Designing developable surfaces in Rhino

The shapes in the photos with flared bows are fundamentally not developable so there are no "developable directions", whatever that means.

 

The "little steps", are they coincident or tangential to the curvature that they are not bending in? (the curvature along the width of the strip) Or somewhere in between.

 

As I understand your question, they are 'coincidental', but I am not sure that is important.

To explain to you and D Cockey what I mean by "developable directions" they are the direction that a thin strip of plywood or timber will lay down without stress over a geometric shape. As plywood and strip timber can only bend in "developable directions", its not that complicated.

In a concave half sphere, you would have ~ 360 "developable directions" for a strip of plywood. In a concave bow design, there are normally at least two - which is good if you need two layers of plywood. A "non developable direction" would be encountered if you tried to lay a strip over hull section where it would split if forced to lie flat. This is much more common on strip hulls than when using short strips of ply, but you can encounter them ( across the bow stem is an obvious one, intermediate chines is another)

Dont get caught up in any kind of definitions - as they boil down to "the builder lays the strips down in a convenient direction to verify that they will lie evenly across the surface"

When building, especially in plywood strips, its mostly obvious, and not worth prior drafting. The most work you can make of it is to cut out test strips of mdf, to see how wide you can go on different sections before hacking up the expensive plywood.

 

So far correct.

Now "developable" is being used in a non-standard way which greatly complicates comunications.

A spherical surface is non-developable by any normal definition of developable. It curves in two directions. A flat sheet of plywood can not be made to exactly conform to a spherical surface without stretching, which can lead to splits or buckles. If its narrow enough it can be bent so that it is close to the surface, and the narrower the strip the closer it will be.

 

Its not the hull shape that is developable, its the material being laid. OK - what do you call it "Layable", "Fittable", "Flattable" ???

Simply, in a complex shape, there are often limited places where single plane material like plywood can be placed accurately enough ( near enough ) to fill in the shape.

What term do you prefer to describe the direction that a one plane material can be laid to fit onto a compound curved shape ?

 

I don't think I'm explaining this well. In order for the strips to lay coincidental the veneer would have to be doing a very small about of stretching. If the veneer doesn't stretch at all then the strips would be lying tangential. In the second situation fairing would be sanding off the edges of both edges of the strips with less sanding in the middle.

 

Hmmm, dont forget that the veneers are usually run in 45 degree layers, so there might be a bit if both.

I know from a building point of view, the ideal is to avoid the work in special shaping. Its always easier to "fill in" areas with a few 'stealer planks' than try to shape a lot of pieces smaller to fit them into spaces. If you are lucky, you will try to do both layers so that you dont need to reduce the 'standard strip' very much.

The biggest challenge is if you are doing a clear wooden finish, in which case the top layer usually runs in the traditional 'planked' horizontal direction, so that you have to do the underlying layer more or less vertical.

I suppose you could get a program to calculate the 'optimum' layout of both layers to reduce the amount of special shaping, but its really only half an hours test placing when starting the planking, to suit the particular hull.

I know Delftship does steel plating layout. If it lets you specify the 'standard' plate size as narrow strips, maybe that would be one way to get a calculation done.

Maybe you could do a couple of rough sketches to show what you want to achieve ?