Any tricks to torturing plywood??

[Dale R. Hamilton]

I'm about to fasten the 1/4" okoume ply sides to the hull frames of Ray Sargeants Sport Runabout. Because there is a considerable tumble home, there is also a hell of a radius to bend the plywood. I have it sort-of clamped in place- up to the point where the ply is threatening to split. I guess I'm hoping if I leave it in place a day or two- it will get the idea and I'll be able to crank it the remaining inch or so. Any tricks that might help? I'll post pix after the holiday.

[Garrett Lowell]

I would try a couple of pots of hot water. Or you could hold a sawzall in front of the piece giving you a hard time and say something like "Perhaps this will refresh your memory!"

[mmd]

An observation that comes too late, is mostly irrelevant, and would be too expensive, but...

Plywood is such interesting stuff to engineer.

Plywood will usually bend more fairly to a curve "on the bias" (when the axis of the curved surface is 45 degrees to the ply orientation rather than in-line with the wood fibres of on set of plies), but because the ply has greater tensile strength in this orientation it resists bending more, making it harder to wrap around tight curves. Oops! Back to the bending problem!

When on the bias, the stress on all the plies' fibres is at the same 45-degree angle; when on-axis the stress is perpendicular to half the ply fibres and parallel to the other half. From an engineering perspective, since wood fibres are stronger in tension parallel to the grain than perpendicular to it, when laid on-axis only half the fibres in the sheet of plywood are able to resist the tension of bending to their fullest strength, so the ply bends easier. The bias-laid plywood contributes tensile strength from all plies, therefore making it more difficult to bend, but also it spreads the stresses more evenly throughout its entire matrix, making it adopt a more even, fair curve around the bend it is being forced into. This even distribution of stresses also makes the bias-laid ply hull stronger longetudinally and transversally than if laid in-line with the centreline of the boat. This means that with bias-laid plywood a thinner hull can be built for the same strength, achieving a reduction in weight which is almost always a good thing.

However, such construction is somewhat wasteful of materials and therefore expensive. The solution? How about multiple thin layers rather than one thick one?

Gee, if making the skin out of two thin layers of ply set on the bias is fairer and stronger, why not multiple plies of solid wood veneers?

PRESTO! - diagonally-laid cold moulding! Best strength-to-weight ratio of any reasonably accessible and priced home-build method, and better than most commercial materials and processes short of pre-preg aramids and carbon fibres. Whoo-eee, Bucky! If'n ya wants to go fast and light, this is the way to go! The penalty? Larger-than-normal labour hours to build. But then a home build boat doesn't require paid labour, so what the hey?

But this doesn't help your problem.

Sometimes judicious application of heat will make plywood more pliable. Maybe a paint-stripper heat gun will be of some help without introducing the moisture that Garrett's hot water would. Just heat it up slowly - too much sudden thermal stress will damage the outer plies.

G'd luck.

[Dale R. Hamilton]

ahh, thanks mmd- appreciate the discourse on plywood- but of course this stuff had to be scarfed UP to 16'- not much chance of utilizing bias. Still the suggestion of heat seems pretty good. I have it clamped now- a little heat might help. Thanks for the suggestion.

[Bill Perkins]

Dale you can drap heavy towling over the ply and then pour boiling water on that untill it's saturated .

[George Roberts]

Potlatch Plywood (or what ever their name is) will custom make plywood. To 8'x60' or some outrageous size.

Perhaps a vacuum would help.

Bending plywood (wigglewood) might help.

[Bruce Hooke]

Originally posted by George Roberts:
Bending plywood (wigglewood) might help.The bending plywood I've seen is not anything like marine grade. Also, its structural properties are rather different from standard plywood so a substitution would not be wise in my opinion...

[kc8pql]

Originally posted by Bill Perkins:
Dale you can drap heavy towling over the ply and then pour boiling water on that untill it's saturated .That's what I'd do. You'll also get to find out if it really was made with waterproof glue.
:eek: :eek:

[pipefitter]

If you could get a ratchet strap around it it will bend it more evenly. Even a couple of them spaced apart a couple feet or so.The strap will help distribute the stress evenly across the face of the ply. The trick is putting the ratchet mechanism in the right place. Afer a couple tries it will tell you where it needs to be.
If it is real extreme you can slip ¼" battens under where the straps contact the ply to help give the ply surface a strongback so to speak. Can also use this method to sneak it around as you fasten it.
If you dont want to bother with the straps,rope used as spanish windlasses work also.
Another thing you may consider is clamping the plywood in the center of the radius and pulling it down from center out both ways. You can hook two ratchet straps together with one ratchet on one side and one on the opposite so as to pull it evenly from center to edge both directions.

