mounting screws in cored materials

[Sandero]

Gord,

Did you mean that fiber glass cannot hold threads? I believe West believes that they can.

The problem is the pitch and depth of the threads, the pilot hole and the number of threads engaged. Why wouldn't GRP have no withdrawal strength? My own experience with tapping screws in thick GRP is that the head can break off from screwing the down way more often than them spinning around and not holding.

[ssullivan]

Originally Posted by GordMay

1. Screws should never be used as a structural fastening on a boat.
2. Screws will not hold (more than a label/nameplate) in fibreglass.
3. A sandwich cored assembly (FRG-Foam-FRG), is, for fastening purposes, as fibreglass (see 2 above).
4. A veneered fiberboard, chipboard, and/or particleboard (cabinet door?) is, for fastening purposes, as fibreglass (see 2 above).

As Sean indicates; use a bolted fastening wherever possible, and a structural adhesive (5200) where nuts & bolts not practicable.

Now how come mine didn't come out as clear?? ha ha

This is a much better phrased response.

[GordMay]

Originally Posted by defjef

Gord,

Did you mean that fiber glass cannot hold threads? I believe West believes that they can.
The problem is the pitch and depth of the threads, the pilot hole and the number of threads engaged. Why wouldn't GRP have no withdrawal strength? My own experience with tapping screws in thick GRP is that the head can break off from screwing the down way more often than them spinning around and not holding.

I expect that West Systems might be referring to epoxy’s ability to hold a thread, not an FRG laminate. A specific West citation might help clarify their position (link, direct quote).

The conventional purpose of the epoxy plug, that Del & others describe in sandwich (cored) assemblies, is to provide a moisture barrier*, and crush resistance for through-bolted fastenings in compression. These plugs may very well hold a screw thread, but I’ll stand by my advice that screws serve no structural purpose on a boat.

* By the same token, the raw edges of cut/drilled FRG laminates should be “sealed” with epoxy.

A thicker laminate of GRP will hold a screw better than will a thinner one; but neither will hold to any significant structural specification (in blind attachment).
Perhaps the breaking screw-head was a result of binding forces (high installation or insertion torque required), but not due to the inherent pull-out resistance of an FRG laminate/screw combination. The fibrous nature of fibreglass may contribute to both ”binding” and poor pull-out resistance.

[Joli]

We have found that if you lay a strip of 1/16" butyl tape in the center of the rim of a hatch and then use polysulfide one either side you get a "shim" from the tape so the polysulfide does not get squeezed out. The butyl tape also seals the hatch in the event the poly fails. Sorry no photos.

Originally Posted by schoonerdog

By the way, great pictures. I've rebedded about 4 hatches on my boat and I dremel out the core surrounding the hatch itself as well and fill that with the colloidial filler and epoxy mixture to create a solid epoxy gasket around the entire hatch. It is a pain but I found that my hatches when they leaked would penetrate into the foam from around the gasket. I'm also going to be putting shims underneath the hatches when they are rebedded to insure I've got a good amount of sealant. I hadn't done this before and the weight of the hatch coming down I think removed too much of the sealant around the rim and a few may have to be rerebedded.
I like your technique with the syringe, what consistancy do you make the mixture? It looks far neater than smearing in a peanut butter consistancy mix (which is what I was doing).

[Sandero]

Anything which requires solid fixing with lots of force should use bolts with backing plates, but I have some instruments and even the panel for the instruments mounted with tapping screws and they have been OK for 22 yrs so far.