Backing plates

Tim O

Tim O

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I'd like to start replacing the backing plates on a few deck fittings and rubbing strake. In particular I need to do a pushpit mounting and a wooden coachroof handrail plus a few others. Not at this point any shroud/stay fittings'

Many of mine seem to be blocks of ply so I'd appreciate any thoughts on the relative merits of marine ply versus 316 stainless.

Or what about a combination of both or is that unnecessary overkill?

What thickness of each would folks recommend?

How easy is sheet stainless to cut to size?...and drill? and can anyone reccommend a good online supplier?

Cheers
 
What size boat? A 40 footer will need more robust fittings than a 20 footer.

I've got a mix on my boat. Some are ply, some 316, and some ply/steel. In locations, where I can rely on the plates remaining dry, painted mild steel is fine. It is also easier to work.

Stainless is easy to cut with an angle grinder with the right blade. Drilling is another matter, but I've found cobalt drills to be best.
 
i used 5mm thick 316 stainless to reinforce the rudder mounts on our Jaguar 25. One plate on the inside and one on the outside for each swivel mount. I would have used 4mm thick plate to be able to put a slight curve into the plate but it was not available in the size that I wanted. I sourced the plate (and a new 16mm dia rod) from the Metals4U internet site and service was fine. Cutting and drilling the plate was no great issue, however, drilling the original swivel brackets to 16mm diameter took many re-sharpenings of a new drill. (There are no bearings in a Jaguar rudder mount, just a clearance hole for the rudder pin in the mounting brackets, which I found quite surprising.

The benefit of 316 as a backing plate is that once in place it does not require further maintenance - varnishing etc. although it is not as pretty as well finished marine ply.
 
Oops sorry yes meant to say she's a 24 footer Atlanta Faroes 24.....sloop rigged bilge keel

Most of the fittings I'm thinking of would be hidden anyway so cosmetics not an issue really....more thinking about long-term strength and reducing need for future maintenance

Will check out the metals4u site - thanks
 
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Why anyone would spend time and money installing timber as a backing plate is a complete mystery to me.

It rots, it crushes and is just not up to the job.

Stainless steel or even Aluminium plate is far better.

Just make sure the edges on both sides are well rounded, as are any corners and that the plate has enough suitable filler to form a good load spreading pad.
A mix of epoxy resin and Micro-Fibres works for me.

Just cover the plate with cling wrap first then the filler, this ensures it can be removed easily for inspection later if required.

Good luck and fair winds. :)
 
oldsaltoz said:
Why anyone would spend time and money installing timber as a backing plate is a complete mystery to me.
Click to expand...

It all depends surely? Sometimes wood is fine and it may be better to have a bit of wood when it is going to be visible down below.

Two examples were mentioned by Tim O. It may well be that wood is best for the coachroof handrail; after all a hand rail is not subject to especially high loadings. For the pushpit, where the loadings due to leverage of the uprights may be higher, it may be that steel is best.

I'm not sure why Tim O is replacing the backing plates. If it is just routine maintenance and there is nothing obviously wrong then why not replace like for like.

On the other point, the plates don't have to be stainless. The biggest backing plates on my boat are mild steel. They hold the keel on. I've got MS plates elsewhere too.
 
I think wood / plywood used to be used for backing pads, precisely because it DID crush. The problem with putting a thick metal backing plate on to the inside of the laid-up surface of a fibreglass hull is that it is likely to apply very high pressure at a relatively few points and might create stress cracks on the outside surface. As Oldsaltoz says though, we have better alternatives these days.

Drilling stainless needs a lot of pressure on the drill bit and much slower speeds than normal hand drills turn at. As soon as it overheats, you're stuffed! Cobalt drills certainly help, but good coolant, low speed and high pressure is the key.
 
As said by Oldsaltoz a plate of stainless liberally coated in an epoxy microfibers mix will ensure perfect contact with the laminate.I've also used solid fiberglass laminate backplates and plywood sheathed in fiberglass.It is very important to apply the said layer of putty(epoxy is the best) between the plate and deck.
 
oldsaltoz said:
Why anyone would spend time and money installing timber as a backing plate is a complete mystery to me.

It rots, it crushes and is just not up to the job.
You mean like this:

DSCF4065.JPG


After some heavy weather off the Devon coast we found the eye bolt had lifted about 10mm from the deck. The ply pad had rotted, as had the steel backing washer, over 28 years, and this was not visible due to a bulkhead. (22 foot boat).
Click to expand...
Click to expand...
 
I make FRP backing plates, or use FRP pultruded plate, bedded on epoxy or bulked resin. Agree with other posters that steel can be too stiff, set up stress concentrations at the plate edges. Much easier to fab, drill and bond in place.
 
You mean like this:

After some heavy weather off the Devon coast we found the eye bolt had lifted about 10mm from the deck. The ply pad had rotted, as had the steel backing washer, over 28 years, and this was not visible due to a bulkhead. (22 foot boat).
Click to expand...

If it lasted 28 years that's hardly damming evidence of the inadequacy of wood.
 
I've used polypropylene chopping boards from the local supermarket where the loads aren't high, cheap, readily available, easy to cut and shape using wood working tools, doesn't rot and flexes a bit. Use a double thickness if more strength is required. To Bed-in I've used Sikaflex or similar.
 
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