[ 07-01-2005, 05:18 PM: Message edited by: pipefitter ]

[Gary E]

Bending Plywood is forcing it where it does not want to go, half of the cross section is in tension the other half in compression. Now as we have been told here by so many people that the glue line in the plywood stops moisture penetration, so just how much good does wetting one side wether it be the convex side in tension or the concave side in compression since it's stopped at the first glue line anyway?

[Bruce Hooke]

Originally posted by Gary E:
Bending Plywood is forcing it where it does not want to go, half of the cross section is in tension the other half in compression. Now as we have been told here by so many people that the glue line in the plywood stops moisture penetration, so just how much good does wetting one side wether it be the convex side in tension or the concave side in compression since it's stopped at the first glue line anyway?It does seem to do some good...it probably does more good on, say, 1/4" 3-ply plywood than on 1/2" 9-ply plywood. It is after all the outer layers that have to deform the most. Also, a major part of the point of water is the heat, which can migrate across glue lines. With that in mind I would certainly try mmd's suggestion of straight heat first before going to lots of water...

[Tom Lathrop]

I'm not familiar with the boat you are building but have done extreme tumblehome on runabouts. The failure mode I noticed was separation of plies on the glue line near the top edge. This was on a stitch & glue boat of my own design. In this case, S&G was the enemy since the chine was securely anchored in place before bending began. This meant that all of the movement was required on the top edge of the panel.

The suggestion to secure the center of the panel first and then bend both directions is a good one since the differential compression/tension stress of bending will be distributed in both directions and less likely to rupture the panel. If the boat is built on a mold, this will be pretty easy but more difficult in the usual fold up S&G construction.

The accepted method to bend any wood is to start the bend at the point of minimum radius first and work toward the softer curvature. This also minimizes local stresses in the material. Sometimes this is not possible though.

I like to use some form of stress relief cover on extreme tortued surfaces. Banding helps as well as wood battens or a sacrificial top layer of plywood. Unless the side panel is to be fixed to a substantial sheer clamp/inwhale, it will try to curl in toward the transom in plan view so a temporary reinforcement to the sheer clamp will be necessary until the deck goes on.

Heat does help to bend plywood but less than most think and it is nearly impossible to get the heat over a large enough area to do your job. Enough heat to ease the plywood and the point where it scorches is pretty close also.

[ 07-01-2005, 09:42 PM: Message edited by: Tom Lathrop ]

[pipefitter]

I know this is going to sound crude but sometimes you just have to get dirty and ugly. The best way to evenly heat a sheet of ply is to put it in the sun with a piece of roofing felt rolled out on it.Dont ask me how I know this. smile.gif

[Mrleft8]

Just put it in a room with my mother-in-law... After about an hour it'll conform to any shape you want just to get out of there! :D

[BillyBudd]

I did several things for tough curves in plywood. First, I went to two sheets of ply instead of one. So, for me it was 2 sheets of 1/4" instead of the specified 1/2". On the first sheet, got one end in place with (in my case) a butt block and epoxy and let that set up overnight. Then, using a system that has glasscloth and epoxy over the ply, drove as many temporary sheetrock screws into the ply and into temporary cleats in the boat. That got the shape started. Next I drilled holes for a Spanish windlass in the ply (doing two sides at the same time, right?), brought the rope through and twisted and twisted. This brought the unwilling plywood into the forms. Set lots more sheetrock screws, epoxies everything up and let it set up. Took off the rope and the screws the next day and then epoxied the 2nd sheet to the first doing all of the screwing and windlass stuff yet again. Removed everything and--voila!--plywood in place.

Alternatives might include cutting ply into strips and applying these (size according to what will bend without major forced warping), and doing this in 2 layers -- on the bias would be good, but at the very least having joints fall over solid pieces. I've not done this but recall that Dudley Dix has a system for getting rounded compound curved bilges out of ply doing something along this line. An old WB mag had an article on this by Dix. He has a website too.