Building of Aluminium Boat . Mounting of hull framing structure

Discussion in 'Boat Design' started by an2reir, Feb 27, 2019.

  1. DCockey
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    DCockey Senior Member

    I may have figured out what is going on here. Andrei - Did you and Lowell use DevSrf to create the entire bottom surface? If not how did you create the remainder of the bottom?
     
  2. an2reir
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    an2reir fifty boat designs


    Hi Gonzo, yes I do think so, if the FEA loadcase is setup properly with restraints at the rear and perhaps progressively applying more restraints as you would weld the metal plate and the force maybe in three sequences. In regards developability of bottom plate I think the fact that my cutting file is similar with that unrolled from a bottom surface made by the guy from Mc Neel is conclusive together with the fact that a scale model 1:10 has been built in steel. basically if theres a scale model in steel and bottom plate has bent on that bottom I think boat should be buildable allright.
     
  3. an2reir
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    an2reir fifty boat designs

    Hi David, I do not know what software did use Lowell to make the bottom plate, but his unrolled surface is similar to mine. I presume he may be working with an edition that is more advanced than the one available for download. What he said is one on my rails did show a number of control points that were not placed as he would consider their placement as optiomal. I think he did slightly change the position/and/ or number of control points . I think one may use Dev Surf and make slightly different surfaces depending on how many control points degree ; where the control points are placed on the rail curves ? I confirm I did the bottom and topsides with DevSrf on a Rhino 4, the chines by sweep two rails. Here attached is a new bottom plate I just made right now in DevSrf by rebuilding the chine and keel curves to a lesser number of control points ( 10 ) . I recently did learn to use Paneling Tools and I did work on complex developable surfaces and sets of cutting files. This new tool does have its own unroll dev surf. By practicing the Paneling Tools I learned that the unroll of Paneling Tools is more strict than the unroll in Rhino, meaning if a surface does unroll in paneling tools you may assign a high degree of reliability to its developabileness quality. This is how I cross tested at the beginning my bottom followed by the building of the steel model
     

    Attached Files:

    Last edited: Mar 2, 2019
  4. Barry
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    Barry Senior Member

    This is not correct in the bow area. Developable surfaces do not mean that at 90 degrees to the keel, which is what transverse means (?) that the side of the plates are straight.
    A cone is a developable surface and while there is a straight portion between both the arcs at each end and a curve throughout.

    I believe Mr E or DC has mentioned this several times.
     
    Last edited: Mar 3, 2019
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  5. DCockey
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    DCockey Senior Member

    Andrei - Does the bottom surface created by DevSrf and shown in the attachment to post #78 use the keel and chine curves in your attachment to post #74?
     
  6. an2reir
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    an2reir fifty boat designs


    Affirmative. But I did change the number of points now to 10 to both chine and keel lines.
     
  7. DCockey
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    DCockey Senior Member

    The surface illustrated in your attachment to post #78 is not an "exact" developable surface. It is close to an exact developable surface but is not the exact surface even though it was created by DevSrf, Lowell obtained similar results, and it unrolls. With the warped bottom the ruling lines aft (transverse isocurve) should be much more skewed. It may or may not be close enough for your purposes.

    I created 3D keel and chine curves based on your attachment in post #74, and then determined the "exact" ruling lines between those curves. I'll include those results in another post and discuss the implications.
     
  8. an2reir
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    an2reir fifty boat designs




    Hi David, thank you for your feedback yes I do agree and I do undersand. I look forward for your further posts.
     
  9. gonzo
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    gonzo Senior Member

    I think that maybe I should've referred to my previous post. It narrows the comment to a developable surface that would follow the frames; that is the designed shape.
     
  10. DCockey
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    DCockey Senior Member

    Developable bottom surfaces based on the keel and chine curves drawing attached in Andrei's post #74. Forward station curves are perhaps closer to straight than someone experience with developable surface hulls might expect, though not as close to straight as reported by Andrei for his surface. The aft portion of the bottom is not uniquely determined by the keel and chine curves. Two developable alternatives for that portion of the bottom were created.

