Enlarging an existing design

Discussion in 'Boat Design' started by simonawatts, Oct 16, 2009.

  1. simonawatts
    Joined: Oct 2009
    Posts: 9
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: San Francisco

    simonawatts Junior Member

    Some years ago I produced plans and a building manual for an eleven and a half foot Norwegian pram. Jacques, a Belgian woodworker, built one of these and liked it so much he wants to build a 5-meter (16 foot) version--about a 30% increase. I'm not sure how to advise him and thought there might be somebody out there with the necessary experience? Please keep your comments concise as I may forward them directly to Jacques.
    Simon Watts
  2. lewisboats
    Joined: Oct 2002
    Posts: 2,329
    Likes: 127, Points: 0, Legacy Rep: 1603
    Location: Iowa

    lewisboats Obsessed Member

    I'll leave this one to you Paul...
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 479, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A 30% enlargement is just too much to expect from the laws of mechanical similitude and comparison (relativity). It would be kind it everything scaled up nice and uniform, but it's not the case and things quickly get out of hand. It would be your best advise to select a design of the size (volume) he actually needs, possibly one that needs just a little "tweaking", say less then 10% in length difference from the original, certainly no more then 15%.


    In this thread it's discuss it in a little more detail.
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,811
    Likes: 769, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect


    do you mean straight forward scale up by 30%?...in naval architectural terms, a geosim?

    If you keep everything the same, then scaling up by 30% is easy, you just multiply all your linear dimensions by 1.3.

    BUT, your volume will increase by 1.3^3 = 2.2, this is a significantly larger boat, being more than twice the displacement. Unless of course that is what you want.

    Perhaps better narrow down what you really mean by 30%, or just geosim up (scale) a bit, then as PAR noted above, just tweak it slightly, ie increase length uniformly etc etc
  5. HJS
    Joined: Nov 2008
    Posts: 357
    Likes: 58, Points: 28, Legacy Rep: 288
    Location: 59 45 51 N 019 02 15 E

    HJS Member


    Try this excel spreadsheet

    Attached Files:

  6. gonzo
    Joined: Aug 2002
    Posts: 15,124
    Likes: 899, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If you designed the boat why is it so difficult to design another longer? Strech the station spacing by 30% , increase the freeboard a bit and re-fair the lines

  7. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 148, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    As Gonzo points out, if you designed one and it worked well, it shouldn't be too complex a job to design something similar a bit bigger.

    The Barkla scaling factors, applied to your original hull, should produce a shape with comparable performance in a somewhat larger (or smaller) size. Of course, the vessel's scantlings will have to be re-calculated for the greater loads encountered in a larger hull.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.