Does scaling a hull effect more than scantlings

Discussion in 'Boat Design' started by Sheldon, Sep 18, 2011.

  1. Sheldon
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    Sheldon New Member

    Does scaling a full displacement hull 20% effect more than the scantling. I have found plans for a 28 foot trawler that would meet my needs perfectly if it was 34'.

    Thanks in advance.

    Sheldon
     
  2. PAR
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    PAR Yacht Designer/Builder

    It affects everything, stability, mass, scantlings, literally everything except the size of the humans that will crew it. If you can rescale the station molds to give you the length you desire, then 20% is about the max you'd want to do this. If you are also looking for the additional beam, then you'd be much better off finding a 34' design more suited.
     
  3. masalai
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    masalai masalai

    EVERYTHING... one way or another (larger and smaller)
     
  4. Sheldon
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    Sheldon New Member

    I was wanting to scale all dimensions. I guess what I really want to know is will the boat be unsafe if scantiling are increased and the boat still floats at the designed waterline (taking the new scale into account) through the use of ballast in the keel.

    I'll keep looking for a existing 34' plan (I want stich and glue).

    Sheldon
     
  5. PAR
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    PAR Yacht Designer/Builder

    Sheldon, this is a very common request of designers. "Can I have this boat, but bigger or smaller" and the answer is yes, but it'll be a new design, unless you can live with stretching the station mold spacing. It gets complicated, but simply put, things don't scale as would seem logical at first glance. For example if you take a 20' boat and double it proportionally, you've increased it's stability 16 times, it's mass 8 times and it's area is cubed. This is a far cry from the simple doubling asked for. This is why boats get thinner as they grow in length, have less proportional amounts of wetted surface or sail area and are generally lighter for their length.

    There aren't many taped seam (stitch and glue) builds and I know of none that use stitch and glue techniques in this size, nor 28' for that matter. This is why you need a 34' design and also why the price of yachts seems to jump exponentially with modest increases in length. An 18' boat is only 25% shorter then a 22' 6" boat, but the 22' 6" boat may weigh 2 or 3 times as much, not 25% more.
     
  6. Sheldon
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    Sheldon New Member

    Thanks for the insight and education. There are several 28' boats in this type of construction if anyone cares. This is the one I like http://www.bateau.com/studyplans/TW28_study.htm?prod=TW28 (Wanted to scale everything except the superstructure to make it a walkaround and provide more space for liveaboard)

    Thanks again.
     
  7. PAR
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    PAR Yacht Designer/Builder

    I'm familiar with this design and the designer's work. This isn't a typical "stitch and glue", but a taped seam build over a jig. Stitching is required on this type of build, where as on other builds, it is. Building on a jig means you can fasten directly to the jig, rather then fold the panels together with wire or wire ties as usually seen in a stitch and glue build.

    If interested in this design contact Jacques Mertens and see if he has or can draw up a 34' version.
     
  8. DCockey
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    DCockey Senior Member

    Sam Devlin has designed and built a number of larger stitch and glue boats, up to 50 ft or so in length. He also does custom design work. http://www.devlinboat.com/ Some of Devlin's designs use molds in addition to bulkheads to help shape the hull.

    As in many other aspects of boatbuilding, there is not complete agreement on whether a boat is "stitch and glue" if any molds are used which do not remain in the final structure.
     
  9. viking north
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    viking north VINLAND

    This has the lines of a modified Novi (Down Easter) Lobs sta (lobster) yacht, nice looking craft-you might want to contact TAD, a bit of a guru on this style--Geo.
     

  10. PAR
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    PAR Yacht Designer/Builder

    Stitch and glue is a technique to align panels. The build method is "taped seam" and this is where the confusion begins as most interchange the two, which isn't quite right. A taped seam build doesn't need to be stitched, but a stitch and glue project is a taped seam build. Personally, I've long ago given up on stitches and use duct tape, ratchet straps, brads, Spanish windlasses, what ever it takes, so long as I don't have to drill a thousand holes, along the mating edges of the panels.
     
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