Theoretical hull lines and plate thickness

Discussion in 'Boat Design' started by ldigas, Aug 18, 2011.

  1. ldigas
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    ldigas Senior Member

    In your area of the world (please state roughly if you can) what is the convention used - does plating thickness come on the outer site, or on the inner side of sections in hull lines?
     
  2. tunnels

    tunnels Previous Member

    What kind of a question is that !!What do you mean ??
    DOSENT MAKE ANY SENSE :confused:
     
  3. BATAAN
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    BATAAN Senior Member

    In the US wooden ship building the lines are to inside of plank. This is all the builder cares about and in the past, lines were for the frames and not the outside of planking.
    I don't know a great deal about today's metal construction, but the builder wants to know the outside of the frame, while the designer is concerned with the total displacement including plank or plate.
     
  4. tunnels

    tunnels Previous Member

    WE LIVE AND LEARN SOMETHING NEW EVERYDAY !!:p
    Now i understand sort of what the question was about .
    Thank you !!:D

    All measurements are to the outside finished surfaces as thats what the drawing lines are all about ! or we would not be using autocad and the likes to get fine detail and critical measurments for performance craft .
    I spent ages doing all my drawings by hand ,before the days of computers and it was always the outside surface that was the critical one . :)
     
  5. Alik
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    Alik Senior Member

    For steel and aluminium lines plan corresponds to inner surface of plating; for wood and composite - outside surface.
     
  6. ldigas
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    ldigas Senior Member

    @Alik - yes, that was my understanding too until recently, when I received lines and construction drawings for a steel yacht, on which plating went "inside" the theoretical form.

    Which prompted me to ask this question.

    Personally, I see the advantages of both approaches, but still wanted to see what is the accepted convention. I see already in your and @BATAAN's approach, that the
    general agreement is still to be reached.
     
  7. BATAAN
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    BATAAN Senior Member

    Builder cares about the frame and not the plate, because what he has to get right is the frame and the plate follows that as correct.
    Builder is not concerned with performance computations, that's the designer's job.
    Designer cares about the final outside dimension because he has to figure all the performance variables from total displacement.
    A good designer in steel or wood gives the builder a framing plan that does not include the plate or planks. This way the builder does not have to compensate for plate thickness at odd angles and make possible mistakes.
     
  8. tunnels

    tunnels Previous Member

    HAVING BUILT A FEW RACING YACHTS AND LOTS POWERBOATS OVER MY LIFE TIME we have always worked the skin outside as the final measurments . In tha case of performance yachts even the glass skin and paint is taken into consideration !!:(
     
  9. BATAAN
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    BATAAN Senior Member

    The usual shipyard situation is "Here, build this design." A price bid is requested. The builder figures his costs and bids the job. He cares little about the final shape and size, beyond the frame he has to build accurately, then clad with a skin.
    What use is anything outside the frame to the builder here? So he can second-guess the designer?
    He is doing no calculation, as it has all been done, and all he requires is the plate specs to clad his frame.
    A designer giving the builder hull lines to outside of plate is just making the build more expensive as the builder has to deduct plate thickness in the lofting, a potential source for error, unless he is furnished patterns for frames, and why does he need the lines to outside then?
    If the yard/builder is designing the boat, it's a different thing entirely, and total displacement must be used in calculations.
    If the designer doesn't know how to keep costs down by these sensible and ancient methods, get another.
    For the history of some of this, read Chapelle about half models, or "Design and Construction of Small Craft" by R. Munro Smith, A.M.I.N.A., from the Technical Section, Association of Engineering and Shipbuilding Draughtsmen, London, 1924. This gives great insight into steel vessel lofting, bidding and construction.
     
  10. BATAAN
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    BATAAN Senior Member

    Why did you do this if you were building someone else's design, unless they just didn't furnish you the deducted lines as they should?
    The lofting process has plenty of tricks to deduct plank/plate/glass skin/paint thickness, but these take time, are subject to error and not necessary when the designer does it for you, and more accurately.
     
  11. philSweet
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    philSweet Senior Member

    Well, if you plan to pull the hull out of a female mold, its nice to know the outside offsets. If you peel it off a plug or bend it around frames or stringers, its nice to know the inner offsets. So its rather more a local issue than continental.
     
  12. BATAAN
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    BATAAN Senior Member

    Seems to depend on what you are building and what method you plan to use.
    The old wooden ship designer's half-models (as opposed to display half models) are to inside of plank to keep things simple, accurate and cheap.
    To design any carefully-engineered more modern vessel obviously the total finished displacement and lines are what are used in development.
    The builder, however, isn't developing the design and needs to know what the frame or mold looks like exactly, and if the designer can furnish this data, the build time is less. Mold is the hull lines to outside of gelcoat. Frame is a complicated structure inside of any skin, no matter what material, so is quite different from hull lines.
    If the builder must deduct skin thickness on the loft floor it is not absolutely accurate (see Vaitses LOFTING), while the designer can use his computer power to supply appropriate templates, patterns or cutting guides in making this correction with great accuracy to ensure the builder follows the designer's intent as closely as possible.
    All of the modern builds I have been around lately have been computer generated steel frames cut with plasma or water jets, to inside of plate, so someone has to get to that point and it might as well be the supplier of drawings.
     
  13. Landlubber
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    Landlubber Senior Member

    ...I am with Alik...that is the way I have known for decades, but of course, we are supposed to be able to read, and SOME designers actually do tell us.....
     
  14. BATAAN
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    BATAAN Senior Member

    A complete set of plans from a good designer who knows how to build is a great joy, and leaves nothing to chance or the imagination, but answers every question in advance.
    A wooden boatbuilder needs to know what the inner surface of the planking looks like, because that is what he lofts and sets up. Typically for a 26' yacht of conventional construction, at least one inch must be deducted from the actual hull lines to obtain this. On a large craft it approaches 2". Every carefully faired body section must have this plank thickness dimension adjusted with a compass and batten, basically throwing all your fairing out a little bit.
    The old shipbuilders knew this perfectly well as they were designing something to build themselves by carving a half model, so the frame was all they cared about in design, since they weren't using drawings. Thousands of ships were successfully designed and built this way up until the 1920s.
    Look at most of the Smithsonian drawings of fishing schooners and that is how they are presented, not to outside of plank, because Chapelle was taking the lines off of builder's half models that were intended to be used that way.
    Later schooners came from yacht designers using paper and battens, but were still delivered to the Essex and Gloucester builders to inside of plank as they were used to it.
    Any modern designer does not use this method, but a pen or computer to draw to the outside of final paint, the final desired shape, and figures displacement and stability with much more accuracy than the old methods.
    But he also wants the builder to reproduce that design as closely as possible. The builder must then deduct plank or shell thickness, which takes time and introduces error, so it would make sense to give the builder a set of framing lines to inside of skin and avoid this.
     

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  15. DCockey
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    DCockey Senior Member

    A designer who specializes in high end, moderate to large size custom sailboats told me a couple of years ago that all the builders of such boats in Maine wanted CAD files of the frames/mold surface/etc to build from, not a set of traditional lines and offsets. (Lines and offsets are good to have for visualization.) If they were given just the lines and offsets then someone would build a 3D CAD model to use for the building. Is that common or are there still yards who prefer to start with a set of lines and offsets and then loft the boat on a lofting floor?

    If the shape of the boat hull (not just the lines) is in a 3D "math"/CAD model, then going from the outside to the inside (deducting plank or sheel thickness) or vice-versa is generally straight forward and quick, and it can be done essentially exactly (if the particular software used is up to it).

    Starting with inside or outside first can matter is if you want certain features/joints/lines to be straight or planar. I've run across this in wood construction. For example if you want the outside rabbet line to be straight then the inside rabbet/bearding line needs to be curved unless the boat has constant deadrise.
     
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