Screws in GRP

[BobPeters]

When attaching things like winch handle pockets on to the white plastic cockpit of my new Bavaria, should I just drill a hole and use a wood screw, or should I seal with some sort of silicone, etc. ? Glad of any recommendations.

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[extravert]

Wood screws are no good because they have no thread near the head, and the thread shape is wrong anyway. Use short stainless self tapping screws which will have thread right the way to the head. Cockpit layup is usually thin, make sure you don't go through to anything you shouldn't on the other side.

Screws in GRP and generally a bad idea for anything permanent or load bearing. They are a cheap quick fix used by sloppy boatbuilders. Screws expand the hole they are in, which shatters GRP and will fail sooner or later. Through bolting is much better, even for small non-load bearing attachments like a winch handle pocket, but you may not be able to get to the back to put on the nut. You should always use some support behind the nut, a sizeable pad if load bearing, a penny washer if not (as in your case). If your layup is foam or balsa cored then do not do up the nut at all tightly, you will crack your gelcoat. Finger tight and then a little bit more is all that is required in this application.

A bit of gunge to stop water getting in is required. B&Q type silicone sealant is not really the right stuff. Better is marine sealant like Sikaflex, but it is expensive and kept for long once open it goes hard in the tube, making it more expensive still. Cheap though compared to getting water into a core and damaging it.

I have heard that an opened tube of Sikaflex will stay useable if kept in a freezer. Not tried this though. Anyone know if it works?

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[rusty_nail]

I use sykaflex to seal all screw fittings

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[oldsaltoz]

I agree with Extravert and would add the following.

Forget any screws in GRP, even heating them first is risky, they will cause cracking.

When drilling it is most important that the edges are sealed, drilling an over size hole, filling it and drilling to the required size is a very good practice; when drilling foam, remove some or the foam (A bent hard wire in a drill makes easy work of this) fill the hole and re drill to required size, this will ensure no water enters the core material.

When fitting load pads, mix a batch of resin and fibres to form a grout, this will ensure the load is evenly spread and will help prevent stress cracking later, put a dab of wax on the bolts and don't fully tighten till the groat has gone off.

Well fitted and maintained craft last longer, look better and sell for more.

I hope this helps. . . . . .



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[Wire_Less]

Sikaflex is the great, but beware when removing fittings in years to come it can pull gel coat with it.

A tip I was given by a boat builder was to smear the boat surface thinly with vaseline so the sealer just seals rather than sticking.

If using self tappers the correct drill size is also very important.

Good luck

Tim

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[Evadne]

Sikaflex also comes in smaller "squeezable" tubes, that only cost half as much as 10 times the volume in a gun-tube. Squeezable tubes seem to stay un-solid for a bit longer though.

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[ccscott49]

Anybody tried drilling and tapping threads into the GRP and using stainless machine screws? Try a slightly larger than normal tapping size drill, it works for me, for non load bearing items, just a smear of sikalflex is enough, I usually put a washer between the item and GRP, to allow water to run away from behind and not sit there festering.

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[colvic]

I take the nozzle off the tube, wrap cling fill round the fitting and put the nozzle back on. The silkaflex in the nozzle sets within three days but the contents of the tube have been useable upto three weeks later. Just pull the solid silkaflex out of the nozzle, remove cling film and away you go.

Found the black lasts longer than white.



Phil

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[chippie]

To attach winch handle pockets you could glue them on with sika flex or the like.
the pockets could be held in place with duct tape until set. ( depending on the available surface area of the pocket to glue to)

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[theia]

countersink the hole before applying sikaflex to ensure a watertight seal

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