Plywood, double curvature impossible ?

[Steggen]

In my mind making a double curvature on a sheet of plywood is impossible, or is it?
Take a look at the pictures in the links below and see. How is this done ?
http://classics.robbeberking.de/comm...l/li-00053.jpg
http://classics.robbeberking.de/comm...l/li-00058.jpg
http://classics.robbeberking.de/commuter/index.php

[hokiefan]

Plywood can be "tortured" to a small degree of compound curvature. But those sections are probably conic sections, which if used well can form a boat that appears to have compound curvature. Some design software is set up to work with conic sections specifically to allow pretty boats to be built from plywood.

Cheers,

Bobby

[wizbang 13]

I like to put a bit of " impossible"bend in my ply boats.
But somewhere on the boat will be a dimple, a large shallow dimple.

[Oldad]

Doable, I have tortured the h@ll out of plywood using straps and screws and anchored it all in place with epoxy.
Amazing what you can do. I am sure, however, there are limits.

[David G]

Another way it can be approximated is by using a 'dart' - just like sewing. But I think Bobby's answer is the germane one here.

[Oldad]

Darts "work" but they do not yield a fair curve. It all depends on the design and how much. The sides of my Ninegret/Handy Billy definitely curve in two dimensions in the same sheet of ply. The tumblehoe on the transom was created with a ratchet and a strap around the entire hull cranked a few clicks each day until the desired effect was achieved. Then I loosened the strap. applied thickened epoxy to the mating surfaces and cranked her down and left it a day or two til the epoxy cured, then taped the joint with 12 oz biax which when done yielded a fair curve—in two directions.

[Tom Lathrop]

Yes plywood can be tortured into compound surfaces but I have only worked with 3mm ply in this way. Even a bit thicker or more plies must be far more difficult as even 3mm can be risky. I would guess that over 3 plys would not be practical for more than a little compounding based on this experience. This project was forming 16' amas of a lightweight trimaran and these surfaces had a lot of compounding and while some cracking was experienced, the resulting hulls stood up fine in hard if only limited time service racing. Many other simpler forming jobs have been done with 4mm such as a base for a compound planked foredeck but still only 3 ply.

[htom]

Plywood can be tortured -- slightly -- into non-developable surfaces. I think your examples are what might be called compound developable, a series of developable surfaces connected at lines where your eye does not notice those lines.

A developable surface has to fit onto some conic section; a different surface on a different conic section may share a three-space curve with a portion of the first and those surfaces may be connected at that line even though there is no single conic section that those two connected surfaces will fit on.

[upchurchmr]

There must be 2 dozen threads on this subject.
Go to the Gougeon web site.
Download the "Gougeon Brothers on Boat Construction" for free.
There is a whole chapter on how to make double curvature in plywood, with several boats shown in detail as examples.
If you can ever find a Tornado Catamaran (former Olympic sailboat) the whole class started as a compounded plywood (4mm Okoume) boat.
There are copies of the plans on line.

The Gougeons show a distinct limit to the compounding possible and a design methodology.

They are not the only people to have designed and built compound curvature plywood boats.

Edit, in the past I owned a wooden plywood Tornado. It worked well and was quite stiff under sail.

[Russell Brown]

The links at the beginning of this thread show some unusually clean and beautiful boat building and yes, there are some serious compound curves happening. The third link shows that it's a long and narrow boat so the longitudinal curve is less than it appears. This is why it is possible to build multihull hulls this way, because most of the curvature is happening in the crossways direction. A Tornado hull is a good example. Very straight in the longitudinal direction with almost round cross sections in the bilge.
Tom L, I usually agree with everything you post, but I'm not sure that compounding gets much harder as thickness increases. Not that it's easy with thin plywood, but as long as the ply can take the crossways curves, thicker ply may compound as easily as thin. Keeping fair lines at the joints is another issue. Tortured ply (or stressform) gets around that with full length panels, but this is a very esoteric method and only works on skinny hulls. I have never used this method, but I very much appreciate the results.
Russell

[MN Dave]

Doable, I have tortured the h@ll out of plywood using straps and screws and anchored it all in place with epoxy.
Amazing what you can do. I am sure, however, there are limits.

I'd say that this took quite a bit of persuasion pretty much as you describe. (2nd link) As Russel pointed out, it is a long boat, and the


The thickness limit for tortured ply is somewhere around 5mm. 4mm has been used for canoes.

There must be 2 dozen threads on this subject.

A least. Like http://forum.woodenboat.com/showthre...irchbark-Style for instance.

