Adding Waterlines in Rhino

Discussion in 'Software' started by AdamBB, Feb 5, 2014.

  1. Hampus
    Joined: Jan 2010
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    Location: Sweden

    Hampus Junior Member

    Thank you for the insight SukiSolo. When i was talking about mm precision i didn't mean precision of parts, but that of the lines and the fairness. You'll have the same problem building a fair hull matching the lines exactly in real life as you will building a 3D model from your 2D lines, to an exact match. Any such fairing mistakes, be they due to a mistake by the designer or due to the inherent inaccuracy of a pencil drawing on an A2 sized drafting film, would be corrected by the loftsman as he lofts the full size patterns.

    Now, what you say about the difficulties of creating an exact 3D model from your lines is interesting. I'm struggling with that problem. I guess there are two ways of going about the hull design? One woukd be to first do your lines and then build a 3D hull from them. In this case, the 3D hull wouldn't be an exact match and would be pretty much useless for anything but show and tell, right? The other way would be to create the 3D hull and then build your lines using the hull. In this case, both hull and lines would be useful. Parts could be created with an exact match to the inside of the hull and then machined. Correct?

    I'm being taught to do it creating the lines first. I have to admit that I prefer that way as I feel more in control of the shape I give the hull. I guess that also depends on the software you use. I use Rhino, straight up, but I guess it would be a bit different working with Orca or any other hull modelling plugin or application.

    What are your thoughts on the different approaches to hull design?

    /Hampus
     
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    In the real world nothing is an exact match; there is always some tolerance whether it is explicit or implicit. It may be the accuracy with which the comparison is made, or the precision with which the lines are specified. If the lines are defined by a set of offsets specified to 1 mm then a surface with 0.5 mm of the lines would be an "exact" match.

    A 3D digital hull surface can be created which will match a set of lines within any practical tolerance (1 part in a million or better) assuming an appropriate set of tools (software) is used. This is certainly possible with Rhino and other software. (My understanding is it may not be feasible with some software such as DelftShip/FreeShip.) So it comes down to what is meant by "exact" and the corresponding question of how precisely are the lines defined.

    Even if the surface does not match the lines "exactly" it my still be very useful depending on the magnitude of the discrepancy between it and the surface. How closely does the actual boat need to match the lines? The requirement can't be any closer than the accuracy with which the boat can be built. Hydrostatic calculations do not require an "exact" surface. Look at the methods and assumptions traditionally used for hydrostatic calculations; they are not "exact". Likewise CFD and other performance modeling does not require a "exact" surface.

    Correct. Once a 3D surface is available what is a traditional set of lines needed for beyond an illustration of the hull shape?

    I work in Rhino. For a new hull design I create some initial curves which guide surface creation. I use the fewest number of curves needed to define the desired hull shape.

    If I'm starting with a furnished set of lines/offsets then I create a surface from the lines/offsets which matches the lines/offset within the desired tolerance and which is fair to the desired degree. It is an iterative process. Sometimes it isn't possible to achieve both the desired match and fairness simultaneously, and then tradeoffs are needed.
     

  3. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Well said David, I'll second your comments.

    Hampus please note the last comments - the minimum number of curves to create the surface. Absolutely key to a fair result. Once you have a good model you generate all the other data, offsets etc required to build it or for other purposes. Unlike older days of drawing to scale, one of the freedoms of 3D is everything is full size. Oh! and never scale to a third ;)
     
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