Alternatives to self tappers?

[RobF]

I'm looking to put a couple of jamming cleats on my boat. My preference would be to use bolts with large washers behind the nuts to keep it secure, but the internal moulding prevents access. I'm aware I could secure the cleats with self tappers into the fibreglass (the load on the cleats is unlikely to exceed 50Kg), but was wondering if there were any alternatives which would be a bit more secure than self tappers (e.g. rawlplugs or the style of fixings you get for plasterboard walls).

Thanks

 

[PuffTheMagicDragon]

Depending on your skill with fibreglass, you could cut a channel in the top moulding (deck or coaming?) where the cleat is to go, dimensioned such that it would be covered by the jamming cleat. You could then rebuild the GRP with a stainless flatbar embedded in the new lay-up. Once cured it is then a matter of drilling and tapping to take the securing bolts, the tapped flatbar acting like an embedded captive nut. Only you can decide whether it is worth the trouble.

 

[William_H]

A few thoughts come to mind. Firstly drill an approriate sized hole and tap a proper thread into the GRP. This will be a lot better than self tapper. If you doubt the strength of just tapped screws then make a flat plate of SS larger than the cleat and bolted to the cleat on top of GRP with more holes and screws tapped into the GRP.
The channel idea by Puff could allow you to slip a ss plate bigger than the channel under the GRP. Tap the holes you need for the cleat into the SS plate before slipping in. With more than 2 holes in the cleat/plate you can use wire to guide the plate to the right place then fit bolts into the other holes. Then unthread the wire.
However my guess from my own boat is tha tthe cabn top is reasonably thick such that tapped holes with screws will be fine. Indeed I always tap a hole fr a through bolt because it means the screw will hold tight while I fit a nut underneath. Doesn't need 2 people one outside one inside. good luck olewill

 

[KINGFISHER 9]

I've just been pondering a similar problem ... how to fit a flagstaff socket with 3 holes into a glassfibre coaming ... screws or bolts? ... assuming I can get to the other side for the nuts and use a backing plate .. would that be necessary with a socket?

 

[macd]

I've just been pondering a similar problem ... how to fit a flagstaff socket with 3 holes into a glassfibre coaming ... screws or bolts? ... assuming I can get to the other side for the nuts and use a backing plate .. would that be necessary with a socket?

Screws would be fine for that. Not much load on a flagstaff, and three fasteners to share it -- until someone uses it as a handhold. So you decide which you want to yield first

 

[PuffTheMagicDragon]

I've just been pondering a similar problem ... how to fit a flagstaff socket with 3 holes into a glassfibre coaming ... screws or bolts? ... assuming I can get to the other side for the nuts and use a backing plate .. would that be necessary with a socket?

With all the flapping that a decently-sized flag will do if there is more than just a F2 breeze I would definitely go for proper bolts, nuts and backing plate. I would bet that, sooner or later, screws or self-tappers would tend to shake loose.

 

[William_H]

I've just been pondering a similar problem ... how to fit a flagstaff socket with 3 holes into a glassfibre coaming ... screws or bolts? ... assuming I can get to the other side for the nuts and use a backing plate .. would that be necessary with a socket?

Use bolts and tap a thread into the GRP. Then if you can fit nuts and big washers under the nuts. If you can't get in there to add nuts then don't and see how long it lasts before becoming loose. If it does try again to fit nuts or retap with bigger bolts.

Or make up a backing plate of SS or ali. Tap the holes in the plate. Drill the clearance sized holes in the GRP. Use some old rigging wire as a snake to get from the holes to where you can reach inside the hull. Use the sake to run a piece of cord or wire through 2 holes in the plate the 2 ends going back to the inside of the stern and out the 2 corresponding holes Now when you pull the cord for the outside the plate should be pulled up to the inside of the GRP and in location so that you can get the screw into the remaining vacnt hole. Tighten this up so that plate is in place. Pull the string or wire out and fit the remianing 2 screws. good luck olewill

 

[rogerthebodger]

Or make up a backing plate of SS or ali. Tap the holes in the plate. Drill the clearance sized holes in the GRP. Use some old rigging wire as a snake to get from the holes to where you can reach inside the hull. Use the sake to run a piece of cord or wire through 2 holes in the plate the 2 ends going back to the inside of the stern and out the 2 corresponding holes Now when you pull the cord for the outside the plate should be pulled up to the inside of the GRP and in location so that you can get the screw into the remaining vacant hole. Tighten this up so that plate is in place. Pull the string or wire out and fit the remaining 2 screws. good luck olewill

That's how I do all fittings on my GRP boats (tender and motor boat). On my steel sail boat I weld the stainless steel " backing plate" on top of the deck of hull. I do it this way as I then have no through holes in the deck or hull so no leaks.

 

[SAMYL]

You can get special Rivetnuts for GRP. Ebay or fastening supplies supply them.

Much better than self tappers or a threaded hole.

 

[Gordonmc]

You can get special Rivetnuts for GRP. Ebay or fastening supplies supply them.

Much better than self tappers or a threaded hole.

That's the way I would go. Rivnuts come in stainless steel as well as aluminium. Used with a setting tool they are pretty well bulletproof.

 

[lw395]

Screws would be fine for that. Not much load on a flagstaff, and three fasteners to share it -- until someone uses it as a handhold. So you decide which you want to yield first

Indeed.
Someone will always be able to break it, do you want a pulled-out 6g screw or a broken flagstaff?