Hello, I have used Rhino professionally for 10 years and always come across this issue but I have been able to fudge… until today!
I am trying to make a hull model with 100% adherence to original, faired lines/ table of offsets. I have tried Sweep, Srf from Network of curves, loft, and blend but have had less than satisfactory results. The surface always seems to adhere to a given section, then bellow out just a short distance afterwards to conform again to another section. The cycle repeats itself. I have tried to see if it is just a graphics issue, but the sections (note the completely unfair waterline in the third picture below) relay the same incontinuity.
Don’t build everything from one surface. I’d say you’d need at least 3 surfaces for what you’ve shown, the radius at the bow being the split point. If you have straight surfaces down the sides I’d use another set of surfaces for that area too.
Single span surfaces are where the degree of the surfaces is one less than the number of control points. These surfaces are super-smooth.
Id’ use curvature/tangency matching at the join.
It could take quite a bit of tweaking to hit the original curves though, assuming the faired originals were actually fair in the first place! I’ve seen plenty of wobbly info in my time…
My guess is the tight radius of the chine at the stem, combined with no curves other than the chine and sheer defining the surface aft of the stem is the source of the problem.
Several possible ways to try improving the surface:
A) Try building one or more waterlines which pass through the stations and the stem. Adjust the waterlines, adding knots using InsertKnot if needed until the have the desired shape between the stem and the first station. Then create the surface using NetworkSrf.
B) Copy the first station curve to just aft of the stem where the chine transitions from tight to almost straight. Position the bottom of the curve on the chine, then use Scale3D to stretch so the top of the curve intersects the sheer, Try creating a surface with the added station. If that doesn’t work then create two surfaces, one from the stem to the added station curve, and the second from the added station curve aft.
C) A combination of A and B, adjusting the new waterlines and/or new station curve so that they intersect. Then create the surface using NetworkSrf.
Edit: Based on images above looks like you already have the curves mirrored.
Build a surface from the first station aft. Make sure you like it.
Mirror the sheer and chine curves and the surface, Join the original sheer curve and mirrored sheer curve, and the original chine curve and mirrored chine curve.
Create a surface between the original surface and mirrored surface. Use the joined curves, the stem profile and the edges of the surfaces (not the station curve), Use curvature continuity at both edges.
What do the lines represent on the last photo, going from knuckle to chine? Are those some construction lines? Seems like with that very shaped nose , maybe that 1st foot or so should be its own surface . Perhaps 2 rail sweep as was mentioned.
Apologies for the late reply.
What is the source of the offsets? How were they faired/lofted?
If.you are using lofted lines then the construction material must be taken into consideration.
I have lofted patrol boats full scale and an aircraft carrier usimg Rhino. In both cases the design office offsets were in error by several centimetres.
As someone else has said it is important not to let the computer fair the surface, or attempt to use too few surfaces.
Looking at the images; the stem plate is critical because it appears to have a radius in plan thus the curve of centres combined with the stem profile will give the tangent line on the shell. This provides the foremost edge of the shell surface -not the stem!
I hope this is helpful. You need to think aboit how you would build it and when you get it right it is obvious (and easy)
Sorry to be blunt but this is the problem, expectations versus reality.
Having done many dozens of these, it is basically impossible to get a fair result without spending a lot of time on it. Even then it will still not 100% match the original offsets/lines plan.
But, the other advise so far is as good as you’ll get.
Do the hull sides first (ignoring the bow area). Try to fair the sheer and chine to use with a sweep2. Even getting the sheer and chine fair and hitting all the points will be tricky though.
Extrude the bow profile transversely and blendsrf between it and hull side. Keep things as simple as possible.
H Ben
With respect the advice will never achieve your objective i.e. accurate. Having spent 7 months bashing my head against the proverbial brick wall to create a full round bilge fair hull in Rhino.
It all comes down to control of the surface created from the lines, however then one has to rebuild the surface for it to be useable, which will distort the shape.
It appears you only have a preliminary set of lines. Nowhere near enough to achieve your objective. There is an extremely useful way of using Orcad3D that helps no end, although Orcad3D do not publish the fact, so I presumed when I informed them that they did not know the capability. Do you have Orcad3D? If not they have a trial.
Please either PM or post your lines, and I will assist.
I disagree.
The issue is with the objective and the word ‘accurate’.
Do you mean that Rhino cannot be used to loft a ship hull?
The the answer is emphatically yes. The limitation lies with the experience of the loftsman.
Rhino is used to loft warship hulls, in my case the tolerance was to 1.5mm. for a 300+m ship
In this case the design offsets were used as a starting point to achieve a fair, lofted hull.
The design offsets are rarely accurate. Errors of several inches are to be expected. The main area being with the stem radii, stem profile and stem sections; another is around the skeg/cutup.
This is tremendously hard work and takes weeks if not months. Some of the comments above reflect the approaches.
However the tendency is to expect the software to do the work ‘automatically’. This approach will not loft a hull and will produce artwork rather than the baseline upon which to build the ship.
As I implied earlier the loftsman must take into account the material to be used by the builder. Only has the the use of moulds and (plastic) lamination allowed the use of what I would call unlofted, but possibly fair, shapes.
I disagree.
The issue is with the objective and the word ‘accurate’.
Do you mean that Rhino cannot be used to loft a ship hull?
