Holes in plywood affect on strength

[rangebowdrie]

I guess you could make one with holes, one without, and test them to destruction.
Figure out some way to hold them and start piling on the sandbags, like they used to do for airplane wings.

 

[wet feet]

Did the designer provide a table showing such things as prismatic coefficient,block coefficient and MCT?If he did,the latter would be useful to you for calculating the effect of the alteration.Failing that,you might consider calculating the waterplane area to determine the pounds per inch of immersion.With this information you can get a good idea of the effect the changes will have and how much weight you need to lose for your desired change.If you get serious about precisely determining the effect and magnitude of a given load there are a few free FEA programs you can download and it will only require a few hours in front of a computer to get some kind of indication.Obviously you will need some good estimates of the loads to be applied and the safety margin you consider adequate,this is one area where "less is more" doesn't apply.

Ad hoc's comment about the gain and the amount of effort required are exactly right;it is really hard to get significant weight out of an existing boat.

 

[jehardiman]

IIRC, the big issue with plywood lighting holes, without reinforcement, is the number of layers. In order to have full isotropic strength you need to have many layers at different angles. This is why aircraft plywood is so expensive.

 

[fallguy]

I also have a diesel tank in the forward section that probably weighs 15 pounds and can host 14 gallons, although only had 4-5 gallons this fall.

Also have propane locker up there. The propane tanks are 26 pounds each and probably 10-12 pounds of steel parts. Took one tank out when I experienced the trim issues, but the tanks can't stay there, and I kind of want to drop to one max.

But these moves are all just to offset the anchor pulplit..so no reason to get all excited. They need to move backwards!

 

[gonzo]

As a wild guess, the weight you would take off would be the equivalent to two cans of beer. In Wisconsin terms, that may be worthwhile. Not so much in the rest of the world.

 

[Paulo Neueschwander]

If you want to hand calculate the panel strength and also check how the holes can impact it in the end I suggest to use the book "Roak's Fomulas for Stress and Strain".
For instance, as @Ad Hoc said about the panel stability in shear the book gives the following:

And also about the stress concentration:

The book covers many boundary conditions, so you need to look what fits you best.

On the other hand you must know the load type and intensity over your structure.

 

[jehardiman]

If I suggest to use the book "Roak's Fomulas for Stress and Strain".

Yeah, but plywood is non-isotropic; the reasons that it actually comes is several "strength" directions (along, across, 45).

 

[fallguy]

As a wild guess, the weight you would take off would be the equivalent to two cans of beer. In Wisconsin terms, that may be worthwhile. Not so much in the rest of the world.

I watched a great video years ago of a Romanian? woman spending weeks to remove 10kg from her catamaran. I thought, wow, she is insane, 3 weeks projects for 22 pounds. Then, at the end of the video, she says, my reward is I can now stow 10 bottles of wine in my footlocker. I love her.

 

[Paulo Neueschwander]

You are right @jehardiman. But the equations are to find the stresses in the plate. And, off course, you need to compare the results with the material strength you're using also considering a safety factor, so one really need to take the material orientation into account. Exception is stability that needs the elastic modulus for the calculation from the beginning .

Besides, sometimes it's needed to consider different directions strength even in metallic structures. For instance, MMPDS shows the following example for aluminium.

I found the strength table according to Eurocode that could help in the final safety margin assessment. This shows the point @jehardiman did about non-isotropic materials.

 

[tpenfield]

Very little strength of a structural member lies at the core. Mostly in the outer edges since that is where the tension and compression forces are the greatest. ( yes we learned that in engineering school ) . That is why you see airplane structural members with lots of holes in the center.

For this particular application, I do subscribe to the "two cans of beer" comment in that the amount of weight that you can reduce is not going to be of significance.

Perhaps other aspects of the design or choice of material are making it heavier than you would like? If you do decide to make the cut-outs, one larger cut-out as @Tops illustrated would be the way to go IMO.

 

[fallguy]

There are a few other places I could apply this concept; thus the query.

The advent of modern composites, the cost of fuels, and the desire to push the envelope on weight is certainly not limited to aircraft.

The mass of each plug would be 0.163 ounces per square inch, plus the glass which is 0.065 or 0.228 ounces per square inch. Using 2" circles; each circle is 3.1415•0.228 ounces or 0.716 ounces. So, 20 cuts, two panels is 40•0.716 or 28 ounces which is a pint AND a can of beer.

If you cut half holes in the margins, another can worth.

But, I have some unglassed 3/4 plywood in the salon that could also get some 2" holes, two 60" wide pieces, so say 20 holes or 0.2445 oz per hole or 48 ounces per side or 96 oz or 6 pints of beer!

So, while the question was specific; it has a broader reach.

By the time I am done, I will be able to get a whole extra 12 pack onboard.

 

[KD8NPB]

Just an FYI ; 1/2” 6 lb density PET foam is roughly half the price of 1/2” plywood.

weight will be about 1/3
Stiffness will be about the same

 

[fallguy]

Just an FYI ; 1/2” 6 lb density PET foam is roughly half the price of 1/2” plywood.

weight will be about 1/3
Stiffness will be about the same

I wanted ply because I did not want the section to sag or crush under a heavy head sea impact, but it should have been foam; for sure.

 

[KD8NPB]

I wanted ply because I did not want the section to sag or crush under a heavy head sea impact, but it should have been foam; for sure.

Increase face and backer laminate thickness slightly.