Backing plate size calculation

[Gixer]

There’s loads of threads on here about backing plate material and thickness but nothing about size. Is there a calculation for size of the pad to spread the load?
Example, I’m fitting some cam cleats with MWL of 68kg on grp of around 5mm thickness. I’ve bought G10 sheet to use as backing which is also 5mm thick.
I’m guessing there too many variables but thought I’d ask. Any thoughts?

 

[doug748]

I'm no engineer but have knocked around boats a bit.

That shear load is so tiny you don't need much, going up a notch you may want to fit larger washers. It's nice to feel you have done a good job so making plates out of stainless, Tufnol or any other reasonable plastic will probably make you feel better - it does me. For this job I really don't think the size matters at all.

.

 

[[70521]]

As big as you can fit in the space. Why skimp when you can over engineer?

 

[Stemar]

While I'm no engineer, I reckon penny washers would probably be fine, but a good lump of your G10 between the washers and the deck would be better, and plenty srong enough. Fix it in place with thickened epoxy.

If it's a cored deck, that's an additional complication. There are plenty of how-tos on You Tube, but briefly, drill your holes well oversize, fill with thickened epoxy and drill again the right size, so the core is sealed against moisture ingress. That will also stop the load distorting the core.

 

[Gixer]

I don't have a cored deck so should be straight forward. thank you guys.

 

[Bouba]

While I'm no engineer, I reckon penny washers would probably be fine, but a good lump of your G10 between the washers and the deck would be better, and plenty srong enough. Fix it in place with thickened epoxy.

If it's a cored deck, that's an additional complication. There are plenty of how-tos on You Tube, but briefly, drill your holes well oversize, fill with thickened epoxy and drill again the right size, so the core is sealed against moisture ingress. That will also stop the load distorting the core.

That’s clever !

 

[Stemar]

That’s clever !

It is, but it's not my clever

 

[rogerthebodger]

If you are looking for a way to calculate the size of a backing plate you need to determine the sheer force of the base (deck material and the loads that are to be resisted, not an easy task.

You can then calculate the force required to resist the load imposed and therefor the size of the plate needed to resist that load based on the shear force of the deck material.

Simple to someone who studied strength of materials in mechanical Engineering

 

[Baggywrinkle]

Don't forget to bevel the gelcoat around the holes to prevent cracking and use sealant/adhesive on the plate, cleats, and bolts. Use existing fixings with similar loads to estimate backing plate size.

 

[billcole]

One thing worth doing is to chamfer off the edges of your backing to avoid concentrating high stress on the grp along the edges of the plate

 

[PabloPicasso]

To fit a bolt to a cored deck;
1 Drill right size hole in right place
2 Use a small allen key in a drill to open out the core bewteen the grp skins. Vacuum out detrius
3 plug bottm of hole and fill gap with thickened wpoxy
4 once set redrill hole of correct size
5 countersink top to give space for sealant.

I prefer to uae butyl tape for above the waterline sealing, such as windows and deck fittings

 

[zoidberg]

"That’s clever !"

"It is, but it's not my clever"

Attributed to the Gougeon Brothers - see their decades-old 'how to' book - and detailed guidance from e.g. Wessex Resins of Romsey.