Blind hole fastener suggestions?

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DavidMcMullan

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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?
 
Those things impose the load over a small area, and may only be suitable if the outer skin is thick, otherwise distortion and breakout are possible. The normal way of fixing anything like an outboard bracket is to put a substantial load bearing piece on the inside, which means gaining access. Of you cannot do that I suggest seeing if you can mount the storage bracket elsewhere. The other disadvantage of those toggles is that they have to be inserted through a much bigger hole than the actual bolt, which is both a weakness and a possible source of water ingress.
 
DavidMcMullan said:
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?
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I would not use those. If there is no way into that void at all and you really can't find another location then suggest one of these in the inner moulding and then fit propper backing pad and bolts.

Www.solocoastalsailing.co.uk
 
I will confess to using plastic plasterboard fixings to fix the toe straps on my GRP Minisail.
Plasplugs type of thing?
It worked fine.

Other games you can play include threading a bolt with a penny washer on a wire and drawing it into place from wherever you have access. Cutting a slot and poking in a drilled and tapped metal plate.
You could use rivnuts.
You could use Wellnuts. These are rubber things with a tapped insert, they fit in the hole and expand. They are used on motorbike fairing and drive me around the bend TBH. (the insert is brass, corrodes to the screw and then shears away from the rubber, so they don't come undone).
You can epoxy on a block of wood or other material with bolts through the block or simply screw into the block. Size the block to suit the load.
You could use pop rivets. People will suck their teeth at this but it works!

I would think the snaptoggle would work fine, provided the GRP skin is thick enough to take the load over the area of the toggle. If it's not a dinghy with wafer thin GRP, I expect you'll be fine. It's better than self tappers and it's surprising what loads they will take.
 
Or make up a board exceeding the size of the bracket base and attach it to the outside of the GRP. Use lots of epoxy for strength over a large area. Attach the board with screws tapped into the GRP or if you must use self tappers. Just to hold the board in place while epoxy hardens. Obviously make arrangements to attach the bracket to the board first. ol'will
 
DavidMcMullan said:
The area on the stern has a double skin
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Given the loads an OB is likely to impose, I'd be tempted to go drill the internal skin, maybe 50mm diameter, and epoxy a bit of pipe to fill the gap between the skins, fill the pipe with epoxy and drill through that for proper bolts with nylock nuts.

You say that it's a storage bracket so, presumably, it's a secondary engine. Do yourself a favour, if you can. and get something like this
Trem-Aluminium-Outboard-Motor-Bracket-Plastic-Pad-upto-20HP.jpg

You might be very glad of the ability to drop it down and drive off in a hurry one day.
 
lw395 said:
It's a 5HP 2T, not a V8...
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Its not just its weight you have to consider but the forces imposed by the motion of the boat, a 25KG outboard mounted so that its centre of gravity is a few inches away from the fixing points will impose some very powerful forces on those fixings if the boat is being driven in a choppy sea.
 
So drill a 3mm hole and see how thick the GRP is. Put a small fastener in and try it for strength.
A lot of elaborate stuff being proposed which probably won't be any stronger.
 
I presume it is somewhere to put the tender outboard rather than a mounting to be used for propulsion?
I would be reluctant to drill holes into a blind area, there is the possibility of leaking, is the void foam filled?
I wonder if a pad could be glued on with enough strength? Can you drill right through two faces and bond in a slieve?
 
Stemar said:
Given the loads an OB is likely to impose, I'd be tempted to go drill the internal skin, maybe 50mm diameter, and epoxy a bit of pipe to fill the gap between the skins, fill the pipe with epoxy and drill through that for proper bolts with nylock nuts.

You say that it's a storage bracket so, presumably, it's a secondary engine. Do yourself a favour, if you can. and get something like this
Trem-Aluminium-Outboard-Motor-Bracket-Plastic-Pad-upto-20HP.jpg

You might be very glad of the ability to drop it down and drive off in a hurry one day.
Click to expand...

I too was thinking about having suitable pipe between the two GRP skins ...
 
Thanks for the different suggestions. Yes, it is a bracket for storing the outboard, when not in use on the dinghy. My boat is a 38ft Nimbus. The inner skin surface is curved behind where I want to fit the bracket, so difficult to mount an access hatch there. Will check if I could fit one offset, with hand/spanner access to the required area. But may try the snaptoggles, if I can find a supplier of the stainless type, who sells in small numbers.
 
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.
 
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