Developed Surfaces????

Surfaces CNC and Models

Hi Chris,
I am new to this site but not to boats. There is a product called BobCAD that can generate a surface from the section views of a boat. I have been using DesignDAC with elipse fuctions as they are used a lot in pretty boats. They can be imported to BobCAD via DXF files and machined with the model upside down, Keel up.
If I can help let me know. I have a CNC router I got to make small boat hulls.
Best Regards,
Bruce
bhltermann@sealeze.com

 

How much can you torture Ply?

What sort of gaussian curvature could I tolerate when designing a hull to be built in 3-4mm tortured ply?

If I place a sheet of playwood on a flat surface but add a distance in the middle, the gaussian curvature of the playwood does not as far as I understand it equal zero if the all edges of the sheet touch the flat surface.

Where could I find information on how much gaussian curvature is achievable?

Br

Patrik

 

Patrik

It's not a matter of how much Gaussian curvature you can tolerate, it is dependent on the amount of elongation required in the panel once it is developed.

When you take develop a doubly curved panel with positive Gaussian curvature, center of the panel will need to be stretched to form the curved shape, whereas with a negative Gaussian curvature panel the edges of the panel will need to stretch.

As you make the panel larger, the amount of elongation required increases. If the amount of elongation needed is less than the natural elasticity of the panel material, the panel will can be regarded as being developable for practical purposes, even though the Gaussian curvature is non-zero. It is these elongation or strain values that you need top look at.

As panel sizes get smaller it becomes easier to accomodate the elongations required, so with steel or aluminium vessels it is often possible to handle double curvature by breaking the panel up into smaller plates. This is not really viable for a plywood boat where typically you will scarph together smaller sheets of ply to make one piece that can cover the whole panel.

So, in summary, you can't tell much from the magnitude of Gaussian curvature, you really need to develop out the panel in a program such as Workshop and look at whether the maximum strain values fall below certain values.

Typically for steel and aluminium I recommend minimum strains in the developed plate of less than 0.25%, although some Maxsurf users have had success up to 0.4%. I don't have a value to recommend for plywood, but expect that it would be slightly higher than this.

Hope this helps

Andrew

 

Note that I did finally get around to sending in the developable surfaces article to our kind host and it is posted in the article area.

As regards torturing ply, the answer to "how much" is to calculate the rolling shear stress based on the distortion and to keep it below an allowable, (which is actually frightening low - WPA allows 75 psi). If you exceed the rolling shear stress, the panels delaminate as you bend them in.

 

I wonder how TouchCAD is with this stuff.

I've certainly heard that the devsurf function in MultiSurf works as well as anything at taking two random chine, keel/rabbet, or sheer curves and producing sections that will make the surface developable. What it doesn't do is help you match the two longitudinal curves so that the surface is truly conic (meaning a patch from a general cone or a general cylinder). This can be done, of course, in a 2D CAD program using traditional drafting methods.

The "Mistral" designs that race in the Classic Moth development class (primarily in the mid-atlantic & S.E. United States), and the new varient known as a "Mousetrap" are impresive tortured plywood shapes, for whatever that's worth.