Plywood daggerboard and rolling shear

Re material selection. Plywood is far from an optimal material for cantilever structures (ridders and daggerboards) since about half the material doesn't contribute any longitudinal strength (or stiffness). The plywood has the advantage of being readily available, easy to work and particularly if the plies are of equal stiffness the glue lines help keep the section symmetrical and fair.
An alternative that is better suited in many cases is to use LVL lumber instead of plywood. The equal thickness plies are all in the right direction and you can usually purchase just how much need since the stuff is sold by the foot.
A caveat, recently the quality of material seems to be declining and I've abandoned using wood except for wood flour in my boatbuilding.

 

Yes and no.
Yes, you can actually add enough fiberglass to take all the loads. But at that point the plywood is just a heavy core, you can just as well use something lighter like foam, because the engineering is for a cored structure.
No you can't add just a little fiberglass to "distribute loads", the outermost layer takes most of the force. If that layer fails the underlying plywood has to resist on its own or you have a cascading failure.
Your choice is therefore structural glass with a core or structural wood with some glass for waterproofing and scratch resistance.

Rolling shear is the least of your problems, in practice the wood fails before the glue. If you bend a sheet of plywood to failure and examine the break you will see the layers still firmly attached at the break point. If the glue fails first it would look like a book, still glued together at the spine and separated at the break. Quality plywood just doesn't fail at the glueline from bending, it's the wood next to it that breaks.

As mentioned plywood isn't the ideal material for a daggerboard, especially a profiled one. It's why most people make wooden daggerboards and rudders from glued staves instead.

 

Yes, it's pretty much the same size. I just truncated it little more so the trailing edge won't cut into the trunk of hitting something. And he trailing edge is kammtail shape.

 

it's not ideal material for a daggerboard, but It would be convenient as I have some marine plywood at hand.

Rumars. How about cyclic loading over time?

 

Do you have any dimensional constraint?

Plywood is also not very easy to handform like foams..

 

What about it? Look if you don't trust the glue the manufacturer put in the plywood look for something else, there's nothing you can do about it. Your concerns are misplaced, build quality (voids) and sufficiently strong and stiff wood species are what you should worry about first. If you don't trust the glue the only thing you can do is make your own plywood. You can use the epoxy or the resorcinol brand you like and trust most.

Or forget about plywood beeing structural and just use it as a core for a fiberglass or carbon board and accept the weight penalty. Or go steel or any of the other options.

 

Fallguy
Yes, 70mm thick naca 0010 foil. 4x15mm plywood sheets glued together would leave around 5mm space for UD fibers on both sides on the thickest part of the board. I would profile the sheets with CNC before gluing.

Rumars
I'm not worried about the glue. As far as I know, the rolling shear failure happens in the wood grain itself -the part running the "wrong direction". Over time as the board bend it slowly grinds the wood fibers.

Even considering the plywood just as a core, it still has to handle the shear forces.

 

Well, I haven’t built daggerboards, but your plan seems a little odd to me.

If I were building the shape from plywood (I would not), I’d glue them together with epoxy putty first under 9-10” vac and this would create a 1mm bondline. If the plywood starts out at 15mm, by the time you precoat the wood and apply putties; it will add 3mm to the layup. Then you are at 63mm and only have 3.5mm left for glass. So, then you put the part on the cnc and cut post glue and that way it can also be trimmed down enough for glass. After cnc trim, you precoat it again to avoid taking resins from the glass which it will like to do with the endgrains. Then mechanical key before any glasswork.

Since I’m not a composites engineer, I won’t delve into the glass choice much, but I think a UNI alone sounds incorrect. The reason is you get very little strength in the horizontal axis this way and the tailing end might deflect thus.

Rumars is really wise here and I recommend you pay attention to his comments. I was only here to offer what little I can and stir the conversation.

By the way, a 3.5mm glass area is really only enough for about 3 layers of glass and also probably ought be done under vac since your margins are small. If you glue it together first; you can trim. And the way to achieve a 1mm bondline is putty both sides with a 1.5mm or 1/16” vee trowel before applying vac. Make sure to use locating pins to keep the plywood from sliding under pressure.

 

The whole board will be sheathed with +-45 fiberglass. And the thickness will be taken into account when routing the shape. The board is also slightly curved, that's why I want to machine the flat sheets, and then bend and glue it under a vacuum(in some curved form to keep the radius constant). The trunk will be build around the board with adequate clearance.

I respect all the inputs. And I know here are many people with a lot of expertise, that's why I came here.

 

Why +- fiber orientation? It should be a mix of mainly vertical unidirectional with some horizontal fibers.

 

You’ll be machining it twice that way. Or handforming and making it less than perfect.

 

I have wondered whether the rolling shear failure can be prevented by cutting the plywood into many narrow strips, and stacking them up so that half the fibres run parallel to the span, as usual, and half not parallel to chord, but parallel to the thickness of the foil. It would involve more work glueing strips, but if the point is to use up existing material and get the strongest structure out of it, would this help?

Perhaps for shear strength, it would be even better to cut the plywood at a 45 degree angle, though then it would be difficult to use the wood efficiently.

In both cases, the plywood would be a core and whatever skin is put around it would be expected to provide much of the strength.

 

You are creating an I-beam, if the uni glass caps don't have enough strength to carry all the loads you can just as well skip them. Highest shear load will be on the neutral axis, wich in your drawing seems to be an epoxy glue line. Analyze it to see if there is enough surface area to resist the projected loads (wich I think it easily will). The rest of the ply is the web of the beam and the alternating grain orientation is not problematic.

If you want to see what a plywood alone board will do you have to analize it as you would a solid glass laminate, decompose it into individual veneer and glue layers and use the wood properties data instead of glass.

 

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If you change the vertical part of the beam (I-JOIST) to plywood, it will be the strongest wooden beam ever.