    3D keel and chine curves were created based on the 2D drawing of the keel and chine curves in Andrei's post #74. The length of the bottom as scaled to 5000 mm. (The maximum deviation from the curve implied by the drawing is less than 8 mm, and the average deviation is probably less than 2 mm.) "Exact" ruling lines were determined between the 3D keel and chine curves using the method described at Developable surface - exact ruling lines from edge curves https://discourse.mcneel.com/t/developable-surface-exact-ruling-lines-from-edge-curves/73928 (In general this method produces ruling lines with with twist significantly less than 0.1 degree.) Those ruling lines are shown below and in the attachment. Note that the aft most ruling line goes from the aft end of the chine curve to forward of mid-ships of the keel curve. This is due to the "warped" shape of the bottom. The keel and chine curves and the ruling lines are shown below. A developable surface was created using the keel and chine curves and the ruling lines. Station curves and buttocks are also shown. Ruling lines and the bottom aft of the ruling lines shown are not uniquely determined by the keel and chine curves, and two alternatives for that portion of the bottom will be discussed in another post.

    BDNForeRL.jpg BDNForeLines.jpg BDNForeStat.jpg BDNForeButt.jpg
     
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  11. DCockey
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    DCockey Senior Member

    An aft triangular section of the bottom is not uniquely determined by the keel and chine curves and developable surface assumption due to the warped bottom. The designer has to decide how to shape that portion of the bottom. Creating a conical surface with the apex at the aft end of the chine is a simple method to create the remainder of the bottom. The resulting surface has a small radius "kink" running diagonally across the bottom with a planar triangle behind it. The two portions of the bottom will be tangent but the curvature will be discontinuous between them.While this is technically developable it is likely to be difficult to bend plate to the small radius and curvature discontinuity, particularly close to the aft end of the chine where the radius approaches zero. It also is may not be desirable for good water flow and boat performance.
    BDNConeRL.jpg BDNConeLines.jpg BDNConeStat.jpg BDNConeButt.jpg
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Typically thorough work by DCockey, the OP will note that drawing pretty well corresponds with the actual build, where you could put your hand in the gap, at the station at the turn of the forefoot, it seemingly being in the neighborhood of about 3 cm, or about an inch and a quarter. One wonders whether the best plan with plating these boats isn't to have the keel and chine as the guide, and let the sectional shape be guided by the "natural" shape the plate takes up when draped over it, in intimate contact with all points along both chine and keel. That way, minimizing the stresses of fitting, too. Shape wise though, I think it is still a good hull.
     
  13. DCockey
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    DCockey Senior Member

    An alternative to the conical surface aft is curving the bottom of the transom and creating a developable surface between that curve and the aft portion of the chine. The curve at the bottom of the transom is arbitrary but needs to be tangent to the forward portion of the bottom (with that surface extended) for the two portions of the bottom to be tangent. If the transom curve is curvature continuous with the forward portion of the bottom then the entire bottom will be curvature continuous. This particular transom curve was designed to distribute the curvature of the bottom and minimize the maximum curvature (minimum radius). Note that the stations aft have significant curvature.

    BDNSmoothButt.jpg BDNSmoothRL.jpg BDNSmoothLines.jpg BDNSmoothStat.jpg
     
  14. DCockey
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    DCockey Senior Member

    Presumably there is a "minimum energy" shape if the plate is constrained to to contact the keel and chine but free at the transom. The second alternative is an attempt to approximate that minimum energy shape.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Beyond my ken to approximate it in a CAD drawing, but I like the "minimum energy shape", if it both fulfils the desired hydrodynamic shape, and facilitates easier construction. I think you have established that the original straight-sectioned drawing can't be met, in practice, and it has become what can be done, and in which way, which is some progress. The curved bottom aft is the preferred shape, I think, but not so easy to get the plate to conform, perhaps.
     
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