# Re: Developable Surfaces

DELFTship forum Hull modeling Developable Surfaces Re: Developable Surfaces

#36291
Peter Edmonds
Participant

I’m brand new to DELFTship and its surface modelling, but come with a substantial familiarity with hull shaping from a background as a working Naval Architect.
I have some superficial observations on the previous postings, particularly dealing with the forefoot. Go back to the conic sections, which is what we (self included) were using prior to the computer applications.
Fundamental position – if you can get a series of straight lines (ruling lines in HULLFORM terminology) on the plate surface, it can be fully developable. Consider these straight lines as local press/roll lines for the shell plate where it has to go through a machine, rather than pulling a light plate into place. (I started in steel shipbuilding.)
Flat plate is dead easy – doesn’t require any analysis.
Cylinder elements are rarely an issue – we can deal with successive cylinder elements (along the plate) with changing local radius. A surface with straight line sections at a constant deadrise angle is one implementation of the cylinder elements.
Conical elements are the more advanced geometry. Where this works in the small vessels around the forefoot is to put the cone vertex (or a succession of vertices) below and forward of the forefoot, and on the opposite side of the vessel to the shell plating. Many years ago I built a 12 foot plywood yacht (Gwen 12) which had this geometry, with the chine meeting the sheer line around 1 foot aft of the stem. The chine angle was practically flat where the chine hit the sheer (required for a continuous fair line for the sheer structure). Bottom sections with an increasing deadrise angle going forward, and a small outwards curvature are representative of this.
I believe an understanding of, and feel for the geometry will be of assistance in setting up shell surfaces that will develop.
For many years I have been using Blue Peter HULLFORM for my hull shaping and analysis. This emulates the battens and weights in that one can fair and manipulate individual longitudinal lines in elevation or plan. It was a good training in shell development as it generated the ruling lines (straight on the plate surface) between the two nominated adjacent longitudinal lines.
I hope this is of some value in understanding what is happening is shell development.
Now we come to the issues facing me.
My first DELFTship (and only one so far) hull model is the bottom plating for a small rigid collar boat, where the collar is built from aluminium plate, rolled and then welded into cylinders. The cylinder elements are then mitre joined to form the collar. This is just SO easy in SolidWorks, where I am developing my hull model.
I set up an indicative shape for this bottom structure, with flat plate monohedron aft, and conical plating forward. There was a tangent line between these surfaces, running from an aft position outboard to a further forward position at the centre profile. This was done with pencil and paper at low precision, using nominal cones and generator lines.
I set up a centre profile line, a pseudo chine line, and a nominally mid plate line. The pseudo chine line was to run within the cylinder, so that the bottom surface would be trimmed by its intersection with the cylinder within SolidWorks. The mid plate line became a non-crease line inside DELFTship.
My issue was that I was not able to generate green and reduce red in the developable display by small movements of the mid-plate controls in or out. This was counter-intuitive, as I knew I was dealing with a surface close to developable.
Part of the issue was that areas within what I intended as flat plate (monohedron deadrise) were indicating as red – curvature must have been carrying across, contrary to my design intent.
What do I need to do here to get a developable surface within DELFTship? I haven’t seen how to pass the straight ruling lines into the DELFTship model.
Subsequent to the DELFTship attempt, I generated offsets for the pseudo chine and centre profile lines at nominal transverse section positions into HULLFORM, faired them, then did the plate development. This gave me the full range of output – rollout shape with coordinates, transverse sections, and the 3D points of the two ends (at the longitudinal lines) of the ruling lines. These latter I have moved into SolidWorks as 3D points.
If anyone can see a way of addressing this issue I can forward my current DELFTship model, and the orthogonal views of the 3D lines within SolidWorks. Please contact with e-mail address.