The the answer is emphatically yes. The limitation lies with the experience of the loftsman.
Rhino is used to loft warship hulls, in my case the tolerance was to 1.5mm. for a 300+m ship
In this case the design offsets were used as a starting point to achieve a fair, lofted hull.
The design offsets are rarely accurate. Errors of several inches are to be expected. The main area being with the stem radii, stem profile and stem sections; another is around the skeg/cutup.
This is tremendously hard work and takes weeks if not months. Some of the comments above reflect the approaches.
However the tendency is to expect the software to do the work ‘automatically’. This approach will not loft a hull and will produce artwork rather than the baseline upon which to build the ship.
As I implied earlier the loftsman must take into account the material to be used by the builder. Only has the the use of moulds and (plastic) lamination allowed the use of what I would call unlofted, but possibly fair, shapes.
Visit Topic or reply to this email to respond
In Reply To
Douglas1
May 31
H Ben With respect the advice will never achieve your objective i.e. accurate. Having spent 7 months bashing my head against the proverbial brick wall to create a full round bilge fair hull in Rhino. It all comes down to control of the surface created from the lines, however then one has to rebuild…
Good afternoon whoever you are Strobe!
I am not the slightest bit interested in what you create or how you do it, or the time it takes you!
If you care to read ‘understand that is’ it is perfectly clear that I have created a full round bilge hull in Rhino that meets the displacement stability Holy Grail criteria……… Therefore …………. The comment.
Do you mean that Rhino cannot be used to loft a ship hull? Is totally irrelevant!
I also state:
‘Having spent 7 months bashing my head against the proverbial brick wall to create a full round bilge fair hull in Rhino. Therefore it should be obvious that the following comment is unwarranted
The limitation lies with the experience of the loftsman.
However for your information using my procedure any vessel can be created in a week, by a fairly competent Rhino user. Therefore instead of wasting my time I suggest you figure it out! Or carry on taking weeks if not months.
As for ‘the loftsman must take into account the material to be used’ that with respect is utter bunkum.
Regards
Douglas
woah!
my intention was to further the art/science not to start a war!
I assumed that contributors would be as interested as I in the value of second hand experience so as to reduce time wastage.
In the interest of peace, therefore, I bow to your experience.
Changing the subject a bit, when I replied I wasn’t sure so I checked. Orca is related to Rhino; in the old days it was a plugin.
It had the advantage in this instance, that the offset coordinates could be easily adjusted at one go rather than in discrete construction planes.
If I can assist anyone please PM me directly since I will not respond to this topic in order to be abused.
Since I may’ve also offended the original poster, I should clarify my position.
I don’t know why this problem is so hard to solve. If it is assumed that the original lines/offsets were developed off a fair hull then it should be possible to re-create a fair hull model.
Perhaps the mathematical models were different, ie. NURBS versus something else.
Maxsurf has a genetic algorithm approach to fine-tune a surface to a set of offsets, but the user needs to manually get it close first, and there is still a tolerance limit between resultant surface and original offsets. Fairness does not come into this functionality, except for the original control point mesh, only closeness to offsets is really considered.
Perhaps for this hull a networksrf approach would work after generating some fair waterlines also. These should help guide the networksrf.
Even generating fair sections, sheer, keel and waterlines can often be tricky with offsets, so generating a fair surface is an order of magnitude harder again.
May I ask why you need it to be so accurate? How accurate does it really need to be? Since you mentioned that you have fudged this for years, you understand the complications that can arise.
Thank you, Everyone-
Everyone’s advice has been a huge help. To broadly answer people’s questions and state the results:
a) The offsets and lines are from one of Ray Hunt’s popular designs, penned in 1967. It is my company’s 50th anniversary, which Ray established alongside our current chairman and I thought it would be a fun exercise to put one of the early designs to a digital model. As for the request for accuracy and adherence to the Ur-text, I am sure all of you in the industry strive for the same thing despite the reality of what happens once a craftsman’s tool, exotherm from resins or heat from a welder hit the parts.
b) The lines are pretty darn fair as far as a spline and batten are concerned. The lines were put to sheet by truly one of the best naval archs out there and the boats were built by a really high end builder. That said, in the old days there was always room for a builder to fair, say, the bow cone into concavity of the forward topsides or the deadrise into our radiused keel.
c) I have taken mcvltd’s advice and built from multiple surfaces.I didn’t make the bowcone separate, but rather incorporated it into the fairing of the forward topside. This was the original intent after all… to be faired by hand at that point. The bow cone has been distorted to within 1/16" to accommodate the smooth surface.
d) David was right about one of the sources of the problem. Strobe, Ncik and Douglas all of your suggestions helped a lot too (Don’t worry Ncik no offense was taken). I welcome the discourse and don’t mind at all when things are bluntly stated.
e) Overall this has been a success, albeit without having absolute 100% strict adherence. The station that is highlighted below was the maximum deviation (about 1/8") from the middle of the curve to where the surface ended up. That said, I’ll take it. Everywhere else in the hull it matches beautifully. Perhaps it is a coincidence, but the incontinuity of the topsides appears to start with station spacing goes from full stations to half stations. That said, it is also the point on the hull that has the most tortured shape.
Anyway, thank you to all. I appreciate everyone’s help. It is great to bring this classic to life in model space!
Sincerely
Ben Stoddard
