Developable hull

Discussion in 'Software' started by Newhill, Sep 26, 2013.

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nukisenSenior Member

One thing i suppose you have to take in the calculation. As far as I know Freship makes the unfolding with the length of inside thickness of the plates. So if you have a curvature in a sheet with radius you have to offset the lines a bit. For an example.
You have a curvature with inside radius 2000mm and with a curvature length of 3142mm.
This will be inside the thickness. But when you bend material it will not bend and stretch excactly in this curve. A normal length would be the length about 30% inside the material. Yes this is depending on how stiff material you do use and even the bending method. In this case I would need a sheet with a width about 3147mm if the thickness of the sheet is 10mm. the bending steals 5mm sheetlength in this case. The more narrow the bend is and higher thickness will make this different decrease and vice versa.
I wanted to tell this so you dont cut and recognise you miss 5 or 10 mm when you want to weld it together.

A good suggestion is to have the sheet a bit overlength and then cut it after mounting.
Almost always easier to cut than make insertion in the material afterwards.

Last edited: Oct 22, 2013
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nukisenSenior Member

Imagine what happens when we bended crosscurvatures for the bulb to a ship. The worse i parsitipated in is a forebulb with thickness 40mm. I can not remember what ship it was and why it had this thickness. The only I remember is that it was not an icebreaker. The man who made the folding is not an amateur. Thats for sure.

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TANSLSenior Member

In this forum there have been several discussions on this topic and have expressed very different opinions about it.
I have worked many years in shipyards that built steel vessels. Always, in all cases, we took into account ONLY the inner side of the plates. The plates were developed according to the length of the inner side, cut with numerical control machines and curved using test templates that reproduce the inside of the plate.
This has several advantages, but perhaps the most important is that the inside of the hull has a smooth, continuous, surface, wherein the bottom transverse, web frames, profiles, etc. . fit perfectly .
Therefore, when creating the surfaces of the hull, the inner face is created without taking into account the thickness then the hull´s plates will have. Among other things because, when carrying out these works , it is normal not to have realized yet the scantling calculations and therefore we do not know the thicknesses of the various parts

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nukisenSenior Member

To answer more about what you asked about. This sheet in the hull you posted I think will be no problem to develope. Just lengthen 10 mm and stretch the width about 10mm also and I think you will be very close to reality. And I think you can almost grind and sand the overdimensioned plate in place.

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nukisenSenior Member

Thanks TANSL.
I do agree but not all the way.
In the shipyard I work we did have the overdimensioned plates where we were not sure if the length was enough. The fore was one of these. As we in a sharp bend 30mm plate Over 6meter we would have a gap of 30 mm. Yes you are able to put ceramic and weld a gap of 15mm in each joint. But why do that when you dont need to in the begining?
The smooth bends doesnt makes this differents and then I do agree.

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TANSLSenior Member

Nukisen, each shipyard has its own standards of work and its "hobbies". But I've never seen, since programs appeared CAD/CAM, which were added mm to a developed plate.
Another issue different : If there has been to place plates of 40 mm in a bulb is because something was done wrong.

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nukisenSenior Member

Yes because we did build the next one in the same serie. Then not this thickness in the bulb. So I guess you do have a point there.

Almost every shipyards does have its own standards.

If it was an Ice breaker it would be even thicker. But as far as I know this one was built for mediteranian water.

Maybe it was the program styrbjörn that did add the thickness. The program that today is named Aveva. I am not sure.

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NewhillJunior Member

Problem is solved (i think). Removed the bottom and added a deck (not visible in the picture). As you can see, the colouring has moved to the sheer.
It's just a "flaw" in the program.

Thank you all for your valuable advises. This forum is a "goldmine" for a newbie like me

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PARYacht Designer/Builder

It's not a flaw in the program, it's the edge strains in the panel, being shown.

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TANSLSenior Member

????????????????????

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PARYacht Designer/Builder

Really? Where are the strains concentrated, when you join two curved panels together, along their edges? It's in fact the very limiting factor, to successful developed panel design.

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TANSLSenior Member

Speaking of metal constructions (similar to any other material) :
A plate, to acquire a permanent deformation must be taken beyond its elastic limit. Once deformed, stabilized stresses are not in it.
If the process of joining two plates produces stresses on them, it is necessary to proceed to a stress relief process. It is nonsense to build a structure with big internal tensions (this applies to any material, even when building with wood paneling). So, for example, there are some certain rules for welding plates, edge preparation, welding sequences, and various other things.
In any case, only one study by, for example, finite element can give an idea of the stresses to which the panel is subjected. The 3D modeling programs do not carry out this study. Red lines in Newhill model ARE NOT "strains concentrations".

Last edited: Oct 25, 2013
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The LoftsmanThe Loftsman

Shell plate Development!

Have to add to this that in all my experience plates were always developed to the moulded side as you say the inside of the material, then you also have to take the plate expansion into account which is slightly different to the developed shape!!!!!
That is why it took Loftsmen many years to perfect this art.

Cheers

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The LoftsmanThe Loftsman

Lofting

The difference between developed plates and expanded plates is the result that any plate just flattened by a computer program without taking expansion into consideration will be short in both length and breadth.
Hence we also had to use Bow tables and learn and learn. That is why Lofting was termed as "The black Art" but that's another story

Cheers

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The LoftsmanThe Loftsman

Heat line forming

When used by experienced operators heat line forming will produce a shape with very little stressed material and to a far better shape than any form of rolling..

Cheers

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