What thickness ply for making backing pads ?

[Boo2]

Hi,

I need to buy some ply anyway and want to also use the leftovers for making backing pads for mounting poles, davits, cleats and thru-hulls. What thickness ply would be best for this application ?

Thanks,

boo2

 

[Tranona]

12mm is a common size for backing plates.

 

[saltylegs]

+ 1

 

[oldsaltoz]

Hi,

I need to buy some ply anyway and want to also use the leftovers for making backing pads for mounting poles, davits, cleats and thru-hulls. What thickness ply would be best for this application ?

Thanks,

boo2

I avoid timber backing as it rots or is too soft anyway, check out your local scrap metal merchant for stainless or alloy.

Good luck and fair winds.

 

[mitiempo]

Stainless, alloy, or ideally fiberglass backing plates epoxied in for real added strength.

 

[sailorman]

12mm is a common size for backing plates.

+ a s/s plate or s/s penny washers

 

[Seajet]

+ a s/s plate or s/s penny washers

I'm with sailorman 100%, a 12mm wooden pad to allow a tiny bit of give - and to conform to slightly uneven surfaces - with a stainless plate or at least penny washers is the way to go.

 

[aitchem]

I found an unprotected ply pad on my boat, totally crumbled in my fingers.
The glassed in ones were OK though.
My philosophy would be to soak it in epoxy and cover it with a mat of glass, minimum.

 

[rogerthebodger]

I would use stainless steel as said above but I would drill and tap so the outside fittings can be fitted with machine screws to allow removal without the need to get inside under any lining.

 

[Norman_E]

I would use stainless steel as said above but I would drill and tap so the outside fittings can be fitted with machine screws to allow removal without the need to get inside under any lining.

For full fixing strength you need the theaded hole to be in steel of the same thickness or greater than the core diameter of the thread. For holding down winches or cleats, that means a pretty thick piece of steel, otherwise you need to use nuts, preferably nyloc types.

 

[savageseadog]

For full fixing strength you need the theaded hole to be in steel of the same thickness or greater than the core diameter of the thread. For holding down winches or cleats, that means a pretty thick piece of steel, otherwise you need to use nuts, preferably nyloc types.

Could fix captive nuts like Rivnuts or weld standard nuts?

I've only ever used ply by the way

 

[Boathook]

I would suggest a mix of all the replies.
12mm ply, soaked in epoxy, bonded in place, covered with grp matt and the fitting bolted through using penny washers and nylocs. Also if possible, oversize the hole in the ply and fill up with epoxy and then redrill to the correct size. This helps to reduce water ingress in to the ply.

 

[mitiempo]

There is no reason for "give". The backing plate, whatever material is used, should be solidly attached to the boat and not compressible. If there is give the attached hardware will move and you don't want that. If there will be side loads as with a stanchion the backing plate should not only be epoxied to the underside but should be considerably larger than the stanchion base.

 

[rogerthebodger]

For full fixing strength you need the theaded hole to be in steel of the same thickness or greater than the core diameter of the thread. For holding down winches or cleats, that means a pretty thick piece of steel, otherwise you need to use nuts, preferably nyloc types.

yes if the load is trying to strip the thread.

You also need to consider the crushing load of the backing if it's a softer material like ply, timber or GRP. You also need to consider that penny/fender washers tend to quite thin and will tend to sink into softer backing material before you get to near the tensile strength of the fastening .

If using just washers under softer material the shear load of the host material / backing pad could also be very much less than the tensile strength of the fastening

The thickness of threaded material is normally at least 80 % of the diameter of the fastening providing both are the same material.

I would also not use aluminium due the requirement for a much thicker backing pad due to the difference in strengths of the different materials and as in most cases stainless steel screws will be used and the potential of the stainless screw corroding in the aluminium plate would be quite high preventing the subsequent removal negating the use of a threaded backing plate.

