What fasteners for Rope Clutches

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Snowgoose-1

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I am unable to access the grp underneath due to a double skin (not foam sandwich).

Blind bolts, self tappers , or glued in threaded inserts seem likely candidates .

As the pull is largely lateral, is there any reason why I just can't drill and tap the fibreglass and use stainless threaded bolts ?

TIA
 
When I fitted cleats to my double skin dingy I made-up stainless-steel plates with threaded holes and maneuvered them in place using string through the hole drilled into the skin

If you cannot get inside cut a slot for the flat to pass through between the holes that will be covered by the rope clutch

Don't forget the cluch will have to take the full load you can exert by the winch
 
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Deck fittings on my boat are also fitted above an inner liner. With washers and nuts underneath (in the void).

Underneath large holes were drilled into the inner skin which are covered with tight fitting, varnished teak discs, or little rectangles.

They stand out but they look ok given that they are clearly functional, I have removed many of them to discover that the fittings have been leaking into the void above the ceiling.
 
When I did the same thing, I drilled both layers to the size of my bolts, then just the cabin top slightly oversize to take a bit of tube. I bolted everything through the tube, which prevents the two being drawn together, with SS penny washers and dome nuts. It worked fine, and I thought it looked fine from underneath.
 
Consider interscrews (aka sex bolts) but cut the threaded outer sleeve to fit the space and bolt from both sides, a variation on Stemars suggestion. There are a fairly large range of design of Interscrews and suppliers are focussed at different applications - so search wide. Another option is to buy the interscrews, which come as a 2 part set, dispense with the male portion and replace with something that has the high tensile strength needed for the application (clutches) (you effectively just then use a threaded tube to keep the inside and outside skins apart.). Some interscrews allow the male portion to be thread right through the female section, others are restricted. You will still need the backing plates/penny washers - but if you source stainless interscrews and use polished stainless backing plates - they look pretty neat.

Jonathan

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Poignard's suggestion is good but it may be that there is no filler material in-between the two layers of the double skin. Hard to know without what boat etc it is which might throw more light on the method of contruction although a hole drilled might reveal.
As Pandos says of his fittings you could cut out a panel in the inner lining to fit a backing plate or similar and the cover up those apertures with nice varnished ply (or other material) cover
 
My boat also has a fibre glass lining below the coachroof. About 20 years ago a block of 5 spinlock clutches came into my possession and as a temporary measure I drilled the coachroof, tapped the holes and screwed them down (8mm IIRC). They are still there. The same year a double clutch appeared and was attached the same way but with epoxy putty pushed into the drilled holes before redrilling and tapping. These clutches carry foresail halyard loads, pole downhaul, Cunningham and Vang (5 block) and spinnaker halyard and pole downhaul (2 block). There is no sign of stress after all this time.
My logic includes the idea that a block with multiple hold-down screws (10 for the 5 and 4 for the 2 block) with shear loading and very small moment is more than adequate. I might be less enthusiastic about a single clutch with a high load.
 
Have successfully used long hexagonal stainless stud connectors bedded in resin to accept bolts from one side .Easy to rough up the flats with a hacksaw to prevent pullout.
 
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