stretching dingy plans?

Discussion in 'Boat Design' started by minno, Dec 11, 2015.

  1. minno
    Joined: Aug 2014
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    Location: Canada

    minno Junior Member

    Hi All

    I'm looking at stretching the hull of a set of 3 panel stitch and glue dingy plans by 40%-60% LOA, freeboard by 3", and leaving the beam the same.

    the idea is to multiply all length measurements by 1.4-1.6, the width measurements of the bottom panel by 0.95 and add three inches to the width measurements of the side panels.

    not sure if it matters but row/sail only, it'll never have a motor on it.

    I'm only interested in an idea of how it will change hull performance, or info on how to calculate it myself, even a complete list of the acronyms used would do, I can google the equations.

    Thanks

    minno
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The short answer is this doesn't work very well. It will be sorta boat shaped, but not the shape the designer wanted or what you are expecting. There is no way to guess performance, other than it will probably not be good, and you have to completely redesign the entire boat. Find a set of plans within an inch or two of what you want. There are literally thousands to choose from. You can globally scale a stitch and glue , but you can't stretch one by stretching the panels.

    Some options for stitch and glue are to preserve the entire shape of the designed boat and extend/extrapolate the transom back an inch or two. You can pretty much do as you please with the shear. And you can scale the entire hull, including ply thickness, by a fixed percentage.

    If you switch to plank on frame, then the panels are fitted as you go and you have more freedom as a builder as far as stretching things. But even here, stretches are usually kept to under 10%. There's simply no need to ever do more.
     
  3. minno
    Joined: Aug 2014
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    minno Junior Member

    bought what I thought, doesn't keep one from hoping though :)

    would adding a few inches to the freeboard mess with the shape of the boat too?

    minno
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You can raise the freeboard a good bit and though the weight will decrease capasity and increase windage, you'll be fine. You can stretch the design as much as 15% by re-spacing the station molds or frames at the larger dimension, but once you go past this, the hull form needs to be looked for suitability in the longer length. So if you have station molds on 24" centers (as an example), you can re-space them to 27 5/8" and get a 15% stretch, but this is the practical limit.
     
  5. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    But if you stretch a sheet plywood design as suggested by PAR the shapes of the panels which fit the stretched shape will not be simple stretched versions of the panels which fit the original shape.
     

  6. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    That's correct, but a stretch will not have as adverse an effect on the outcome as a proportional enlargement. It's very rare for me to recommend a proportional enlargement, but a 15% or less re-spacing of station molds can be acceptable on most designs.
     
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