How to design a straight edge on a developable surface

[DCockey]

I posted a note on how to design a straight edge on a developable surface on the "Boat Design" forum. Straight Edges for Developed Surfaces - Method for Designing

It uses the ShortPath command in Rhino which draws the shortest path between two points on a developable surface. Does other software commands have the same capability to determine the shortest path? Does any other software have another method for mapping a straight line on an unrolled surface to a developable surface?

[DCockey]

To clarify, what I'm talking about is a curve which goes across the ruling lines of a developable surface and is straight when the surface is unrolled, not how to determine the ruling lines.

[Gilbert]

Surely you must know this. Any two points on an unrolled developable surface are or can be connected by a straight line. I think you have to clarify your question.

[DCockey]

Originally Posted by Gilbert

Surely you must know this. Any two points on an unrolled developable surface are or can be connected by a straight line. I think you have to clarify your question.

Yes, I know that. The corresponding line on the curved, rolled surface will not be straight unless it happens to coincide with a ruling line. It will be curved, and will be the shortest path on the rolled surface between the ends of the curve.

My questions are:

Does other software than Rhino have the capability to determine the shortest length curve between two points on a developable surface which is not flat?

Does any software have a method for showing the curve on a curved developed surface corresponding to a straight line on a flat, unrolled surface?

[Felix Muehlhoff]

Yes, NX and Catia can do this. I'm sure there are also other software that can do this.

Regards

Felix

[DCockey]

An example of what I'm interested in doing is designing a plywood boat so that the sheer and/or chine edges of the side panels will be straight when the sides are unrolled flat. In general these edges will intersect the ruling lines. Another another way to express it is to determine a curve on a developed surface which will be a straight line when the surface is unrolled.

A special case sometimes seen in simple plywood boat design is if the ruling lines are parallel and lie in the section planes perpendicular to the centerplane, and the sheer and/or chine are perpendicular to the ruling lines which means they will be a straight line when viewed from the ends as in the section view of a lines drawing. This is obviously very restrictive though.

I've been looking for a way to develop the more general case. That's where the realization that the shortest curve between two paths on a developed surface will unroll to a straight line. Since Rhino has the ShortPath command it's straight forward to develop a preliminary design, unroll the preliminary side panels, and if the sheer and/or chine are close to straight then go back to the preliminary design and create a new sheer and/or chine using the ShortPath command.

If software has a simple way to map a curve or line from the unrolled surface to the rolled surface then it would simple to draw the desired un-rolled edge shape on the unrolled surface and then map it back to the design shape. Unfortunately I haven't been able to find that capability in Rhino.

[ancient kayaker]

I have used sheer planks with straight sheer and chine edges in several of my canoes. I go a little further and keep the sheer angle constant. The primary reason is to maximize material usage and simplify construction; a decent looking canoe with a respectable performance can be obtained from a single 4 x 8 ply sheet using this approach - single sheet designs are an obscure but intersting offshoot from boat design. I have also applied the same system to a sailboat, with excellent results.

It is a very restrictive approach as you noted. However, a significant advantage is the ability to produce a design without recourse to computer software or advanced drafting skills.