# Semicircular hull shape in Freeship

Discussion in 'Software' started by Elgato, Dec 29, 2022.

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### ElgatoJunior Member

How do you create an semicircular hull shape in Freeship? I am playing around with various hull shapes and would like to create a catamaran hull with a constant station radius throughout. semicircular hull midsections with vertical or near vertical hull sides, and tapering off in the ends with short transition zones to straight end sections with the same constant radius so that the forward section get increasingly sharp bottom, whereas the stern tapers off upwards or diagonally, meeting a flatbottom Malcolm Tennant CS-stern. Buildwise it would correspond to bending a massive flat sheet to a U-form (and fixating the radius along the hull), then cutting out the midline forward and a couple of cuts at the stern, and the some Kelsall cuts in both transition zones for the double bend.

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### kapnDSenior Member

A picture is wort ten thousand wordsâ€¦

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### ElgatoJunior Member

Something like this: all the stations' curvatures have the same radius. (Lengthwise, the midsection is constant for about half the boat length)

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### HJSMember

What is it you want to achieve?
Is this a sailing or power catamaran?
JS

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### ElgatoJunior Member

To answer the first two questions: I figure on could make a simple hull build with good finish by laminating a sheet on a table and the bend it along the keel line with no constraints, that would produce a U-shape with constant radius (depending on the sheet properties of course). If this radius is fixed at this stage, before the tapering off at the ends, no further transverse bending/shaping would be required, only longitudinally. A little along the lines of late Kelsall's KSS. When I play around in Freeship I can produce a lot of shapes with sharp and rounded shapes but I haven't found the way to mimic a (half) U-shape. It will be a power cat.

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### philSweetSenior Member

Start with the cylinder tool and then modify the midship cross section, then extrude the midsection to the full length of the boat. The displacements at the bow and stern can be achieved by transforms station by station. The particular implementation of subdivision surfaces in Freeship was chosen so it can render perfect circles. But you need a regular grid for it to work.

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### ElgatoJunior Member

Thank you very much! I saw that there existed a cylinder tool but was unsure if it could be used for hull design (and analysis) and how to use it, but now I feel confident to put some effort in it and give it a try!

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### HJSMember

If it is to be a power cat, it should be designed as a powercat, a split power boat.

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Last edited: Jan 2, 2023
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### HJSMember

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### wet feetSenior Member

Are you aware of the Constant Camber building method?It may give rise to a shape that isn't so constrained.

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### ElgatoJunior Member

Sure, I know Constant Camber. I have limited but existing practical boat building experience, but regarding hull building methods, I think I am aware of all methods used to date. My knowlegde gap right now is CAD/Freeship, and more specifically how to create a smooth, constant radius bottom.

Last edited: Jan 3, 2023
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### ElgatoJunior Member

Yes, that is a simple way to build a hull with good finish, hard chine with flat boards bending only longitudinally. But there will be many, many meters of taping the boards together and external finishing. That could be compared to a half tube with no post-sheet taping or finish works required for most of the hull, a forward double curved transition zone (with a meter or so of taping and of a lot of finishing), and a straight stem with taping / finishing only in the midline. I believe the latter could be less work, but in any case, I would like to be able to design it in Freeship.

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### ElgatoJunior Member

I disagree. A power cat could be designed as a split powerboat, but there are many other solutions, depending on displacement, bottom area, requirements of speed, power, efficiency/gas consumption etc. But I don't want to discuss design in this thread.

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### ElgatoJunior Member

Ok, I got stuck right there. Very frustrating, but I don't want to give up. I would need a detailed step by step instruction (for dummies ), so please?

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### HJSMember

Build a one-meter model and see how your method with a flat surface can possibly be applied. The Constant Camber method is something completely different.
JS

Last edited: Jan 3, 2023
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