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Thread: Lofty ambitions

  1. #1
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    Default Lofty ambitions

    Stopping short of drawing the boat out full size, how does one determine the "coordinates" of the frame shapes etc from a scale drawing? The lofting charts(?) I've seen have an accuracy of about 3mm (eighths of an inch) give or take. SOHCAHTOA & Pythagoras don't come even close enough. Does it really matter? Are corrections usually made during construction? (I'm thinking small plywood on frames boat here not a heavily-timbered colossus.)

    Thanks
    Norm
    Once upon a time, they all lived happily ever after.

  2. #2
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    Default Re: Lofty ambitions

    You could draw it at a smaller size and then scale them up. You could scale directly from the drawings, but they are likely warped and out of scale due to the printing process. So use the table of offsets and loft it at 1/4 size or whatever. Then take your measurements and multiply them by 4.

    Or just loft it full size, it is not a very big job compared to building a boat and will give you much valuable information and insight about the hull.

    Not sure what you mean by "lofting charts." Are you referring to the table of offsets, or the lines plan? It sounds to me by your question that drawing it full size would be a very valuable experience.

  3. #3
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    Default Re: Lofty ambitions

    I find that to be hard to answer, there being so many variables in construction methods. Do you have a specific hull in mind? Got pictures?

    Lofting works full size and is often accurate enough, for me anyway, if lofted half size, but that's because I'm willing to make whatever small modifications are needed as I build, an approach which won't work for every construction method or every builder.

    It helps to have experience and an open mind.

  4. #4
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    Default Re: Lofty ambitions

    Thanks for that. Yes, "Table of Offsets" danged if I could remember the proper name. Cheers
    Once upon a time, they all lived happily ever after.

  5. #5
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    Nelson, New Zealand
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    Default Re: Lofty ambitions

    The table of offsets is compiled by the designer by measuring the scale drawing and either using a scale ruler or calculating the offset by multiplying the measured dimension by the scale factor. Even a small (say 15 foot boat) drawn on a drafting board three feet long will have small inaccuracies in it simply because of the limitations of measuring the width of a pencil line and reading the dimension from a ruler (either a scale ruler or a normal one).

    An accuracy to 1/8" is fine and likely to be "lost" when transferring the lofted lines to the material and in cutting/fairing the components.

    Naturally, if the boat is drawn or designed with software, the table of offsets is likely to be accurate to 1mm or about 1/25th of an inch, so all bets off for that!

    From "how does one determine the "coordinates" of the frame shapes etc", I take it that you are asking about how to construct the full size drawing from the table of offsets?
    Essentially you draw a grid representing the waterlines, sections and buttocks and plot the dimensions from the table of offsets onto the grid and "join the dots" with battens.

    Just about every book about boatbuilding describes how to loft the full size boat from the table of offsets, some better than others. Two I would recommend is Lofting by Allan Vaitses (from Anazon here: https://www.amazon.com/Lofting-Allan.../dp/0937822558) or The Gougeon Brothers on Boat Construction which is particularly good and detailed and is available free here:

    http://www.westsystem.com/wp-content...k-061205-1.pdf

  6. #6
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    Default Re: Lofty ambitions

    I think you may find this video by Lou of "tips from a shipwright" beneficial in gaining an understanding

    Building the TotalBoat Sport Dory: Episode 3 - Table of Offsets

    https://m.youtube.com/watch?v=9w9SUkO7fzo

  7. #7
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    Default Re: Lofty ambitions

    Dynamite Payson was a great proponent of building scale models to double-check the numbers, and to identify & iron out any construction snags. Instant Boatbuilding With Dynamite Payson (LINK)shows how to do it.

    Hope the voyage is a long one.
    May there be many a summer morning when,
    with what pleasure, what joy,
    you come into harbors seen for the first time...

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    (Keeley - Sherrard translation)

  8. #8
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    Default Re: Lofty ambitions

    Quote Originally Posted by telenorth View Post
    I think you may find this video by Lou of "tips from a shipwright" beneficial in gaining an understanding

    Building the TotalBoat Sport Dory: Episode 3 - Table of Offsets

    https://m.youtube.com/watch?v=9w9SUkO7fzo

    Excellent video (series of) just what I needed, thanks.
    If it makes my question any clearer, I have the "bones" of a boat plan at about tenth scale (small drawing board) and need make a table of offsets from it to get the show on the road/water. I think I almost have it sussed. Thanks
    Once upon a time, they all lived happily ever after.

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