One other point is that if the backing plate is to be glassed over on the underside the backing could be mild steel as rusting would be unlikely, but I would either paint it or use zinc plated steel in this case.

 

[Deleted member 36384]

You may be aware of this resource but if not its a great source for all sorts of methods of fastening in GRP: -

http://www.westsystem.com/ss/fiberglass-boat-repair-and-restoration/

I have used 12 mm ply, stainless plate and washers depending on the item to be fastened. I removed a ply pad (about 10 mm) from below a U Bolt and it was soft. Hence, I would recommend good marine ply as a backing pad.

I sail a 41' yacht and the spinnaker sheet block U Bolts have 10 mm backing pads, (now) marine ply backings. It appears to be strong enough. The underside of the deck is quite uneven so the new pad is bedded in thickened epoxy. Penny washers are behind the nuts.

The yacht's shrouds are secured onto quite large eyebolts straight through the deck shelf and these are backed by stainless plates bedded onto a sealant . This is not a good idea as the irregular under deck causes leaks. So they too will be bedded on thickened epoxy but set initially over plastic film so the pad can be removed again. I think plywood would be unsuitable in this application as any compression would loosen the shroud, even slightly.

Hope this helps.

 

[sailorman]

yes if the load is trying to strip the thread.

You also need to consider the crushing load of the backing if it's a softer material like ply, timber or GRP. You also need to consider that penny/fender washers tend to quite thin and will tend to sink into softer backing material before you get to near the tensile strength of the fastening .

If using just washers under softer material the shear load of the host material / backing pad could also be very much less than the tensile strength of the fastening

The thickness of threaded material is normally at least 80 % of the diameter of the fastening providing both are the same material.

I would also not use aluminium due the requirement for a much thicker backing pad due to the difference in strengths of the different materials and as in most cases stainless steel screws will be used and the potential of the stainless screw corroding in the aluminium plate would be quite high preventing the subsequent removal negating the use of a threaded backing plate.

One other point is that if the backing plate is to be glassed over on the underside the backing could be mild steel as rusting would be unlikely, but I would either paint it or use zinc plated steel in this case.

Most items through bolted are with the bolts being "In shear" & not a direct pull

 

[rogerthebodger]

Most items through bolted are with the bolts being "In shear" & not a direct pull

Not always it depends on the detail design.

My main anchor bollard the center of my for deck has 4 bolts through the deck. The bolts do have a shear component but the aft 2 also have a direct pull element as the base is trying to pivot about the front edge due to the offset pull of the anchor line. Same with my mooring bollards on my side deck

If the chain plates are bolted through the deck as my old boat was these bolts are in direct pull but if they are bolted through the hull side then the bolts are in shear.

So as I say it depends on the detail design.

 

[sailorman]

Not always it depends on the detail design.

My main anchor bollard the center of my for deck has 4 bolts through the deck. The bolts do have a shear component but the aft 2 also have a direct pull element as the base is trying to pivot about the front edge due to the offset pull of the anchor line. Same with my mooring bollards on my side deck

If the chain plates are bolted through the deck as my old boat was these bolts are in direct pull but if they are bolted through the hull side then the bolts are in shear.

So as I say it depends on the detail design.

thank you for pointing that out

 

[smonard]

I have been turned onto using nylon kitchen cutting boards. You can get them cheap from places like Lidl or those poundstretcher type places. They are about 10mm. Ideally I'd have a stainless plate under that but realistically I'll just use penny washers. I took a few rotten bits of ply out and some big blocks of aluminium maybe 10 or 12mm thick. They were a bu**er to get off, they'd corroded badly and wouldn't let go of the screws. For chainplates I'll put a bug blob of sikaflex under the nylon backing plate and wait a couple of days before tightening them down.
I'd really like to use tufnol as it is the best material ever made.

 

[mitiempo]

A grp backing pad epoxied in place will make the deck stronger. Any other material that is not glassed in place will function as a backer but the deck will not be any stronger.