how to design in 3D

Discussion in 'Software' started by 1gerry, Jul 27, 2013.

  1. 1gerry
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    1gerry Junior Member

    Hi All,

    Thanks for bringing the topic back to the software/ how to get the proper shape.

    I have noticed the following:
    If i look a section lines in plan view, i can get fairly straight lines going to the bow. Refer to post #16, plan view. But then if I analyze the longitudinal isocurves with curvature combs, I can see that the curvature is not nice an even, but there are bumps. On the other hand, if I get a nice evenly spread curvature with the combs, the section lines in plan view look a bit more like a water drop shape, meaning that the bow area curves inwards a bit, instead of being straigth.
    Is this normal?

    Thanks
     
  2. DCockey
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    DCockey Senior Member

    What are the curves on the bottom in the post #16 plan view - isolines?

    the section lines in plan view - are these what are commonly referred to a waterlines (cuts through the hull surface parallel to the nominal waterplane)?

    Isolines of surfaces shouldn't have bumps if the surface is fair.

    A convex station/section amidships and a vertical stem frequently results in a hollow forefoot.

    How did you create the surface?

    How are you modifying it?
     
  3. rwatson
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    rwatson Senior Member

    No, they are an important part of them though - as you well know. But this isnt the topic of the thread, so I wont get all detailed here.
     
  4. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Quite so.

    You can draw a diagonal through the area that disturbs you, and find an alteration of the respective frame section (or/and its neighbours).
    A slight inclination of the stem also will reduce the hollow effect; if it pains you to lose length in waterline - add it at the transom.
    Also playing with the keel rocker forward can contribute.
     
  5. DavidJ
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    DavidJ Senior Member

    I was going to say the same thing about the waterlines. 1gerry I believe you are using the term "section" in the drafting sense where you take a section through something. And you are probably even using Rhino's "section" tool to make this slice. But careful when you use that term in the nautical world. It made your post unnecessarily difficult to read because I kept imagining a vertical slice when I read section. You are calling an orange an apple.

    Some terms for reading a plan of a boat hull that will make this conversation easier:

    Lines plan - this is the two dimensional plan that defines the shape of the hull. It is made up of a Plan view, a Profile view, and a Body Plan.

    Plan view - the view of the boat looking at it directly from above. Buttocks and Stations appear as straight lines in this view.

    Profile view - the view of the boat looking at it directly from the side. Waterlines and Stations appear as straight lines in this view.

    Body plan - the view of the boat looking at it directly from the ends. Waterlines and buttocks appear as straight lines in this view.

    Stations - transverse (side to side) slices through the boat. The body plan shows these slices on one view. Normally the right side shows the forward stations and the left side shows the aft stations.

    Waterlines - horizontal slices through the boat.

    Buttock lines - vertical slices through the boat in a longitudinal direction.

    Centreline - the line with which the boat is symmetrical about. It appears in both the Plan view and the Body plan.

    Baseline - the baseline. That was a joke. This is the line the designer chooses to call the baseline and knowing what the designer meant when he drew the plans is a big help for others when working on a design later on. It can be the extreme bottom of the boat. Or the bottom of the hull but not including the keel. Or either of those same things but from the inside of plating (moulded dimensions). Or it can be some arbitrary point where the designer felt like putting it. It is a horizontal line like the waterlines.

    BWL - beam on the waterline

    DWL - "design waterline" or "designated waterline". This usually means the same thing as LWL which would be "load waterline" or "length on waterline"

    In a traditional lines plan there are 10 evenly spaced stations spread along the DWL. The first station is number 0 and it is located at the intersection between the DWL and the forward extreme of the vessel. This is called the forward perpendicular (FP). The last station is located at the intersection of the DWL and the aft extreme of the vessel. It is number 10. It is usually called the aft perpendicular (AP) although not always. Naturally station 5 is then located at the midpoint of the waterline. This is called midship and it is designated by a circle with two arcs.

    More stations can be added as the designer feels is necessary to properly represent the shape of the vessel. But the other station numbers will stay the same. For example if you wanted another station between station 1 and 2 you would not renumber but would instead call the new station 1.5. To make things confusing sometimes stations are numbered in opposite order and sometimes the AP is not the aft extreme but the centre of the rudder stock.
     
  6. DavidJ
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    DavidJ Senior Member

    and I completely forgot diagonals HakimKlunker :eek:
     
  7. 1gerry
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    1gerry Junior Member

    Sorry for the confusing post.

    I've made a set of images to illustrate the situation. I hope it helps.

    I don't know if this is simply a design issue, caused by software, or if this is were the surface tweaks come to play.
    Should the ideal hull aim for a combination of "perfect" waterlines, as in Image A, and more even curbature combs as in image D?

    Images A and B reflect the Javelin hull, following all the stations as per the original drawing.
    C & D, me playing with the surfaces.

    Thanks

    Gerry
     

    Attached Files:

  8. DCockey
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    DCockey Senior Member

    The kinks in the curvature comb of the isoline are due to the way you created the surface.

    How many rows and columns of control points does your surface have?

    If you tell us how you created and altered the surface I might be able to suggest how to improve the surface. Or if you attach the .3dm file to your post I'll try to take a look at it.
     
