Rhino- Sweep 2 Rails vs Loft for hull surface?

[Splint]

Hi All,
I'm currently studying 3-D Modeling for the Marine Industry by Cliff W. Estes. I have noticed that the Author is fairly insistent that hull surfaces be created by the loft command and then trimmed. He doesn't go into any explanation why this is his preferred method or why the Sweep 2 Rails command should not be used. I'm sure there is a good reason for this, can anyone shed some light on this?

Thanks in advance
Splint

[CGN]

IMO, everyone finds it's own method, for example, i don't think lofting is the best way i find it to some point the least way i want to model a hull, i prefer model using "freemodeling" (push and pull) using a hull modeler, in regard of timing is easier to obtain profiles that can have "sharp" corners (step on the sheer), because using nurbs is hard to control such shape, so trimming is more convenient.

Using 2 rails sweep it tends to "twist" more than using loft (which you can control and make it a ruled surface for developing). i can't remember if you can "control " 2 rails sweep to be "ruled" (the resultant surface).

and there is another way i have seen use when you have already a set of lines plan which is to turn the 2D frames and waterlines and position this ones on 3D, once on 3D use a combination of frames and waterlines and sheer and apply a network surface so it basically becomes a "patch" modeling type using the wireframe as a reference. (remember that network surfaces have the option to be tangent to adjacent surfaces)

so to confuse more, stay with freeship or any hull modeler and then export to your 3D modeler and trim and cut all you don't need.

cheers

[Chris Ostlind]

Splint,

I have seen Cliff's tutorials before (long time agao) and if I remember correctly, he concentrates on round hull forms and not multichine developed panels. Is that correct?

If you were looking at multichine hulls, I'd suggest that Lofting does not always work for every type of panel you may design. I have found that the Sweep 2 command is far more accurate, though there are some interesting twists in the application.

On that same subject of multichine development... also take a look at what happens when you use the Surface Command box in the left hand menu and select: Surface from 2, 3 or 4 edge curves (also in pull-down menu above)

Chris Ostlind

[Tim B]

Actually, used properly NURBS is EXTREMELY powerful, and it is possible to create most hull-forms from a single surface. I usually create a surface and manipulate it manually by control points. Using the weighting function (which makes NURBS distinct from non-uniform B-Splines) you can produce produce chines with radius, or by introducing a singularity in the knot vector, produce a kink.

If you start with a single surface, you should only need to split it to match keels and shaft exits, prop-tunnels etc.

That said, there are a considerable number of ways to draw a hull. Some work better than others, but with a little trial and error you'll find the best solution for the job in hand. The funny thing about 3D drawing is that you have to forget about fitting sections, buttocks and waterlines, as these become a result of the geometry, not the definitiion of it.

Tim B.

[marshmat]

I wish I had that much skill with NURBS, Tim, to be able to tweak knot vectors like that... every time I try I end up with something that looks like a pretzel on crack.
For developable shapes I tend to use Sweep2Rails; provided that the cross-section curve is a simple straight line and the rail curves are smooth and not radically twisted the resulting panel is usually well within 1% of being perfectly developable. Trying a developable loft often seems to end up with diagonal kinks or folds in the panel, giving it the appearance of an industrial air duct. On chined hulls I tend to end up with a polysurface, with one surface for each panel (makes them easier to unroll, but harder to manipulate, than with TimB's method).

[Raggi_Thor]

For a single chined hull I prefer to start with a flat surface of degree 1 in the transverse direction and degree 3 longitudinally. Then in the front view I rotate, move and nudge the control points to match the estimated midship section, then edit the control points in top view and side view. One important detail is to extend the (half) hull forward and sideways so that when you trim along the keel (x-axis) the bow profile looks good. You can actually move control points longitudinally without any need for retrim.

[Splint]

Thanks Guys,

that's really given me something to work on.

Chris, here's a picture of the four boats in the tutorial.

Cheers
Splint

[Tim B]

Ok, well you can't create a non-standard knot vector (ie. insert a kink) for a surface in Rhino, but never mind. Here are some screenshots of a simple hard-chine powerboat hull made with a single surface with two rows of control points with weighting 1000.

I have also attached the Rhino file so those who wish to examine the hull in more detail.

Tim B.

PLEASE DO NOT USE THIS HULL IN DESIGN WORK!! FAILURE OF THIS HULL TO PASS STABILITY CRITERIA OR PERFORMANCE CRITERIA IS NOT THE CONCERN OF THE AUTHOR. THIS IS AN EXAMPLE OF WHAT CAN BE ACHIEVED USING A SINGLE SURFACE ONLY!!