Plywood daggerboard and rolling shear

If I make a daggerboard from plywood(0/90degree) and laminate UD fiberglass on both sides. If the board is 3m long and sticking out from the trunk only 1m, will it reduce the shear load on the plywood as it's distributed by the UD fibers along the entire length of the board? If so, would this prevent the rolling shear from being an an issue?

Thank you.

 

The load is the load. The design must not fail under the design or expected loads.

What you are asking is will a layer of UD prevent rolling shear. But the question has no answer because we don’t know the loads or the structures.

So, we cannot answer the question.

We can say a layer of UD may reduce the potential for rolling shear failure, but that is about all that can be said. It cannot even be stated as to whether the UD is an optimal means to reduce the potential for shear as other fabrics may perform better.

Just to add ~ the UD does not ‘disperse’ the load. It would increase the plywood’s resistance to rolling shear, but only by adding strength to the entire structure.

As a caveat, I am not an engineer, but a builder. To do a full analysis of the question would require an engineer with more data than I have available. My wish in replying is that I can kick off the query to help with better engagement.

 

Here is a good picture that can help with any discussion. Adding fabric to the structure changes the bending character and reduces the potential for rolling shear by reducing the deflection of the structure.

In order to do this best; you need to know the rolling shear limits of the original and then determine the load with safety margin and then modify the plywood to reduce the deflection.

corrections welcome

 

A picture of the arrangement would be good too. I am visualizing something like this, with the trunk running from hull bottom to deck top:

 

A 3m long board would suggest a reasonable thickness, or many layers of plywood. I would laminate something this size with wood strips.
As above, the loads will dictate core/skin options.

 

Thank you for the replies.

The photo Tops posted is what I'm thinking about, and the daggerboard would be 70mm thick NACA 0010 foil. Maybe my thinking was wrong. Now if I imagine a phone book sticking out of the trunk it would be just as floppy if the shear strength is not there, no matter how long it is inside the trunk.

But in the case of beam Fallguy posted if the beam continues beyond the supports and the bottom and top flanges are relatively stiff material compared to the web. Would the beam length reduce the shear load the bending part of the beam is experiencing? At this point this is just hypothetical.

 

@Steveso Is this more like the size of the (foiled section of the) daggerboard? NACA0010 @ 70mm thick x 1 m deep with a slight rounding to trailing edge

 

You need to revise your thinking, Steve.

The design load on the structure is the design load. The real load is the force of the water against the rudder, but only relevant in determining the design loads.

Changing the shape of the structure to shorter reduces the distance over which deflection occurs. Making the structure stronger increases the resistance to deflection/bending. Of course a short rudder is useless.

The structure is designed and does not affect the load. Think about it like this… if you have a piece of cardboard supported on one end and you apply a load to it of say 3 ounces (bad data, but go with it for now), that load is always the same. The carboard can have layers of tape added, but the 3 oz load is the same load. What changes is how much the cardboard bends.

Tops can help with details, but first get the principle.

 

What you have is a cantilever beam fixed at one end. The maximum stress will be at the edge of the hull where it comes out. Check the calculations here:
Cantilever Beam Calculations: Formulas, Loads & Deflections https://www.engineeringtoolbox.com/cantilever-beams-d_1848.html

 

"Changing the shape of the structure to shorter reduces the distance over which deflection occurs. Making the structure stronger increases the resistance to deflection/bending."
"The structure is designed and does not affect the load."
"The structure is designed and does not affect the load"
I know all this.

Maybe my question was not clear.
I'm talking about making the beam longer beyond the support points(supports stays where they are, and the load stays the same). Not changing the bending moment of the beam.

Does the added length reduce the shear stress?

For the cantilever beam, maybe it doesn't. But how about the beam with two roller supports?

 

Here better illustration for the cantilever beam. Better representing the real life situation with a daggerboard.

 

That is not correct. Making the structure stiffer increases the resistance to bending. Strength and stiffness are two different properties.

 

The way the board fits in the slot will determine the stresses and deflection. A cantilever beam is , theoretically, rigidly attached. There are engineering approximations to solve the questions you ask. Before you can use any of them the properties of the materials and the failure mode need to be determined. For example, in a composite, delamination may be the failure mode. In this case neither material fails in itself.

 

They are very much related. increasing the section modulus will make the beam stronger and also stiffer.

 

You could, but then it wouldn't fit.