Blind hole fastener suggestions?

[LadyInBed]

Here's a thought.
Drill a hole, cut a slot across it ø large enough to push a s/s wing nut through it.
Flatten down the tops of the wings (file or grind), screw in from the wing side of the nut a s/s bolt (so wing nut is inverted).
Push it through the slotted hole, rotate the wing nut through 90° , pull the nut tight against the fiberglass and tighten the bolt.
You could put a bit of epoxy on the wings of the nut to hold them captive, also fill the slots if you wanted to go that far.

 

[rogerthebodger]

I need to fasten an outboard storage bracket on the stern of my GRP motorboat. Engine is a 5HP 2-stroke. The area on the stern has a double skin, approx. 100mm apart, with no access from the other side or from below. So I am considering some kind of blind fastener. Has anyone tried using snaptoggles? They have some with a 304 stainless steel toggle. Any other tried and tested suggestions?

I would cut a mounting block from either thick ply of HDPE maybe 20mm thick that is as big as you can fit at the required location.

I would then cut a square hole in the area that is big enough to get your hand in.

I would then cut some inner blocks/strips from the stainless steel then has tapped holes to mount the outer block onto gluing these stainless blocks/strips inside lining up the holes and bolting on until the glue sets.

The outer block and outboard bracket can then be attached and sealed covering the access hole. This will distribute the stresses over as large as possible.

I have used this method to mount various handles, cleats and rowlocks on my double skin GRP dinghy.

 

[Refueler]

You need to increase the area taking the load.
You could try drilling a relatively small hole in the outer skin, say 5 or 6mm and then using a bit of bent wire to remove the foam or balsa between the skins, to generate cavity say 35 to 45mm in diameter. It doesn't have to be tidy, use a vacuum cleaner to suck out the bits.
Inject some epoxy thickened with micro fibres to fill the cavity, closing it off with some masking tape until it's gone off.
The most difficult bit will be knowing and getting the cavity full, it may need a second small hole at the top of the cavity to inject the epoxy, after first tapping over the first hole.
Then fix the outboard bracket with some heavy gauge short SS self tapping screws.
The epoxy plug will share the load between the inner and outer skin, and spread the load over a larger area.

If I was filling the cavity with epoxy .. I would rather use rivetnuts or expanders pressed into drilled holes ... then machine bolts. ... as per this style ... :

I have re-fixed my stanchions with these and years later ... still rock solid.

 

[Rappey]

remove the foam or balsa between the skins,

Surely it it were filled with balsa or foam then why remove it?
My suggestion would be to fill the possible void with expanding foam, then bolt right through it.

 

[Refueler]

Surely it it were filled with balsa or foam then why remove it?
My suggestion would be to fill the possible void with expanding foam, then bolt right through it.

No .. its like bolting hardware to a cored deck ... get rid of the core and inject resin or similar to create a strong durable area.

 

[Rappey]

Yes... with a 100mm void filled with foam, a decent backing pad from something like kingboard and same on other side it will be very strong .. Have you tried crushing foam between grp? If it crushed easily then yachts foam filled hulls would flatten between the inner and outer hull from wave action ? (yes i know they can over time)
its just another suggestion as there is no wrong or right way to tackle the problem

 

[Refueler]

Yes... with a 100mm void filled with foam, a decent backing pad from something like kingboard and same on other side it will be very strong .. Have you tried crushing foam between grp? If it crushed easily then yachts foam filled hulls would flatten between the inner and outer hull from wave action ? (yes i know they can over time)
its just another suggestion as there is no wrong or right way to tackle the problem

Hull construction overall creates a self supporting shape that as its pressured externally increases its resistance ... the two hulls separated by core material is just that - a core material to allow two materials to stay in alignment with each other and supportive.

Also many hulls are actually not cored same way as a deck ....

Anyway - so by your reckoning all those ways of removing core and filling with resin that boat builders and other experienced boat repair people use are wrong ?

I have to say that my Kormoran 5.5m day cabin sailer ..... has a fixed bracket on the transom bolted to the outer hull skin. BUT on the inside are serious load spreading bearers and coachbolts. I actually cut a large area out of the inner hull cockpit transom so I could work between the skins ... I then fitted a louvred vent over the cutout to allow the aft end of boat to 'breathe' ....
That bracket carries a 4.5 Johnson Twin.

 

[Rappey]

Wow refueller.. your the one who mentioned decks, not me.. your the one talking about removing core , not me.. Your the one mentioning resin, not me ??
and now your bringing boat repairers being wrong into the equation ? I mentioned nothing of the sort..
How do you read so much into a simple post where i suggested another way out of many possibilities could be to use expanding foam?
We are talking about an outboard bracket, not as mentioned earlier mounting a huge v8
Lets not forget it neither hull or deck, just two panels of grp with a possible 100mm void in between..

 

[Refueler]

Wow refueller.. your the one who mentioned decks, not me.. your the one talking about removing core , not me.. Your the one mentioning resin, not me ??
and now your bringing boat repairers being wrong into the equation ? I mentioned nothing of the sort..
How do you read so much into a simple post where i suggested another way out of many possibilities could be to use expanding foam?
We are talking about an outboard bracket, not as mentioned earlier mounting a huge v8
Lets not forget it neither hull or deck, just two panels of grp with a possible 100mm void in between..

No WOW to you ...

YOU posted :

Have you tried crushing foam between grp? If it crushed easily then yachts foam filled hulls would flatten between the inner and outer hull from wave action ?

It was your rubbish statement that prompted my post ....

The removal of core was actually first posted by another NOT me ... I merely support his suggestion .... check out Post #20.

 

[Ian_Edwards]

If I was filling the cavity with epoxy .. I would rather use rivetnuts or expanders pressed into drilled holes ... then machine bolts. ... as per this style ... :

I have re-fixed my stanchions with these and years later ... still rock solid.

Yes, I agree the above would make a good fastening. As an aside, these look more like rawlbolts than rivet nuts, you could also use threaded inserts.
I used threaded inserts to re-fix the stations on the boat this winter and they appear to be very solid, although they haven't had any real world testing yet.

Slotted Self Tapping Threaded Inserts AN 335 External and Internal Thread | eBay
hash=item3b19f12aae:g:Sz8AAOSwXk9bgHtw&var=553131021265

These allow the machine screw to easily be removed., but require a reasonably this and solid base to work well.
But at the end of the day a big self tapper will have plenty of grip for a small outboard storage bracket, there are lots of options which will work, it just depends on how much effort you want to put into the modification.