Twisting 8mm ply?

Of course I know what a radius is, but was confused, how to apply it here. Guess I got it. A twisted plane is composed of circular segments and it's their radius, you are referring to.
Thank you for you comments and help, finding a better concept. The weight penalty for 2 extra mm ply is about 1000g/m² which can also make a pretty serious glass reinforcement. That's the way to go.

 

Couple of points, you definitely don't want to "bulletproof" that boat. It's all extra weight for sailing, launching and general handling.

Sure, there are some advantages to being able to lay glass on a flat, but then the glass will be adding to the tension of the bent wood, instead of resisting it.

 

Bulletproof for .22 rather than .357 ^... 2mm extra ply would have been not more than 8lbs which I tolerate, but anyway I will shave of some weight on the interior. That dinghy is not for sailing fun only and I can see some abuse ahead.

 

If you are not worried about sailing performance etc, attach some longitudinal timber strips to the bottom panels, to both stiffen the bottom, and act as rub rails. That will protect your ply from most scrapes.

 

Stiffness.... the resistance to bending is a function of the cube of the thickness (or depth) of the beam. At 6mm cubed you have 216 and at 8mm cubed you have 512 which is more than twice the stiffness............ A twist on a boat such as the one you describe is about all the torture you want to subject 6mm ply to. If you must have bulletproof then add a lot of glass, internally and externally, to the finished product. For a much larger boat, 8mm ply or more would be workable. It is a matter of twist per unit of length.

 

8mm is overkill and then some. The boat you're looking at is going to be weight sensitive. Increasing skin thickness by 33% is going to jump the weight by close to 30%. If you really want to stiffen it up add a thin layer of glass (like 6 oz) on the outside and 4 oz in the inside. And don't even try to put the glass on the inside unless you put it on wet and immediately install the sheet, or put it on after you do the stich and glue bit. If you let it set up you'll never get it bent into shape. If you doubt me, make up a strip and glass it and let it set up. I'll be so stiff you won't be able to twist it at all. Okoume is soft and you should glass the exterior with 6 oz glass and epoxy to keep it from being dinged when you beach it and scrape it over a rock. But if you did glass it inside and out you could go down to 4 mm Okoume it would be stronger and stiffer than the 6 mm without glass. Here is a table of strength and stiffness for ply with glass. As you can see in the table a 7/32 ply with 4 oz glass on both sides has about the same weight as 1/4 ply, but and has similar stiffness to weight ratio and obviously has good waterproofing of the wood, and oh by the way it's 30% stronger. Water proofing with epoxy requires about the same amount of epoxy as it takes to wet out 4 oz glass, so if you use 4 oz of glass it ends up being the same weight and a lot stronger than if you just waterproof with epoxy.

Trying to strengthen a design like that by increasing the thickness of the plywood is asking for trouble. You'll seriously regret trying to bend 8 mm ply where 6 mm is called for.

 

Wow, thank you very much. That is very comprehensive and that table was missing in my "toolbox", as other basic informations do. Are there web sites to recommend to find them, other tham asking you guys here?

 

I don't know why I noticed, but the first thing I did notice was in the pounds to failure column, with 1 layer of 4oz glass, 1/4" cedar has less strength than 3/16". Now that I look, it's the same for 2 layers of 6oz, the thicker cedar has less strength. 3/16" and 5/16" cedar wit 2 layers of 6oz have the same value of 500 lbs to failure.
The chart seems a little iffy.

 

@Yellowjacket What was the source of your chart?

Looks like an error for 3/16" cedar with 2 layers of 6 oz glass.

 

The data was created by the folks at Epoxy Works. I took their data and added the last three columns on a spreadsheet, but they are just manipulations of their data. I wanted to be able to see the specific performance rather than just the raw data.... Here is a link to the original description of the testing that was conducted. Since it doesn't look like they did multiple tests of each configuration, you shouldn't think of it as gospel, there could well be some scatter for any number of reasons, but it does show the trend pretty well. Also remember that Okoume is softer than marine ply wood or cedar. When you make a sandwich construction the limit in bending strength is the strength of the skin, and the core doesn't contribute as much to the strength of the sandwich. To put it another way, if you coat Okoume it will be closer to strength and stiffness to heavier plywood since the skin is where the stress is. While it isn't definitive and absolute, it is test data taken with a real laboratory material testing machine, so the only variation is actually in the samples, and not in the measurement.

How Tough are They? https://epoxyworks.com/index.php/how-tough-are-they/

As to the data where the two 5/16 panels failed at the same load could well have been variation in panels. The strength and deflection of the 3/16 with two layers of 6 oz and the 5/16 with two layers of 4 oz all fit the pattern, but the failure of the 5/16 with two layers of 6 oz seems premature. The stiffness is higher, but it obviously failed at a lower level than expected. Note that they also didn't list the mode of failure for each panel. We don't know if the failures for any particular panel was interlaminar shear or failure of the skin, or another mode like delamination of the skin from the wood. Since the skins we're talking about here are thin there also could well be variation in the layups, or the bonding. As I said above, without multiple samples it isn't going to be ironclad data, but the general trend is there.

 

Straight milled timber varies one heck of a lot in individual samples.

Ply has the added complexity of differing quality on really thin veneers, and between manufacturers.

Add to that, the ply YOU buy will be nothing like whatever was tested on this chart. You will have to make your own coupons and glass samples if you want to know about your project.

There is ONE case, when you do want to glass before you bend. If you are trying to torture plywood or any wood into a curve, and you start to get fractures, you can apply say a 4oz cloth on the outer radius of the bend. This enables you to torture the ply immensely without fractures, and thin strips of cedar can be bent without steaming.