Scaling a boat

Discussion in 'Sailboats' started by turtle, Feb 15, 2015.

  1. turtle
    Joined: Feb 2015
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    Location: Montreal

    turtle New Member


    I've been looking into building a boat for a while and stumbled upon the aviateur 5.7,53.html

    Looks like a nice and spacious boat for the size, possibly quicker to build due to hard chines and lack of protruding cabin but size might be a bit small.
    If I wanted an extra ~3 ft (for a length of 22ft), I suppose I can:

    -"stretch" by proportionally lengthening the spacing between each frames/stations (and recheck scantlings). It would have final length at ~22 ft and width kept at 8.2 inches

    -Ratio the other dimensions per "Principles of yacht design", where beam and freeboard/depth would be ratioed by (length ratio) ^.7 . It would have the same length of 22 ft but a width ~ 9.2 ft.
    (bigger scantling changes but that's fine too)

    The second option gives me more volume, more form stability but is there a big difference in motion comfort?
    Are either options affecting planning ability or pointing ability?
    What else should I think about?

  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    There are many designs similar to that at 22 feet. It may be cheaper and easier to get one of those. You will also have the added advantage of technical support from the designer. Any major modifications will cause the designer to say you are on your own.
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    You will find very different answers to your questions. Here are some of my observations:
    - If you change the dimensions of the boat, everything changes and it's hard to tell how it will affect the seaworthiness, stability, etc. . You should study what happens, according to the dimensions you want to change. If the original ship was well designed, it is likely that these changes spoil the final result.
    - Changing the separation of frames and breadth you should check all scantlings and may change more things than you expected.
    - The first thing I would do is check what happens changing only the length. Not increase the separation of frames, but insert a cylindrical section in Lpp / 2 or so, 3' length.

  4. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    IME boats don't always scale well. A brilliant design scaled may well become an ordinary one. The less you change the better. We know for example that 12ft racing dinghies do not scale well to 14ft or vv, and your proposed change is approaching that magnitude.

    The other issue is that you probably don't want to scale all parts evenly. If you take the example of a racing dinghy that needs to carry 2 people the same size, the best result for speed if you want to go faster is probably to leave it unchanged after max beam and put allvthe length in a finer bow. Conversely if you need carrying capacity in a freight barge you might simply add an extra parallell section at max beam and leave bow and stern unchanged.

    That said it was not uncommon for trad boatbuilders to space frames a bit farther or closer, but such workboats had v different priorities anyway and frankly were rarely very special.

    Finding a good design the size you actually want is the safer option.
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