Old people can answer this question! : )

Discussion in 'Boat Design' started by netjaws, Mar 13, 2009.

  1. netjaws
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    netjaws Junior Member

    I'm trying to make a geometry file from a table of offsets that looks like it came from the 60's or 70's.

    Each entry is, for example, 4-5-4 - which I take to mean 4'-5 4/8" -because there aren't any occurrences of "8" or anything higher in the third column.

    But in some places, it adds a + or - after it, like "3-1-6+" or "6-11-6-" ... what does this represent? I'm guessing, plus or minus a sixteenth of an inch. Please help.

    And all, have a good weekend!
     
  2. alan white
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    alan white Senior Member

    Plus or minus a sixteenth.
     
  3. duluthboats
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    duluthboats Senior Dreamer

    Alan is right and I'm not that old.
     
  4. PAR
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    PAR Yacht Designer/Builder

    The + or - sign typically means a 1/16", but more often then not it means a "smidge" more or less depending on which it is.

    Offsets produced like this are usually done by hand, so take them with a grain of salt or possibly the whole salt shaker, depending on the designer.

    In other words, these numbers will get you close, but actually fairing is done on the loft floor not on the drawings, plus it's not uncommon to find errors in the offset table.

    This isn't to say the drawings aren't fair, they probably are, but only in the scale they were drawn at. When you take a very sharp pencil line and blow it up to the size the boat will be built in (the drawing scale), it's not a very sharp line any more, but a big fat smudge of a line. This causes subtle errors and unfairness to creep into the drawings. This is one of the reasons you loft a set of lines full size. You'll catch these errors and straighten any unfair lines you sight down.

    Oh please, Whoosh. A mm? No one is building boats to those tolerances. Hell if you can make one side of a 40'er within a 1/4" of the other you're far better then most. A 1/16" is more then accurate enough, likely more then necessary and certainly more then you can see or feel in it's performance, hence the commonly accepted 1/8" limit on offsets.
     
  5. rasorinc
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    rasorinc Senior Member

    Plus or Minus +/- may have been an acceptable error ratio in those dated offsets.
     
  6. alan white
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    alan white Senior Member

    I'd imagine the designer who wrote down those particular offsets meant "up to a sixteenth" (otherwise, only plusses would be needed, and not minuses).
    Probably, you'ld get closer going one third of an eighth by eye.
    Lofting, you'ld be close enough. I don't think absolute accuracy is needed if the boat's lofted. The boat will be true to itself if ever so slightly different than its sisterships.
    So PAR's right, a smidge it is. A sixteenth is close, but a third of an eighth is closer, though I think it is called a scoche or a dight or a smidge. Finer gradations would be named after various hairs...
     
  7. netjaws
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    netjaws Junior Member

    whoosh,

    You can convert my offsets to mm when you pry them from my cold, dead hands

    thanks all for your help, I'll go on the assumption that a "plus" or "minus" equals a +/- one-sixteenth of an inch.
     
  8. PAR
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    PAR Yacht Designer/Builder

    My whole point is, regardless of what the table off offsets says, the lines need to be lofted.

    Pulling station molds from unchecked offsets will result in an unfair boat. The only ways to check the offsets are to loft or plug the offsets into a bit of software and generate lines from them.

    I can pretty much guarantee the resulting hull will not be perfectly fair, if produced exactly as the offsets recommend.

    I've lofted quite a number of boats now and none didn't require at least some adjustment on the loft floor.
     
  9. tom kane
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    tom kane Senior Member

    Maths is never an exact science.
     
  10. Boston

    Boston Previous Member

    just noticed this thread
    my grand dad used a similar number system
    you are right about it being in feet inches and eighths
    with the plus or minus being called out as strong or light depending
    Rasornic got it down
    my grand dad was insane about accuracy
    if he was in the boat house
    it was his way or the highway and that included my dad
    who was bigger than he but not quite as sharp with his tongue
    although you didnt always get the benefit of a warning
    if old Roby called something out strong and it didnt come to him with that extra sixteenth on it
    you might consider ducking depending on the day and who he was pissed off at
     
  11. mydauphin
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    mydauphin Senior Member

    I had a production boat, that I went to have a hard top made for. I measured one side, and width and of I went to builder. When I brought top back there was 2 inch difference from one side to another. I blame top guy and took top back. He showed me it was the boat. Since then I have measured several boats and found even larger errors. I found a 4 inches on a 70 footer.
     
  12. PAR
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    PAR Yacht Designer/Builder

    I've been part of hundreds of builds in some form or another. I've never seen tolerances anything close to the 1/8" typically found on offset tables.

    I once bragged about a 27' fishing boat I'd just built. The sheer was within a 1/4" of it's opposite side, when measured with a tape. I considered this as close as you could expect and am still quite proud of the feat.
     
  13. Landlubber
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    Landlubber Senior Member

    The worst one I saw had a 4" height difference in the topsides, nicely finished, amateur built, he was very proud of his boat, he only found out when doing the rigging, the caps were diffferent.....in the water a blind man could see it.

    But you are right PAR, we never build symmetrical....human error or whatever, they certainly are not true, we all try, but.......

    I am CAD CAM manufacturing alloy plate boats in China at present, they are pretty good, but 1/8".....sorry, no way. In theory of course....but so is the world perfect, in theory....even Communism is......in theory.
     
  14. PAR
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    PAR Yacht Designer/Builder

    When you add up all the pencil thicknesses that get used during a build, an 1/8" is a miracle.
     

  15. mydauphin
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    mydauphin Senior Member

    How about symmetrical underwater? Most boats have more dohickies on one side than another, even weight may not be distributed properly. Surely this has more effect against water resistances than a 1/8 of an inch. To me Boats because of the general weight and slow speed have a larger margin of error than a Aeroplane. But the forces are also much greater. Case in point rudders. A plane rudder is a simple, light thing on a boat the forces are greater. Of course that is comparing comperable size vessels.
     
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