  9. HakimKlunker
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    HakimKlunker Andreas der Juengere

    How COULD you?
    Diagonal is the plane on which we live while sailing :D
     
  10. 1gerry
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    1gerry Junior Member

    I simply created a loft that travels through the 10 stations, since I had the profile of each of them.
     
  11. DCockey
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    DCockey Senior Member

    Too many stations for initial design of the hull surface. With that many stations you'll spend lots of time adjusting the station curves to get a reasonably fair hull, and then it may not be the hull shape you want. The fewer the stations the fairer the surface will be without lots of moving control points. Begin with fewer stations and get a fair shape and make it as close to the final shape you want as you can. Then only add additional stations if really needed for local control.

    The individual stations should have as few control points as possible and still obtain the shape you want.

    Start with no more than 5 stations. Also put splines through the sheer, chine and keel using InterpCrv. Then experiment creating separate side and bottom surfaces. Try both Sweep2 and NetworkSrf. This assumes you are not interested in developable surfaces which make plywood construction easier and sometimes even possible.

    Use a different method for developable surfaces. Create shear, chine and keel curves using 5 stations and InterpCrv. Then ignore the stations; developable surfaces probably won't lie exactly on the stations you've drawn. Use the sheer and chine curves to create the side surfaces, and the chine and keel curves to create the bottom surfaces. Use Loft with Method set to Developable, or DevSrf with Least Twist to create the surfaces. It can be useful to extend the curves and surfaces beyond the center-plane at the bow, and beyond the transom at the stern. Then trim the surfaces at the center-plane and transom.

    Once you have a surface modeled you can quickly and easily create as many stations as desired using Contour, Section or Section Tools. I usually use Contour.
     
  12. DCockey
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    DCockey Senior Member

    The kink in the curvature comb in figure B of Post#37 is due to the shape of the station at that location.
     
  13. Peter Edmonds
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    Peter Edmonds Junior Member

    1gerry

    I am quite interested in the design you have chosen to develop. It does seem to be turning out to be a fairly extreme shape, read by me as a high-performance centreboard skiff. Are you active in this niche of boating? If not, it would appear to be a somewhat offbeat choice for a design.

    As you are building skills in computer hull shaping, I strongly recommend that you use a design with which you are very familiar, ad leave the innovation for later.

    A few years I tried Rhino, and had some instances where surfaces misbehaved. I strongly recommend that you go to a hull design package of your choice. It will give you a lot of useful data - hydrostatic particulars (starting with displacements and centres of buoyancy for nominated waterlines). I have had Rhino described as being very good for sculpting superstructures.

    I used Hullform for years; now in public domain. It is very friendly for working from lifted or other nominated offsets, but initial dealing with the transverse curvatures is a bit tricky. I am moving on only because it doesn't support IGES surfaces, which I want to feed to SolidWorks.

    I am about to build up in Delftship- earlier version free current eminently affordable. Too soon to pass judgement. Beware the knockoff scams on this software - 3dboatdesigns is sold for money, and not supported; an earlier version of what is currently available Look up DELFTSHIP/Freeship in this forum.
     

  14. SukiSolo
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    SukiSolo Senior Member

    I have used Rhino for a few boats now for various jobs. Hull design or internal layout stuff. Having done a double chine small racing craft for plywood build, you can create good surfaces with the tools in there. Agreed the 'tools' are not complete without the add-on package but you can get there even if slightly longhand. If it had dynamic curve of areas stuff I would say it would be good enough, but I don't value the prismatic coefficient as a major item. The curve of areas shape I do value. The Hydrostats Rhino gives are useful though.

    From build experience, I would say why limit your design to what say 5 or 6mm ply will curve to?. Even thinner stuff, say 2 or 3 is limiting. The bow area is the problem bit but get creative, nothing stops you making your own ply locally. If you buy some veneer and scarf it and laminate it onto the undistorted ply you can get almost any curvature you want. Might not be super quick but it works. Having to get my design to be competitive against other FRP moulded boats I went down this route. If you tried to bend 6mm ply (as required by rules) round the FRP competitors bows, it would break. Still class legal though....:)

    As far as the design above goes, to me she looks like her buoyancy centre is too far aft and she will nose dive badly in breeze. Try and get a little more rocker in there - but where I'll let you decide!, but the main thing is to let the water away from those powerful stern sections without getting the boat to lift her rudder heeled and lose control. Take a careful look at an RS100 hull underwater. I will say that Frank Bethwaite in High Performance Sailing was not entirely correct about hull shapes for more conventional to moderate performance small mid weight boats. For this I mean for 4m long or so modest sail area things say 4-5kn upwind to 15-18kn offwind. For skiffs and 'flying machines' he may well be right. I do note a trait amongst some skiff designs for burying their noses because they cannot sink the stern. Likewise sharp V sections up front get boats to nosedive or porpoise, most unpleasant.

    Like 1gerry I am an industrial designer with plenty of surface modelling experience. However I have also sailed for a long time and worked building cold moulded rowing shells. The basic principles of naval architecture are pretty easy to get hold of, it does require a bit of on water experience to make useful judgements about shape. Having sailed lots of different designs of restricted classes N12 and Merlin Rocket wa invaluable for noting the varying behaviour and speed of different design shapes. My own experience in the class I currently sail is that upwind its 10% hull 90%rig and foils but much more hull offwind closer to 60%. I'm just getting a bit old to really be comptitive but my last effort is on the pace fine. Not bad for an 'amateur':D
     
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