Page 2 of 2 FirstFirst 12
Results 36 to 40 of 40

Thread: Penalties for scaling up plans?

  1. #36
    Join Date
    Dec 2015
    Location
    Dorset, UK
    Posts
    860

    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
    Join Date
    Jun 2013
    Posts
    2,329

    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.
    Has BigFella and SkyBlue on ignore.

  3. #38
    Join Date
    Dec 2015
    Location
    Dorset, UK
    Posts
    860

    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
    Join Date
    Jun 2013
    Posts
    2,329

    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.
    Has BigFella and SkyBlue on ignore.

  5. #40
    Join Date
    Dec 2015
    Location
    Dorset, UK
    Posts
    860

    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •