Inside/Molded or Outside/Displacment - Where to Design

Discussion in 'Boat Design' started by DCockey, Jul 7, 2013.

  1. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    PAR - Are the lines plans of your designs drawn to the inside or outside of the planking? Do you initially design the shape of the inside of the planking and then compensate for the planking thickness when doing the hydro calculations, etc; or do you first design the shape of the outside of the planking, and then determine the shape of the molds/frames for the builder to work from directly?
     
    Last edited: Jul 8, 2013
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    I think another point has to be made here that I did not see in the above discussion. And this is from my understanding and experience: When designing metal boats, the thickness of the plating will most likely vary--thinner strakes of plating near the sheer, thicker in the bilges and bottom, thicker yet near the keel. If one were to design these plates to be flush to the outside surface of the plating, the outboard edges of the frames would necessarily have to be notched and cut inward for the different plate thicknesses. It's going to make for very messy and difficult construction because the builder cannot guarantee that any given plate edge is going to fall exactly where the notch or change in edge of the frame is. It could be off by a half inch or more. Also, the frame edges would NOT be fair lines; they'd be discontinous jogged lines. So we design to the inside surface of the plating to make the cut edges of the frames continuous edges and fair. By frames, I mean the transverse frames and bulkheads. The plates land where they may without difficulty of fit from plate to frame or plate to plate. Thickness variance progress outward rather than inward.

    This practice is derived from large metal ship construction. When calculating the hydrostatics and stability to "molded lines" (i.e. to the inside surface of the plating) the error in calculation is small because the plating thickness in relation to the beam of the vessel is small. It is a necessary, although very small, error and should be noted on the calculations.

    For small molded boats, on the other hand, particularly when we use thick fiberglass laminates (or wood and wood laminates), the thickness of the plating also varies from sheer to keel, but the construction method is a lot different. Actually, the thickness junctures can be rather arbitrary and variable if the construction is "messy", shall we say, i.e. not too good on the QC. We build in a mold which necessarily fixes the outside surface. We are not dealing with lots of individual metal frames, nor that many bulkheads--making bulkheads to fit a molded boat is relatively easy. Also, the thickness of the hull laminates or hull skin are relatively greater in relation to the beam of the vessel, so calculating the hydrostatics and stability would generate more error if we calculated to the inside skin. Therefore, drawing lines and calculating to the outside surface of the hull is more accurate, both for building and calculating hydrostatics and stability.

    Yes, it is up to the designer to state explicitly which surface the hull is designed to. In my practice, my metal boats' hull and deck lines are designed to the inside surface of the plating everywhere. This involves noting where the "molded lines" are on all of the construction drawings. Hydrostatics and stability are calculated accordingly. On my wood and compoiste boats, my lines are drawn to the outside surface of the plating. Again, the "molded lines" are noted on the construction drawings where appropriate. Hydrostatics and stability are calculated to the outside lines. Keep these rules straight, and you will not have a problem.

    Eric
     
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  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Eric, thanks for the insightful and informative reply.
     
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