Predicting plywood bending behavior for stitch and glue design

The simple way to design tortured plywood hulls, at least for me, is to build a model. You can then take the offsets off the model and smooth them with a suitable program. The Gougeon Brothers book gives design guidance on how to scale the model. Many successful catamaran and trimaran hulls have been built with this method. The Tornado sailboat was by far the most popular, but I still like the Gougeon's Victor T.

Lohring Miller

 

What kind of car is that in the background center of the photo in post #6?

 

I've got a photostat of Lindsay Lord's conic development booklet (at least that how I recall it right now) buried in a box somewhere at home. You can look up the gist of conic development in chapter 6 of his "Naval Architecture of Planing Hulls" of which several pdf version are floating around the web. FWIW, sheet panels take a curve that minimizes the total stored flexural energy, so a lot of the shape depends on how you restrain the edges. You can actually calculate this with a N-dimensional polynominal mesh if you have the shape (or the forces), but it is just better to make it a developable surface to begin with with curvature based upon the thickness and modlus (Castigliano’s theorems) if doing it by hand.

 

For those interested in math, this paper explains it step by step: http://people.csail.mit.edu/bkph/AIM/AIM-612-OCR.pdf
For some reason I cannot remember, I thought that the resulting minimum-energy curve is a 3th-degree polynomial passing through given control points. In reality it seems that the resulting curve of minimum energy is the one with linearily-varying curvature along the axis of symmetry of the curve, for a simple symmetrical case. The complex non-symmetric curves can imo be divided into smaller symmetrical sub-regions and the above result would be locally valid for each region. The exact shape of the curve, as you pointed out, will depend on the restraints at its ends, but also along the curve.
Tricky.

 

It is a Morris Minor Traveller

https://www.google.co.uk/search?q=m...ASu3IG4Bw&sqi=2&ved=0CDIQsAQ&biw=1920&bih=890

 

For those that use Rhino and other surface modellers you can also use curvature matching techniques vs radii for the construction curves. Look up G1, G2, G3 and G4 continuity. If you use radii the intersections do not have curvature continuity. ie when you bend something around it the intersection has a lot of energy involved. If you curvature match the curves then there is no tangencial intersection but the curvature of the curves is the same at both sides of the intersection so the intersection is much smoother then if a tangent is used. When designing boat hulls I have found that ellipses give the smoothess hull but they don't produce "full" hulls. They produce tippy racing hulls. cheers Peter S

 

Thanks. I almost wish I was a midget, I'd have to have one then.

 

That paper is about curves in space, not surfaces. It's interesting that the result equates directly to a parametric cubic spline using the arc length as the parameter. A very close approximation for most purposes is a parametric cubic spline using chord length between supports as the parameter.

 

Developable surfaces can be created in Rhino using the Loft command with developable option or the DevSrf plug-in with the least twist option. Loft is included in Rhino. DevSrf is a free plug-in which can be downloaded from the Rhino website http://wiki.mcneel.com/labs/devsrf The resulting surfaces with be NURBS surfaces with degree 3 in the curvature direction and degree 1 in the direction parallel to the "ruling" lines. The surfaces will have the "cupping" the original poster inquired about.

 

Hello,

As others answered before, I think that Rhinoceros with the option of building developable surfaces is a great aid in designing boats to be built in plywood, whether stitch & glue or not.

What I learnt a few years ago, and I wrote somewhere that I can not find at the moment, was to use the curvature feature in Rhino, to point all the areas with a radio of curvature bigger than a value given in Gudgeon's book that relates the thickness with the radii of curvature the plywood can be easily bent to. In this way, Rhino would show, for example in red colour, complicated areas, and I could check the areas where I could have problems and take some action if the value of the curvature in there was very deviated from the ratio I knew we could bend the wood.

Also, and trying to answer the original question, now I am just starting to peep at the wonderful world of Grasshoper for Rhino, and I think that it could offer the tools to simulate how plywood can bend, in an easier way that FEM can do it, that for some of us is still very far away and obscure.

Thanks and regards,

 

Gougeon's Victor T cat race, question...

Hi Lohring,

I'm trying to reproduce the patterns of Tornado almost primitive way and using software such as Corel Draw and Sketshup (see attachments).

I did not know the GVT Cat, I was impressed with its smooth lines, but I'm not finding anything about it on the web. Please, do you have more information about plans, drawings or sections of this catamaran?...

Thank you,
Valter.F

View attachment 90590

 

If the axis of the bend is not perpendicular to the center line of the boat, the cross sections will show a curve.

 

FWIW, the Tornado used tortured plywood construction. Double curvature was forced into the sheet by the construction method and hot wetting.

 

Hello guys,

I got these steps in this project:
http://www.thebeachcats.com/Downloads/display/lid/46

I am attaching 3 files and would like a second opinion, ok?





Thanks.
Valter.F

 

I only make strip-built boats, that method is pretty easy. Maybe it could work for you?

I have no experience with stitch and glue.