OK to secure fastener directly into GRP ?

[Boo2]

Hi,

I need to secure some bullets to the side of my coachroof for running the spi pole downhaul back to the cockpit (similar arrangement to the blue line along the coachroof in the pic of a different boat shown below). There is a difficulty getting behind the headlining inside the boat and it has been suggested that since the lines act only as guides then it is acceptable to drill and tap the GRP directly to take the fittings. Does anyone have an experience of this and any suggestions about how to do it properly ? Or is it a complete no-no ?

Many thanks,

Boo2

 

[awol]

As always - it depends. Tapping solid glassfibre where the thickness is about 6x the screw thread pitch will take shear loads but not good on tensile loads. Foam sandwich may need strengthening with epoxy or something. Although the intention is an almost zero load guide the fittings will get stood on, kicked and abused.

 

[macd]

It's permissible if the loads aren't great, with the provisos awol mentions. However, if you just drill and screw, you'll almost certainly shatter a little of the surrounding gelcoat. The hole needs to be counterbored through the gel (although a countersink bit works almost as well). Good GRP is hard, so the tapping size for a given fastener is larger than with, say, hardwood. Otherwise it can grip a screw like stink -- well enough to sheer it off. The counterbore/countersink also acts as a 'reservoir' for bedding compound, helping you get a watertight seal.

Obviously a 'proper job' would involve bolting right through and a backing pad.

 

[laika]

If you decide against tapping the GRP and want to make it look pretty inside without pulling up the headlining, you could consider drilling thorugh the headlining and using big shiny washers and shiny dome nuts.

 

[boatmike]

Absolutely no problem in that application with little load applied. try to drill the right size of tapping hole and counterbore through the gel coat only to avoid star cracks. If you can its a good idea to use a slightly oversize length screw and remove the first couple of threads on a grinder which will give you maximum thread length in the GRP without the point digging through your headlining. A little silicone sealant on the threads would help too....

 

[doug748]

It is another thing to pull out of the deck, stub your toe against, painfully sit on, or remove if you decide to have a change.

Leaks are a bugger to trace behind a fixed headlining.

You might consider trying your downhaul without the guides, maybe you will be happy without.

 

[lw395]

It is another thing to pull out of the deck, stub your toe against, painfully sit on, or remove if you decide to have a change.

Leaks are a bugger to trace behind a fixed headlining.

You might consider trying your downhaul without the guides, maybe you will be happy without.

Some truth in that.
Also, if the line so much as brushes against the GRP when it is tight, the friction is quite surprising.
You could consider a rivnut in GRP for a blind fixing?
But I have found self tappers absolutely fine. Just drill the fitting out a bit and fit nice big ones, and get the hole the right size. I was lucky, I had a section cut out for a window to experiment on, it was pretty thick.