Developable Surface

Hi there , I did design an aluminium boat and I need to check if my boat bottom surface seen in the attached is certain to be a developable surface that will be buildable in aluminium 5mm. I did validate my boat bottom in a number of ways:

  1. The bottom surface does unroll in Rhino " unroll developabale surface" command
  2. The bottom surface does unroll in Paneling Tools " unroll faces" command
  3. The Rhino model units are millimeters. The bottom surface when analysed with the gaussian curvature analysis ; the curvature range between - 0.00001mm and +0.00001mm results in the color green with very slight bluish at the tip of the bottom. When analyse is -0.000001mm- +0.000001mm result is similar slight bluish aat tip and towards center
  4. I did make a scale model in steel scale 1:10 in steel plate thickness 3mm and the bottom did bend corectly on the shape
  5. I did a check by te Rhino command " curvature and I did measure the curvature on an array of 5 points on the FWD section of the bottom surface. The values seem to indicate very little deviations from zero at the Minimum Principal curvature.
    Is the surface a developable surface? Thank you

Note - @andreirochian and I have posted about this at the Boat Design Forum

Two similar but different questions:

  • Is the surface a developable surface as in an exact mathematical developable surface with Gaussian curvature and one principal curvature exactly zero everywhere?
  • Is the surface close enough to a developable surface to be buildable?

An answer to the second question requires welded aluminum boatbuilding expertise which I don’t have.

To answer the first question the surface is very close to but not an exact developable surface. In general the Rhino tools such as DevSrf and DevLoft for creating developable surfaces from two curves generally result in surfaces which are close to but not “exact” developable surfaces.

An exact developable surface has zero twist along the ruling lines. Previous thread with method to check twist along a ruling lines of a surface: Ruling line from edge curves twist check

Previous thread with method for obtaining exact ruling lines from a pair of edge curves: Developable surface - exact ruling lines from edge curves Anyone who wants to use this method should read the entire thread as there is some important information in later posts. I use this method to obtain ruling lines between edge curves, and then use Sweep2 with the edge curves as rails and the ruling lines as sections to obtain a surface. With sufficient exact ruling lines the surface can be as close as desired to an exact developable surface.

Previous thread about creating a developable surface for a boat from edge curves: Developable surface

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Hi David, thank you for your feedback wich is indeed very helpful.
"Anyone who wants to use this method should read the entire thread as there is some important information in later posts. I use this method to obtain ruling lines between edge curves, and then use Sweep2 with the edge curves as rails and the ruling lines as sections to obtain a surface. With sufficient exact ruling lines the surface can be as close as desired to an exact developable surface. " I am now working on this: read your posts and your method and I try to do exactly as per above.

Interesting thread on boatdesign site. I saw an aluminum 24’ center console aluminum deep V fishing/sport boat built in Ft Launderdale that was done in a novel manner. It was sitting upside down still on the building strongback and I was amazed at how fair the topsides and bottom panels were. It was fully welded up along the keel, chine, and sheer and around the transom but I could see no evidence of welds other than the perimeters of each hull panel. You can always see the heat distortion of skip welds done on the inside along the stringers and there was no sign of that on the outside. That led me to crawl inside to have a closer look and I was surprised to see standard “bulb T’s” in use for the longitudinal stringers but they were “upside down” as in that the flange was against the hull and bulb which is usually welded to the hull skin was towards the inside of each frame! It was pretty dark inside the hull and close to sundown and I couldn’t really see the details or why the bulb T’s were capsized from their usual orientation. Before my eyes could adjust to the dimness, I heard tires screech to a halt right next to the hull and could see a golf cart had arrived in haste. My feet were still sticking out from under the hull and I expected some security goon to grab me by my ankles and drag me out from under the boat. I had been accosted at the boatyard just next door the day before and threatened with a beating in the alley for merely taking photos of a developmental hydrofoil watercraft but had managed to talk my way out of that offence. This boat was not within a fenced compound but just up in a corner near a fuel dock that was totally open to the public.
It didn’t occur to me until I just wrote this but why would one do a welded boat project in any sort of proximity to a fuel dock in the first place? I extracted myself from under the boat to find a huge guy sitting in the golf cart who simply asked what I thought of the boat…
He had the golf cart listing like a sailboat in a gale from his weight and must have weighed over 400 hundred pounds. He seemed friendly enough and to really want my opinion on the project. I sort of stammered and said that it looked pretty good but added that the builder must have no idea of how bulb T was meant to be used.
He laughed and handed me a big flashlight and told me to go back and take a closer look. I soon could see that the reason for the inverted T’s was that there was a generous and continuous bead of silicone caulking between the wide flange of the T and the inner surface of the hull. The bulb sort of floated in a longer than needed slot cut into each frame and there were wedges driven into the slot to force the stringer outward. Some stringers were still floating in their slots, some still had the wedges in place, and some had been welded to the frames at the bulb.
When I went back out to the guy in the cart, he pointed out a big drum of silicone with an air powered pump and hoses leading under the boat and he told me that they we doing the last of the pairs of stringers the silicone bonding in the morning and I was welcome to come watch. He told me to jump in the cart and we went back to his office where he poured his next ‘sundowner’ and handed me one as well and showed me photos of the whole build as well as a finished boat that he had displayed last year at the Miami Boat Show.
The hull panels would be designed to be developable but by having them get driven outward by the wedges in the frames forcing the stringers out they could compound the hull panels after they had been welded around their perimeters. There would be a couple of guys on the inside hammering in the wedges at the direction of a couple of guys with a fairing batten on the outside checking the fairness.
They would keep going from frame to frame, stringer to stringer until satisfied with the results. Then they would back off of one stringer per panel and squirt a fat bead of silicone and then wedge the bulb T back to just shy of where it had originally been wedged in order to leave a nice soft pad of the resilient caulking. That mean they could do four stringers a day and let the silicone cure overnight. Once cured they would wedge it back out a bit more to compress the silicone and then weld the bulb T to the frame. After all the stringers had gone through this process the result was a nice fair hull with no weld distortion that rode quietly on its ‘shock absorbing’ silicone bedded grid. No fairing compound needed and no ‘oil canning’ over frames and longitudinals.
They would use a 8" ‘soft pad’ on a low speed polisher and create an ‘engine turning’ pattern that looked like fish scales on the bare aluminum and then used clear Awlgrip over and the boat really turned heads at the show and on the water. It seemed to me like they were able to get maybe 1" of bulge max in the middle of the 24’ x 3.5’ panels and it seemed like a good way to get around trying to predict the subtle arcs as described in the boatdesign thread.
I think the big guy in the golf cart got a patent on the procedure so I called the yard about a year ago as I had a project that I thought we might try it but the fellow had died about three years ago and they didn’t want to discuss the process. Patent is probably expired as well.


Hi Jody, thank you for your feedback; yes I did read your above and the suggestion is right. Yes , indeed I was not planning for the transversal framing to be welded to the hull bottom but just for the longitudinals and yes I was thinking about how possibly to make the building process so the fairness of the hull would be good starting hopefully with this present prototype. My builder is not vastly experienced but does posess good skills and does have the right equimpment. Yes the "floating "
transversal framing structure is the way they do it with series aluminium boats, where the transversal framing would bot be touching the inside of the bottom hull.

You have a nack for telling a story. I work in a small company that builds aluminum boats. I will have to look into those t bulb stringer pieces you talked about.
Do you know of any resources a person could learn about the technique you described?thank you,Mark