trotted out to justify the practice among the unwashed masses. The difference is that the professional designers who do this know full well what the implications of the changes are, and will adjust the design and/or scantlings to compensate for the change in physical size of the boat. Amateur designers can do this too, provided that they have educated themselves, done their due diligence, and understand the pitfalls. But many do not, and the fact that it is entirely too easy to die out on the water causes me to caution those contemplating the process.
    Do bear in mind people, that the designer of the boat that you're intending to stretch or scale up, may have effectively done just that, and have it out near the limits of the various elements of the design.
    Do ask before you commit the materials and labour to the project.

    John Welsford
    An expert is but a beginner with experience.

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