Learning Lofting am I ok with this?

Discussion in 'Metal Boat Building' started by Tesno8, Apr 8, 2008.

  1. Tesno8
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    Tesno8 Junior Member

    Hi guys, am I on the right track here? I drew this at frame 20 based on my table of frames and offset chart. If so I will continue as I am doing for every frame. I am brushing up on my cad skills an thought this would be a neat project.

    Thanks.
     

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  2. Petros
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    Petros Senior Member

    That looks reasonable, but are not these usually dimensioned from the water line level, or perhaps a strong back reference plane? This would only be useful if the hull has no rocker (unless you are just making profiles of the individual frames).

    I would think if you are making templates or profiles, then you would want to dimension between points that area easy to measure from so they can be shop cut with minimal risk of error. In that case always consider how a builder would lay out the material to make the part, and dimension it clearly in a way that would be the most natural or intuitive to lay out the pattern.

    good luck
     
  3. PAR
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    PAR Yacht Designer/Builder

    Another thing to consider, which hasn't been mentioned is how the offsets are presented. In other words, do they represent the exterior dimensions or to the inside of the plate. If to the exterior, you must deduct the plate thickness from the frame dimensions. This is an important consideration, because you can't just subtract the plate thickness from the offset dimension, it has to be "scaled" along the different angles the chines make.
     
  4. yipster
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    yipster designer

    i'd say go metric, learn it in 5 minutes and your live and lofting will be so much easy'r
     
  5. Tesno8
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    Tesno8 Junior Member


    I am glad you noticed that because I am not really sure what "plane" I should be using to offset my height from when developing my sections, though the plan does use the base line in the elevation for all the fwd and aft end point dimensions of the lines.

    Also the plan does not show any hull shape bellow the baseline so I am assuming that I just need to project these or the hull stops at the baseline, but I am not sure??

    Thanks
     
  6. Tesno8
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    Tesno8 Junior Member

    Not sure if I understand the "scaled" idea, I was thinking as you said that just offsetting the plate thickness would work no?; but then again I have never built a boat. I work with steel and fabrication but not hull lines; and now I am doing cad work, though I have always loved boats and this is just for fun but I wish to learn the correct way since its valuable too know.

    Thanks for the tips!
     
  7. Tesno8
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    Tesno8 Junior Member

    :eek:

    LOL, I am going to convert it to all feet inches and decimal form. I grew up on the imperial units of measure so for me and my area and even every job I have ever had, units are always this form.

    But I do understand your point!
     
  8. PAR
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    PAR Yacht Designer/Builder

    The base line should be indicated on the lines drawing. Many times the LWL is the base line, which means the offsets will have a + or - sign to denote if it's above or below the LWL. It's very rare to have the bottom of the boat, dead flat, so that it could serve as a base line. Typically, there'll be some rocker, which means only small portions of bottom will contact a base line. Check your drawings and insure you're using the correct base line.

    1/4" plate (shown for example) set an angle to the previous plate isn't 1/4" at the junction.

    This is the "scaling" I mentioned.
     

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  9. Tesno8
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    Tesno8 Junior Member

    PAR, its marked baseline and I see no + or - signs in the lofting tables. Based on this picture would you say all my lofting lines will be from this baseline? Any other thoughts based on this picture?

    I get your point now about scaling, and yes I would have done it just as you did, but I wanted to be sure! Thanks for explaining that.

    Thanks!
     

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  10. PAR
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    PAR Yacht Designer/Builder

    All of your "heights" would be from the base line. What is shown in the latest drawing, suggests (my assumption) that some lines go below the base line. Is this true? If it is, the designer may have just figured you would go below the base line when necessary, without notation. In these cases you just measure down, instead of up.
     
  11. lazeyjack

    lazeyjack Guest

    the point is that in metric you have one measure, 1504 mm, instead of 5 foot and one eigth or whatever, one mm, is close to loft and build, one sixteenth is a mile away, as in 1/8th, so that means a confusing tape with thirty seconds on it
    If you really want to learn to loft, try find a book boatbuilding by american Howard Chappelle, it has an excellent chapter on lofting, down to developing curved and raked transomes
    By the way UK, AU, NZ we were all imperial, but metric is used almost exclusively now
    I saw copies for sale on ebay
     
  12. Wynand N
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    Wynand N Retired Steelboatbuilder

    No need to learn metric. Millimeter (mm) and meters are most commonly used, and tape measures are calibrated (marked) in these units.
    What is easier than this; lets say 1506.62mm +673mm + 34.4mm - 1762.8mm ? Do this with the Imperial system with inches and fractions.
    BTW, 1000mm = 1 meter.

    It mystifies my mind how the Americans could have put a man on the moon with the Imperial system:?:
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    If I recall it right they paid the bill, but the math was DIN...
     
  14. Tesno8
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    Tesno8 Junior Member


    Sounds reasonable to me, thanks for your help. My co-worker and I are writing a program that automatically reads in a lofting chart and plots all the points for us in 3D. We also wrote a program that will do this in any units, for all the metric users too.

    ;)
     

  15. Tesno8
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    Tesno8 Junior Member

    since 1000mm = 1M then two of those values that you posted above may have needed to be converted from Meter to Millimeter in order to add them. Thats not any different then using decimal inch to add or subtract:

    96.25" + 24" + 240.75 - 12.375"

    So I just did it with the Imperial System.

    Just like you would not mix Meters with mm when calculating values we would not mix fractions, not all that different really, because all fractions convert to decimals.
     
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