Boat hull loft symmetry problems again

WaldPinkler

I feel like I keep running into quirks. I can't get this boat hull to loft + shell symmetrically.

I hope I'm sharing the model here correctly: https://cad.onshape.com/documents/111831e267f05831d19affa5/w/ee585eb42ce339129c741233/e/9ff688a4c6082a249b529902?renderMode=0&uiState=637be2b7566201297846e4cc

The first image is after I shell the hull loft. The inner surface becomes off center. I've double checked my sketch symmetry. This loft is from side to side of a boat. I've tried the loft from the opposite direction (top to bottom) but the underside hull join is too angular.



Here is using thicken.

S1mon

I think you need to make the document public.

WaldPinkler

I'm not sure what to do. There's an icon saying it's public then I click the link.....

https://cad.onshape.com/documents/111831e267f05831d19affa5/v/0433f7857b5e33fc6ac856c8/e/9ff688a4c6082a249b529902?renderMode=0&uiState=637d836c1384457d146335c4

WaldPinkler

I've split the hull down the center then mirrored as a workaround to get a symmetrical model. I can't seem to lower a surface from the top of the hull to make a deck a bit below the top. I can't split a thicken deck against the hull face.

This worked when I didn't split the hull and just made one complete loft.

S1mon

I would highly recommend using Bézier splines instead of the regular splines. They have much better internal continuity. Also, whenever you project a curve onto a curve, it's a little like making a photocopy of a photocopy. It needs to do some approximations which aren't always great.

In this particular case, your Rear mid hull sketch has curvature which accelerates towards the mid plane. When this is offset (also always an approximation), that acceleration towards the ends can make the offsets more bizarre. 



In general, when laying out the control polygon for Béziers, I would recommend following these "golden rules". They're written for Autodesk Alias, but the math underneath is the same.

Looking at the bottom surface, you're lofting to points. This is a degenerate situation for NURBS surfaces. Not that you can’t get away with it, but surfaces want to have four sides. When you offset things that go to degenerate points, they often don’t behave well. The curvature graphs at the bow and stern are both very extreme which means it’s very hard to offset cleanly.

All that said, if you really just want to fix the glitches you have, you might be able to cut the solid thickened model in half and then mirror it. Sometimes that will avoid some of the issues you're dealing with.

steve_shubin

joshua_jenkins063 said:

I can't seem to lower a surface from the top of the hull to make a deck a bit below the top.

https://cad.onshape.com/documents/8033d5e4c8d1c41c02a257f4/w/ec489d974b7d64ffa15b57a9/e/973ce04816a4e60e4dc4dd02

steve_shubin

@joshua_jenkins063

Added a lower deck, and refined things a little bit

S1mon

The curvature of the outside surface is still asymmetric. Normally for symmetric things, I like to model the whole surface across the mirror plane, but in this case, I would be very tempted to model 1/2 the bottom surface with it going to a virtual sharp for the keel. Then mirror and add a fillet or loft to create the much sharper curvature at the keel.

The top surface is also asymmetric. There's no reason to use a loft here. A fill surface would be much better than a degenerate loft.

WaldPinkler

Interesting. There's a deeper level of understanding there. The curvature tool is interesting.

I'll give it another try with bezier curves.

S1mon

It goes way way deeper. People devote their lives to things like lofting boat hulls, or studying the math behind splines, or creating class-A surfaces for automotive design. 

S1mon

"Lofting" comes from the large loft rooms where boat/ship builders would lay out the full sized curves of boat hulls, using splines - thin, flexible strips of wood. 

https://www.core77.com/posts/55368/When-Splines-Were-Physical-Objects

 

WaldPinkler

Bezier made no difference. Just a bit harder to make the curve fit.

I'll cut and mirror. It just seems I run into other issues further down the line like the deck. (I haven't tried the zero offset surface as above though).

WaldPinkler

I've been to sail lofts and never thought about the name.