# Section after Section, How designers get a smooth continious 3d surface ?

Discussion in 'Hydrodynamics and Aerodynamics' started by mustafaumut, Aug 18, 2015.

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### mustafaumutJunior Member

When we look to the line drawings of an sail boat from bottom or from front , we see curves of sections. I am not sure , it is called as section but this is my comment.

I could not understand which collection of formulas resulting to smooth continious curves around hull , there is no bumb or other thing.

For example , we have a herreshoff design , someone removed a section from the plan , how can we redraw it ?

Thank you very much,

Mustafa Umut Sarac
Istanbul

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### NavalSArtichokeSenior Member

There are no formulas, or collection of formulas, making up a boat's lines.
The lines were all drawn by hand and "faired", which means the shape of each curve was adjusted so that intersections in all three views, the body plan (station view), the profile, and the plan, agreed with each other.

If you want to reproduce a given curve, overlay a gridwork on top of it and take offsets from a known reference along the length of the curve. One such gridwork usually is the stations, waterlines, and buttocks which you see drawn on a typical lines plan, like below:

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### mustafaumutJunior Member

I want to ask new question related to two posts above.

I read few pages at google and understood that every calculation starts after the line drawing been put.

Is there a method which hydrodynamic and stability and cfd results been put BEFORE the line drawings drawen ?

Thank you,

Mustafa Umut Sarac
Istanbul

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### NavalSArtichokeSenior Member

Not unless you want to obtain results which are meaningless.

The stability of a vessel depends on its lines to a great extent, and, to a lesser extent, so does its hydrodynamic performance.

As part of the design process for a vessel, you can estimate certain things about the final hull shape, like the block coefficient or the waterplane coefficient, which can be used before the lines are drawn, but the stability of the vessel must still be calculated after the lines are completed to confirm these initial estimates. So, too, for its hydrodynamic performance.

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### philSweetSenior Member

It's a very good question, Mustafa. A serious issue with what you ask is where do you get these requirements from a priori? They are highly derived quantities that are the result of experience with real hulls' geometry. One should not assume that any hull shape that has equal values will perform equally.

With that in mind, the technique is called reverse design. There is a free program called Godzilla written by a member here that will iterate until told to stop (it does not use CFD). It will try to find the lowest drag shape that complies with a large number of possible user defined restrictions. However, the real trick is knowing what those ought to be when it comes to stability and such. It is free to download and worth learning to use IMO.

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### HeimfriedSenior Member

That is correct, but Sir Isaak Newton gave us a formula, which creates from an input of a few points (in x,y-coordinates) a 2-dimensional polynomial curve.

The results should be checked, but in most cases, any calculated point in the interval is on a fair line.

I did an Excel-sheet (2002) which calculates it. It is downloadable as zipped file at
https://www.boote-forum.de/showpost.php?p=3751040&postcount=11
(Blue colored cells = input; blue colored cells to change only; don't touch other cells; reddish cells = output; "Diag" Tabs for checking the calculated polynome graphically; separated tabs for 5 given points (Stützpunkte), 4 points, 3 points)

It should be possible to find the offsets of a lost frame, if you input the offsets of the nearby frames.

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### TANSLSenior Member

The frames can be deduced through the water lines. If you have those lines do not need to know the nearby frames. If you have no water lines, you can not get any additional frame. You can interpolate, as seems to suggest Heimfried, but the result can be very inaccurate. I think the right way is by graphic procedures.
If we talk of a 3D model the situation is totally different.
Nor do I speak of hulls with chine or knuckles, for whom the problem is still more complicated

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### RurudyneSenior Member

Just an aside: Hercules had a gorgeous sheer line, well, lines in general I suppose. Yes, underwater she'd not be modern, but a yacht with all the modern improvements where they can't be seen could be a stunner with those lines where they can be seen.

Of course I write that as one with a way too anemic bank account to even hope to try....

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### SukiSoloSenior Member

As pointed out above, all three views have to 'agree' dimensionally. If they do not, then some part is unfair. If something is missing in one view, it can be recreated by using the one or two views. Basic drafting techniques should let you do this whether on paper or CAD.

As for checking the 'smoothness' of lines and fairing, that is largely a function of the diagonals used. These are often shown as draft curves on lines plans, commonly in the plan/waterline top view.

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### HeimfriedSenior Member

Just to give an example of the application of Newton's formula:

The first picture shows an offset table of a boat.
In the first line are the stations (x-coordinate, longitudinal) given in mm (point means not decimal separator, but serarates thousands).
In the second line shows the hights of the sheer (z-coordinate, vertical) at the stations.

The second pic shows six marked stations and the separately marked number "1.288" is assumed to be lost.

The third pic shows the calculation sheet with the nearby values of the offset table inserted (x1, y1, x2, y2, ... Excel-cells B6 to C10). In cell B28 the station with the lost value is inserted (x = 3.200) and C28 shows the calculated y-coordinate (z = y in this case).

The result of 1.286,6 instead of 1.288 is within the allowance, I think.

The last pic shows a diagram of the Newton polynom to check, if it is fair. The diagramm is normalised (non scale), as it has only the purpose, to prevent false results, caused e.g. by faulty insertion of coordinates.

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### Squidly-DiddlySenior Member

I'd take the frames you have, enter them into one of those computer hull performance programs, and let the computer fill in the blanks.

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### AlexanovSenior Member

Nice lines!

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### AlexanovSenior Member

Ship design process based on iterations. With every new run you try to make surface better and better. Start point of this process depends only from designer experience.

14. ### OleboynowPrevious Member

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### jehardimanSenior Member

Actually, you start the design with the requirements. See the linked post.

http://www.boatdesign.net/forums/boat-design/lines-plan-29882.html#post310553

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