Stainless Steel Captive Nuts

[3571]

Does anyone know of a supplier of captive stainless steel nuts?

I have a radar tower, one of the supports for which was for some bizzare reason fixed to the boat with self tappers straight in to the fibreglass. It has inevitably worked loose in pretty savage weather, and needs fixed right before the entire thing lands around my ears.

There is absolutely no access underneath to use normal backing plates, nuts etc (which will be why self tappers were used).

My options as far as I can see are to cut out and fit an inspection hatch to gain access (4" seems the smallest I can find, which will not look pretty) or find some sort of up to the job stainless captive nut set up or something similar, which I seem unable to do so. Or hope that someone can come up with a better idea.

 

[Simes]

Try drilling an over sized hole into the GRP then Epoxy a S/S nut into the hole. You can then screw directly into the now captive nut!

Simple is good.

Simes

 

[SAMYL]

Or try some of the plastic expanding wall / blind hole fixings that have a bolt or screw into them. Some of the plasterboard type ones might suit. There are many different types available. Look up www.screwfix.com for some ideas.
You can then replace the normal screw or bolt that is used with a stainless one.

 

[nimbusgb]

Or try some of the plastic expanding wall / blind hole fixings that have a bolt or screw into them. Some of the plasterboard type ones might suit. There are many different types available. Look up www.screwfix.com for some ideas.
You can then replace the normal screw or bolt that is used with a stainless one.

But the nut and retainer will be galvanised or passivated mild steel so numless you cover the whole lot in some gunk or epoxy it's all going to fall apart in a year or two.

I fitted two of the dinghy inspection hatches into a coaming to fit my radar arch. Damned handy for wire access, backing bolts etc.

One other option would be a footpad of 3mm s/s plate to match the mounting of the arch big enough to cover a hole made to allow fitting the mounting nuts inside the coaming. Trouble with that is that a 4 inch hole is going to weaken the area.

 

[Norman_E]

You say that there is absolutely no access underneath, which raises two possibilities. There may be access but it is very difficult, and on my boat I have had a couple of situations including needing access to a push-pit mounting bolt where a great deal of dismantling, followed by the use of a special spanner was required. Double jointed arms would also have helped. The other possibility is that for something that needs to be strongly fixed, it may just have been put in the wrong place. A thing like that needs backing plates and moving it may be the best option. If it is a stainless fabrication and its other "legs" are properly fitted, could you have it modified so that the offending "leg" is planted somewhere better?

 

[snowleopard]

The only maker of captive nuts/studs I know is Bighead. They are designed to be bonded into plastic/grp. The trouble is that they won't sell them in quantities of less than 100 and they are around £1 each. I thought I'd use a lot when building my boat but in the end most of them are still in the box. You may get lucky and find a retailer who has some in stock.

If it's any help the ones I have left over are 25mm x M6 studs.

 

[Poignard]

What I would do is to bond the fastenings into the GRP deck with epoxy, using the method shown in the WEST Instruction Manual.

http://www.westsystem.com/ss/bonding-hardware/

and

http://www.westsystem.com/ss/fastener-bonding/

If the deck is too thin I would cut 3/4" off the tower leg and build up an epoxy pad 3/4" thick on which to mount the leg foot.

Properly bonded fasteners in epoxy are very strong. I used the technique to bond some fairleads to my toe-rail and, despite some very heavy loads imposed on them, they have never moved in 12 years.

 

[3571]

Thanks for all the responses gents, cutting 4" Inspection hatches and fitting backing pads to spread the load along with proper nuts is looking the favourite.

 

[Rigger Mortice]

Have a look at "Rivnuts". They are similar to pop rivets, but you end up with a captive nut rather than a rivet. Available in different metals but I've only used alloy ones.

You need a tool which is similar to a pop rivet gun but NOT the same.

I've not used them on fibreglass yet. Others may have a view on their suitability, but I'ld certainly consider them.

 

[CharlesSwallow]

Thanks for all the responses gents, cutting 4" Inspection hatches and fitting backing pads to spread the load along with proper nuts is looking the favourite.

Boater's World in Corfu stock large numbers of these but the only place I've ever seen them in the Uk is at Index Marine. Do a google.

Chas

 

[AndrewB]

You don't mean castelated nuts do you? I got some A4 nuts from here.

 

[CharlesSwallow]

You don't mean castelated nuts do you? I got some A4 nuts from here.

Doesn't sound as if he does to me. A "Castellated" nut is one, as it's name implies that looks like the turret of a castle with cut-outs around the top to facilitate it's being locked by a split-pin inserted through a hole in the bolt. The OP is looking for a type of nut (there are several patterns available) which is fixed to one of the parts to be bolted together.

This is a "Castellated" nut:-

Chas

 

[vyv_cox]

I'm a big fan of Rivnuts but I think it would be tricky to get them installed correctly on fibreglass. The thickness of the material needs to be known fairly accurately so that the stretch and fracture of the Rivnut pin can be preset. If it's too tight it could well crush the GRP. I mounted four in the end casting of my boom, several millimetres thick, and it took me several attempts before I got it right.

 

[Topcat47]

Quite a dilemma. You need to spread the load. If the area under the deck is completely inaccessible as opposed to just really difficult to get to, there is no real alternative to fixing into the grp. You still need a broad base. 5mm SS plate would be my minimum option, although anything over this would be an advantage. You could weld nuts to the reverse side of this, but you'd need clearance holes drilled in the deck to allow it to sit flat.

Epoxy is good stuff but the problem with epoxying a strong baseplate to the deck will be that it (the adhesive) has poor peel strength. You need a good flat surface on the deck, to make sure the epoxy is as thin as possible, matt surfaces (as opposed to bright and shiny) and layering some mat around the edges (also with epoxy) will help. By all means fix the base plate to the deck with self tappers, but these should not be considered as the sole method of securing the base.

When I needed a couple about 15 years ago, bighead were kind enough to supply them FOC but you have to get them behind the decking and they are designed to be moulded into the glass. You have to be able to get to the site from underneath to use them.

If the deck of your boat is of sandwich construction, it's a bit easier as you can cut around the SS plate remove the layer of sandwich material and epoxy the plate to the lower GRP layer. Then build up over the SS plate with mat and epoxy until you have filled the void and finish with some woven cloth extending over the join to provide a finished surface.