Plywood on Ericsons

[goldenstate]

Now here's a backing plate....

View attachment 35963

Given the pictured anchoring arrangement, suppose each bolt diameter is 3/8ths and the plywood is 1/2" thick, then each bolt exerts load on 0.375"*0.5" = .1875" square inches of plywood edge * 900lbs psi shear strength from Matweb (midpoint of range provided above) means the failure would happen at 168.75lbs for a single bolt.

168.75*8 bolts = 1,350lbs for the smaller plate
168.75*14 bolts = 2,362.5lbs for the larger plate.

I expect some engineer can check my math - I think it's in the ballpark, however.

I wonder about how one measures failure loads for custom-fiberglass tabbing. If the plywood flexes out of plane, I think expect it would fail much faster.

 

[Christian Williams]

the failure would happen at 168.75lbs for a single bolt.

I weigh 200 pounds and I think I could hang from a 3/8th stainless bolt in 1/2" plywood without the plywood even noticing.

No?

 

[goldenstate]

the failure would happen at 168.75lbs for a single bolt.

I weigh 200 pounds and I think I could hang from a 3/8th stainless bolt in 1/2" plywood without the plywood even noticing.

No?

I think you could too. If anything these numbers seem very conservative to me.

I have no experience in structural engineering calculations, just trying to interpret the data provided.

There's also some factor that's not being captured: If the 3/8th" hole from which you wanted to hang was drilled 6" away from the bottom edge of the sheet, you would probably feel better about it than if the hole was drilled .25" away from the bottom edge. I don't know how this "adequate spacing" concept is reflected in engineering estimates.

 

[Kenneth K]

It's not just the bolt diameter resisting the forces; the forces are distributed to the entire backing plate which becomes "clamped" to the plywood by the tension in the bolts. A better comparison might be put a 1/2" piece of plywood into a bench vice and see how much force it takes to move it (or to destroy it) in "shear."

 

[Hagar2sail]

I think you could too. If anything these numbers seem very conservative to me.

I have no experience in structural engineering calculations, just trying to interpret the data provided.

There's also some factor that's not being captured: If the 3/8th" hole from which you wanted to hang was drilled 6" away from the bottom edge of the sheet, you would probably feel better about it than if the hole was drilled .25" away from the bottom edge. I don't know how this "adequate spacing" concept is reflected in engineering estimates.

This is called edge distance, and yes it has an important part to play. Basically in shear though a material of a hole, you have 3 failure modes, single shear, block shear, and material failure (through shear). You also can have crushing or dishing failure, but we were smart enough to put a washer or backing plate on both sides right? If so, then the washers or plates, assuming they are strong enough prevent that failure from occurring. Shear stress travels typically at a 30-45 degree angle in the material depending on the material. So you actually have to shear quite a bit of area, assuming as stated above, you stay far enough from the edge. So excuse my terrible phone drawings, the red area is the price of material failing:

single shear:

block shear:

through shear (this is often the failure mode for chainplates)

Generally you are shearing way more than the edge width (3/8) of the hole you have.

 

[Hagar2sail]

Just to keep my conscience clean, washers and other things don’t help you with dishing/bearing failure if the material clamped is the same strength as the clamping material. E.g. most metallic connections. Washers do help with pull through, but that is a different topic.

 

[goldenstate]

Just to keep my conscience clean, washers and other things don’t help you with dishing/bearing failure if the material clamped is the same strength as the clamping material. E.g. most metallic connections. Washers do help with pull through, but that is a different topic.

I knew someone who had more direct experience would chime in if I kept after it!

The idea of the shear stress travelling at a range of a 30-45 degree angle away from the load point makes perfect sense and something I think I knew intuitively but could never have articulated.

It reminds me of the way I have been told that structural engineers model loads under a concrete foundation - that the downward force is transferred in a +/- 45 degree angle range under and away from the concrete slab.

Great explanation and thank you for taking the time to make the little diagrams.

This makes me think more so that I really should use good backer plates when I mount my windvane on my transom....

 

[Hagar2sail]

Haha, you dangled the lure for long enough, I had to bite

 

[nquigley]

///
This makes me think more so that I really should use good backer plates when I mount my windvane on my transom....

We probably need a whole separate sub-forum for windvanes....
What sort(s) of windvane are you looking at?
I know the choice is complicated as it depends on how much of what type of sailing you'll do (solo much?), the design of the boat (e.g., how much 'value' is there in the back-up rudder functionality offered by some designs and not others), whether you also have/need a powered autopilot as well as a vane, etc.
These things are pricy, so it's worthwhile thinking hard about the best option for one's actual anticipated needs.
I've been eyeing the Cape Horn design for some time because its control lines are all under the cockpit, but if you'll need hefty backing plates for mounting the vane on your transom, you're thinking of something else.

 

[nquigley]

Oooh ... just found this thread on wind vanes:

E32-200 - The Mechanical Windvane Market

Summary By the internet-testimony of their customers, the companies listed here seem to produce quality products, based on the positive reviews one finds on-line. Prices for a new unit range from $3,500 to $7,500 and up. Introduction I...

ericsonyachts.org