Backing pad for stemhead fitting

[Cuan]

I'd be grateful for any views/guidance on this.

The backing plate sits under the foredeck and is a large piece of 20mm plywood. The stem head fitting is bolted to this through the deck by stainless steel nuts and bolts on penny washers.

The wood has softened due to water getting at it, so I want to replace it.
What I propose doing is replacing it with is 5mm thick High Density Polyethelene (HDPE) which I understand is the same stuff that plastic chopping boards are made from.

I am thinking about adding a 3mm stainless steel plate cut to the same size as the HDPE backing pad so that the HDPE pad is sandwiched between the deckhead and the stainless steel plate before bolting down onto penny washers.

 

[LadyInBed]

That sounds like a reasonable solution to me.

 

[Sandy]

An excellent suggestion. All my backing pads are metal and I like the idea of something with a bit of flexibility in them then the metal.

 

[rogerthebodger]

Have a look at UHMWPE which is stronger than HDPE

I would also use thicker than 5 mm

https://www.google.co.za/search?ei=.....0.0..0.0.0.......0......gws-wiz.qPWP8TYfx1g

 

[Cuan]

Thanks for the feedback. Went with 10mm HDPE and 3mm 316 stainless in the end

 

[BabaYaga]

An excellent suggestion. All my backing pads are metal and I like the idea of something with a bit of flexibility in them then the metal.

Could someone kindly explain what is the purpose of flexibility in this case? Will it not potentially serve to break the bond of the bedding between fitting and surface?

 

[rogerthebodger]

Could someone kindly explain what is the purpose of flexibility in this case? Will it not potentially serve to break the bond of the bedding between fitting and surface?

The softer material allows the load to be distributed more evenly across the clamping face.

It can also act a a gasket and seal the clamped surface to some extent.

 

[BabaYaga]

The softer material allows the load to be distributed more evenly across the clamping face.

It can also act a a gasket and seal the clamped surface to some extent.

I see, thank you.
To avoid point loads, one other option could be to use some thickened epoxy between steel plate and deck, which is allowed to set before finally doing up the bolts.

 

[rogerthebodger]

I see, thank you.
To avoid point loads, one other option could be to use some thickened epoxy between steel plate and deck, which is allowed to set before finally doing up the bolts.

Yes that is an option.

A piece HDPE is a lot less messy then thickened epoxy particularly when working upside down in a confined space.

 

[BabaYaga]

Yes that is an option.

A piece HDPE is a lot less messy then thickened epoxy particularly when working upside down in a confined space.

Possibly so, but the epoxy is only flexible when this is an advantage – while being formed – and becomes rigid after setting.
I'm not familiar with HDPE, so don't know how much 10mm might be compressed under load. Perhaps any fear of the fitting 'lifting' from the deck surface is unfounded?

 

[rogerthebodger]

Possibly so, but the epoxy is only flexible when this is an advantage – while being formed – and becomes rigid after setting.
I'm not familiar with HDPE, so don't know how much 10mm might be compressed under load. Perhaps any fear of the fitting 'lifting' from the deck surface is unfounded?

Plastic chopping boards are very often made from HDPE

 

[Cuan]

And HDPE is harder than marine ply.