Fitting a deck fitting like this?

[davethedog]

Morning all and as part of my solar arch build project I am looking to add another of these deck fittings (circled in red):



What is the best method of supporting this from the underside, as dont really want it to go just into the fibreglass and think a backing plate of some measure may be required.

Thanks

DTD

 

[Fr J Hackett]

As big a backing plate as you can reasonably fit beneath it in 3mm stainless should do it.

 

[VicS]

A simple backing pad .................... or just penny washers maybe ???

What backing has been used for the existing one(s) ?

 

[davethedog]

thanks guys and will order some stainless sheet then, and some big penny washers too.

 

[fisherman]

HDPE is the latest thing, a boatbuilder tells me that as well as workboat gunnels they use it for backing pads above or below water, and much more. It won't bond with GRP or glue. (I recently used it for frames in a wooden boat). You can't pull a screw out of it, and when drilled and tapped is surprisingly resistant to thread stripping. Of course they buy it by the sheet, but you can find it at plastics suppliers (Plastics online) or in the form of cheap cutting boards, which in truth may be LDPE. Low density not high.

 

[duncan99210]

I've got fittings like that supporting my outboard hoist/antenna pole. I just used large penny washers on the inside of the hull, seals with a smear of sealent. Been there for about five years now with no signs of any damage to the grp.

 

[Lon nan Gruagach]

A backing pad that isn't bonded to the grp will only prevent the bolts pulling out. It will do nothing to stop the fitting punching down through. If the pads above deck (thats the plates with the bolt holes in) are sufficient then the same underneath is a good bet too since the forces experienced pushing down ate likely to be similar magnitude to those pulling up.

 

[Fr J Hackett]

A backing pad that isn't bonded to the grp will only prevent the bolts pulling out. It will do nothing to stop the fitting punching down through. If the pads above deck (thats the plates with the bolt holes in) are sufficient then the same underneath is a good bet too since the forces experienced pushing down ate likely to be similar magnitude to those pulling up.

I can think of no applications that resistance to " punching down and through" is required and if it is the best option is to spread the load above the GRP ie deck with a large plate just the same as below to prevent pulling through. Looking at the small brackets in the photo they are not involved in preventing loading downwards but more supporting a pole of some sort where the ultimate loads will be upwards and twisting when the through bolt would likely deform or pull through the tube wall before even a modest backing plate stressed the hull.

 

[William_H]

If as is likely you can not access the point under the fitting to put penny washers and nuts on bolts there is another way.
Make a plate of suitable size and thickness and drill and tap holes for your bolts. (to match the holes in the GRP.)
You push 2 bits of wire through 2 diagonal holes and hopefully you can fish them through to a point where you can reach.
Push the wires through the correct holes with something like a knot so they can't pull through. Attach another wire or string to the knots long enough to be accessible when plate is in place. Pull the two wires so that the plate is pulled up into position under the GRP. Now you can fit the 2 bolts not having wires and can pull the plate up into position. Cu the wires on the outside and fit the last 2 bolts. Use the strings to retrieve the ends of the wires.
Only problem is if the under side of the GRP is not flat for the backing plate to sit on. You may be able to add a blob of sealer before pulling the plate up. For small irregularities. I hope this might help. olewill

 

[Bristolfashion]

I tend to use 18mm good quality marine ply for backing pads. I also use the largest washer that will fit to spread the load. Obviously cheaper and easier to make than stainless. Is there any reason not to do this. Obviously I seal appropriately and am lucky in my boat that I can keep an eye on them for water penetration or deterioration.

Cheers

Bristol