Backing Plates

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Mwinda

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How big should they be?
I'm planning on fitting two safety eyes (for harnesses) in the cockpit. Easiest access is from the inside of a cockpit locker, although I'm aware that mounting the eyes here means that there is a free edge at the top of the locker (which presumably reduces the strength of that location compared with a place where there is no free edge).
Anyhow - I know the eyes should have a "large" backing plate - but how large is large- 5cm x 5cm, 10cm x 10cm - bigger???
Should the plate be wood or steel?
Cheers
Jonathan
 
Just fitted Seasure harness points and used 12 mm ply twice as big in both directions as the base of the fitting. To an extent it depends on how thick and stiff the GRP panel is. My locker sides have a substantial moulding for water drainage and the seals which make it very stiff.
 
Provided the mounting point is reasonably strong as per above post I would be looking for a roundish plate of stainless steel with both edges well rounded off and at least 6 mm thick.

Also a good idea to add some resin and Micro-fibres between the plate and the glass, this ensures an even loading on the glass.

You could also use 10 or 12 mm aluminum.

I would the use of ply for any backing plate, timber rots.

Avagoodweekend......
 
6mm steel? That's pretty hefty stuff?
What sort of diameter would you suggest? Where do you get that from?
The ply kinda appeals from an ease-of-use perspective. How about using ply and then glassing that over to stop the rot??
Jonathan
 
[ QUOTE ]
6mm steel? That's pretty hefty stuff?
What sort of diameter would you suggest? Where do you get that from?
The ply kinda appeals from an ease-of-use perspective. How about using ply and then glassing that over to stop the rot??
Jonathan

[/ QUOTE ]
Dear Jonathan
Brian (Oldsaltoz)'s advice is usually among the best but in this case I think he is way off. A stainless steel plate wouldn't need to be more than 2 possibly 3 mm thick to adequately spread the load, however so long as it is protected from water ply will last very well. Here in Scotland we use 'paint' to protect plywood from water, I find it quite effective.
 
Tufnol is brilliant stuff for backing plates. Easy as MDF to work, strong as anything and no rot/corrosion problems. Stick it to the boat with Sikaflex or 3M 5200.
 
Welcome on the forums Jonathan. I think the size of the backing pads depends largely on the strength of the grp you will be attatching it to. If you fit them fairly low down away from the edge it will help. I usually use aluminium plate as its much easier to work with than stainless.

The backing pads for the shrouds that bolt through the coachroof on my 25 ft boat are aluminium about 3 mm thick and about 150 x 75 mm size. I should mention the bolts go right through the coachroof and the plywood liner which will also spread the load.No sign of any stress after 35 years .

If I were you I would bond a square of marine ply inside and use a smaller aluminium pad on top of the ply.Then cut the bolts correct length and use dome nuts so your hands and kit in the locker dont get damaged.
 
I use grp .Lay a few layers up (not CSM but woven fabric) on a waxed pane of glass or shiny plastic, use epoxy resin if budget allows. When cured chop up your sheet with diamond disc. Also as mentioned Tufnol is great but expensive.
 
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