[Tom Lathrop]

The links at the beginning of this thread show some unusually clean and beautiful boat building and yes, there are some serious compound curves happening. The third link shows that it's a long and narrow boat so the longitudinal curve is less than it appears. This is why it is possible to build multihull hulls this way, because most of the curvature is happening in the crossways direction. A Tornado hull is a good example. Very straight in the longitudinal direction with almost round cross sections in the bilge.
Tom L, I usually agree with everything you post, but I'm not sure that compounding gets much harder as thickness increases. Not that it's easy with thin plywood, but as long as the ply can take the crossways curves, thicker ply may compound as easily as thin. Keeping fair lines at the joints is another issue. Tortured ply (or stressform) gets around that with full length panels, but this is a very esoteric method and only works on skinny hulls. I have never used this method, but I very much appreciate the results.
Russell

Russell, My comments are based on my personal experience in bending and twisting plywood as thick as 1/2" in the bottom panels of warped plane powerboats at the bow. While these panels often appear to be compound, they are either developable of very nearly so and I recommend safety measures in the form of stress relief battens along the chine. I find that 3/4" ply is not capable of anything that could be called compound in any practical sense and must be laminated from thinner stock. Even 6mm ply takes some critical stress relief battens to avoid rupture in severely twisted bow sections such as nearly plumb stem runabout designs. Localized ruptures, complete failure (complete explosures) as well as delamination along glue lines has been seen is some cases. Its all relative but I am talking mainly along practical lines and, as you said, size and length of the boat are determining factors. On the other hand, there might not have been a single straight line anywhere on the 3mm amas I mentioned.

[W Grabow]

[QUOTE=hokiefan;4088786]Plywood can be "tortured" to a small degree of compound curvature. But those sections are probably conic sections, which if used well can form a boat that appears to have compound curvature. Some design software is set up to work with conic sections specifically to allow pretty boats to be built from plywood. QUOTE]

As the Gougeons state in their manual, compound curvature can be achieved in long slender hulls. I agree with hokiefan; what we are seeing here are conic sections. Even with conic sections, some serious force has to be used to impart significant bend to broad surfaces. The limiting factor is not what can be imagined/designed, but to what curvature practical building materials can be forced to accept. Several conic sections can be connected through common ruling lines to create complex but fair results. I have been using three different connected conic sections per side (topsides) in my recent boats, A "warped" plane bottom can be constructed also by transitioning between three conic developed sections.

[htom]

The conic sections we're talking about here are not only sections of right circular cones, those are merely the simple ones.

[Breakaway]

Check out Ross Lillistone's "Flint" -tortured ply.



Check out post # 92 of this thread: http://forum.woodenboat.com/showthre...ng-boats/page2


Kevin

[capefox]

My next question -- if it is "double tortured, will it check at some point?

[Mrleft8]

Yes.

[Steggen]

Tanks for all input, there is always something new to learn.

Anders

[upchurchmr]

Steggan,

I had a 20' Tornado sailboat of tortured ply.
20 years old.
6oz glass with epoxy, it did not check, split or anything.
Perfectly fine except where the coating was damaged and not fixed with epoxy. 2sq inches.

[S B]

All is not what it seems to be, if you take carefull note of the curve it is made up of simple bends separated by straight sections, the trick is in the cutting not the torture of wood.

[DCBrown]

http://koti.kapsi.fi/hvartial/torture/torture.htm It's explained well here. D.C.

[upchurchmr]

SB,

Are you talking about the Tornado I referenced?

If so I disagree with you completely.

DC,

Your reference is completely wrong.
There were at least several hundred Tornados built in "tortured plywood" with no heat or soaking.

[S B]

SB,

Are you talking about the Tornado I referenced?

If so I disagree with you completely.

DC,

Your reference is completely wrong.
There were at least several hundred Tornados built in "tortured plywood" with no heat or soaking.

I took a closer look, the panel doesn't have any compound curves. Looking at the panel and forgetting the boat lines, it is bent arround an axis parallel with a line, looking at the picture, from the top left hand clamp to the bottom clamp station 3.

[DCBrown]

I don't think any of us are "completely wrong". We all know a sheet of ply can't be bent any which way, there are limits, and I believe my reference is usefull in understanding these limits....D.C.

[upchurchmr]

SD if you are talking about #12 post, I understand. That is not a Tornado. I have no idea about the #12 picture, for that example it might not be compound curvature.
You might be interested in this article and the reference on the plans for the Tornado.
http://www.instructables.com/id/Torn...tructions--79/

DC

The article you reference suggests building "compound curvature" in a method that has failed many times. It does not describe the successful methods that have been proven.
I agree, doing something "wrong" probably will result in a problem.

The one thing that everything written about successful attempts agrees on is that the shapes that can be achieved are very limited, primarily including catamaran and trimaran hulls.

[johnw]

In my mind making a double curvature on a sheet of plywood is impossible, or is it?
Take a look at the pictures in the links below and see. How is this done ?
http://classics.robbeberking.de/comm...l/li-00053.jpg
http://classics.robbeberking.de/comm...l/li-00058.jpg
http://classics.robbeberking.de/commuter/index.php

The bottom looks like a developed section of a cone. The topsides where there's a double curve are being planked in narrower sections of ply.

[DCBrown]

Gougeon Book page 294 "Compounding plywood". Same thing as the reference I posted. No secret. Limited? Yes, what isn't?
Mr Upchurch you must love to argue.....D.C.

[upchurchmr]

I quit.