Surface continuity

[Smircio]

Hi, I've a little question...how can I recognize if a surface is fair? I'm designing a catamaran with a not so simply geometry,maybe just for me...anyway...I'm using catia to build the surfaces but I'm not be sure if I'm doing a good work...can someone give me advice on the way I'm working?
For exemple...which type of continuity between two section have I to give?tangency or continuity?The keel is filleted...naturally I don't know the radius,also becouse is not constant...how can I do?I've build my surface as features...but naturally that are too constrained to the sections...Have I to build on that features others surfaces with the "freestile" or the "Automotive Class A" module?Can someone tell me if there is a way to resolve my problemes?
Thanks

[nero]

Do not use that program. Looks like you have too many control points.

[DaveB]

Hey,

I don't have any experience with that particular program, but any software that uses nurbs these days likely has some decent fairing tools. I think you want to have constant curvature. Here's a manual on fairing in rhino... There should be similar tools in the program that you're using. Surfaces will be fairer if you use fewer control points, however there are a minimum number that are required to define a surface. I've found it most useful to make sections and use porcupine curves to determine fairness. It's an art, not a science, and I think it takes a fair bit of practice to get really good at it...

All the best,

Dave

[Andy]

Nero - Catia can't be that bad - Americas Cup designer Jason Ker uses it and produces some awesome results...however it looks to me also like Smircio has simply used too many control points. Cut them down and keep them ordered in as rectangular a pattern as you can and distributed as evenly as you can and you should'n go too far wrong.

Andy

[nico]

I dont think anyone is using Catia to design a hull shape. Importing a hull from maxsurf (for example) probably. If you dont have hydrostatics (upright and at some angles) you cant do much. What catia should be able to do, is a nice surface.

[nero]

Oops! Meant to type { I do not use that program }

appologies

[CORMERAN]

O you computer dependent guys!

One of the joys of building wood boats: is that, given half a chance, a long
thin piece of wood will give you a beutifully developed FAIR curve.
Secure it at each end and apply gentle pressure, to find the right line.
- Apply PENCIL appropriatly.........

Back in the stone age, we used battens and weights on {shudder !} drafting
boards - to achieve the same results as the shipwrights - on the shop floor.

[DGreenwood]

Yeah..and I have a pair of completely nonfunctional knees from crawling around on a lofting floor(among other things) producing the same mold in three weeks that a CNC router makes in 45 mnutes. Sorry dude... no amount of self rightious justification of the old method will prove to be easier, more accurate or aesthetically pleasing. It is just a matter of enjoying the process---and I still very much enjoy the process. So, if you like using a batten to loft a boat ( as I do) then by all means have a good time. Just don't try to convince people who actualy understand the process that it is still superior.

[CORMERAN]

Dear Mr. DGreenwood,
sorry to hear about your knees.
I've also spent my fair share of time on the lofting floor - so I commiserate
with you.
So I'm not some John Henry, prepared to die, racing a Steam Hammer.
Resisting progress.
I use computers all the time. My clients demand it.

However, clearly - humour and irony - is not always welcome.

Specificaly , my intent IS practical and might be of help.
- To the person who made the request.
- Perhaps, it is useful to them. To make a quick check on the direction they are going, in their design process.
- To spend a few, brief minutes with a batten. - While they wait for delivery of their improved software program.

[DGreenwood]

Humour and Irony are always welcome...except when I have mistaken it for the rantings of another contrariant traditionalist poser. And I am afraid I have done so here.
I am weary of thread after thread being hijacked by " the old way is better" harp. There is no need to justify my love of traditional boats and methods by trying to sell them as the better way. They are not the better way, I just like them...period.
Incredibly (sarcasm)... I like using the latest technology to design and build lightweight Aramid and carbon structures to go insane and dangerous speeds while being pounded to a pulp. For some reason those same tradtionalists think that this must be justified as well?
It gets tiresome and makes me cranky! My apologies.

[CORMERAN]

- To: DGreenwood.

No Worries.... Mate!

- Sometimes I am accused, by my daughters, of being a bit " pompous " and
pontificating more than is neccessary.........
Which can be counter productive, sometimes.

I share your wearyness of some of the discourse that floats through
these forums.
- However, there are also some real gems - that stand out.
- And to my suprise, I have also found that some very expert people
can teach me, who is so wise, some valuable lessons. These people
can often be, suprisingly generous in the passing on of their experience.

Cheers !

[Andrew Mason]

Nero - Catia can't be that bad - Americas Cup designer Jason Ker uses it and produces some awesome results...however it looks to me also like Smircio has simply used too many control points. Cut them down and keep them ordered in as rectangular a pattern as you can and distributed as evenly as you can and you should'n go too far wrong.

Andy


Jason Ker is a Maxsurf user, I doubt he does the hull design in CATIA. I note from Jason's own web site and from this article (http://www.ugs.com/about_us/press/press.shtml?id=3586)that he uses Unigraphics for general CAD, but the article clearly states that he uses Maxsurf for the hull shape definition, and that importing a Maxsurf NURBS model was a major factor in the choice of system.

Smircio, from the look of the image you posted you have created a large number of surfaces whose edges are defined by the section shapes, and you are then relying on the continuity between patches to give you the required longitudinal fairness. This is an inherently flawed approach, and although you appear to have obtained good results, it will be a lot of hard work to make sure that the hull is fair.

A user of Maxsurf (or Autoship, Fastship, Rhino and all the other NURBS based hull design programs) would model this hull using a small number of surfaces, each of which extended from bow to stern, giving much better control of longitudinal fairness. This does mean that getting the surfaces to exactly match the sections you have is not guaranteed, however if these were not designed using a good hull design program they are unlikely to be perfectly fair anyway. If the hull was designed using a NURBS based hull design program, you would be far better off getting an IGES file of the original design and importing it into CATIA.

[Smircio]

Thanx guys for the support...Actually, I've also tried to model the boat using Rhino,but near the stern I don't know how to get a longitudinal fairness and continuity...I've read also the Rhino's marine guide,but I don't know how to built the "phantom" station...I've tried,but the boat is totally filleted with an high curvature near the stern and all over the keel and I didn't like the surface obtained...what do you think?Should be better use Maxsurf?
I'll becoming crazy...:confused:

[nero]

Smircio
If you download TouchCad demo, I will email you a file that will help you to design your hull.

The basic idea is to multiple extrude a crossection line (that represents the crossection of your hull). This crossection line needs to have at least one of the points on the wl and one point on the widest part of the hull.

Then scale the curves to control lines.

Also, I do not set the front and back of the surface on the bow and stern lines. I hold them back and use a sweep of the cross section to finish out the stern.

[Smircio]

Ok, I've downloaded Touchcad.

[nero]

send me a private message with an email address so I can email you the file. I can not post it directly.

[Smircio]

My mail address is:
bypino@yahoo.it