Securing deck hardware when you can't get to the backing

[jbinbi]

Originally Posted by deblen

Just use self drilling self tapping 316 or 18-8 screws for a light duty application such as yours. Apply a daub of sealant/bedding compound to back of bracket.
https://www.mcmaster.com/products/st...ews-for-metal/


https://www.boatus.com/expert-advice...right-fastener

Thanks for these links. Yes, it seems for this job, just using a machine screw will work fine. Since I am putting this on part of the cockpit that is exposed to rain etc., want to make sure it is water tight. What type of sealant should I dab in there/coat the screw with? Another person told me to take a small dab of butyl tape over the hole and just screw through that?

[s/v Jedi]

I recommend against screwing into thin fiberglass. It will lead to leaks and failure a couple years down the line.

Simply buy a piece of 1/8” G10 fiberglass (Amazon), cut a nice fitting piece from it, then mount the pad eye to that using countersink heads on the fiberglass back and acorn nuts at the pad eye.

Now you have something you can either glue down permanently or semipermanently. You can use thin VHB tape, superglue, even good quality hot glue.

[ggray]

You could use a couple of these:

https://www.boltdepot.com/Product-De...CABEgJfDvD_BwE

This would be very strong.
Only thing is you have to bore a larger sized hole that you would need to seal.

[deblen]

Originally Posted by jbinbi

Thanks for these links. Yes, it seems for this job, just using a machine screw will work fine. Since I am putting this on part of the cockpit that is exposed to rain etc., want to make sure it is water tight. What type of sealant should I dab in there/coat the screw with? Another person told me to take a small dab of butyl tape over the hole and just screw through that?

I recommended sheet metal self tapping self self threading screws-not machine screws. Machine screw or bolt threads are too fine for fiberglass less than 1/4" thick & no good for any core.

Fiberglass requires an aggressive coarse thread. Screws should be power driven into fiberglass without drilling for best thread strength.
If fiberglass is less than 1/4" thick,I would go with toggle bolts-per GGray post.
I recommend good butyl (windshield) tape or 4200/sikaflex bedding compound. Screw drill the holes,remove screws,countersink holes slightly to allow compound to form a donut around the screw shank & re-assemble.

I cannot see the advantage/strength of thread inserts,unless they are mechanically "Flared" or some other method of attaching them from the "inaccessible" back side.Perhaps I am missing something.

You indicated that you may want to remove this gear in future & restore area to original. Nothing easier than removing screws,wiping butyl,plugging holes with white fairing compound,gelcoat,sand & buff with 3M cleaner buffing wax-or similar.
Cheers/Len
https://marinehowto.com/bed-it-tape/

[deblen]

The Gougeon Bros. performed experiments on hardware & fasteners.
Check out their free book-pg 129
https://www.westsystem.com/app/uploa...k-061205-1.pdf

[s/v Jedi]

Originally Posted by deblen

I recommended sheet metal self tapping self self threading screws-not machine screws. Machine screw or bolt threads are too fine for fiberglass less than 1/4" thick & no good for any core.

Fiberglass requires an aggressive coarse thread. Screws should be power driven into fiberglass without drilling for best thread strength.
If fiberglass is less than 1/4" thick,I would go with toggle bolts-per GGray post.
I recommend good butyl (windshield) tape or 4200/sikaflex bedding compound. Screw drill the holes,remove screws,countersink holes slightly to allow compound to form a donut around the screw shank & re-assemble.

I cannot see the advantage/strength of thread inserts,unless they are mechanically "Flared" or some other method of attaching them from the "inaccessible" back side.Perhaps I am missing something.

You indicated that you may want to remove this gear in future & restore area to original. Nothing easier than removing screws,wiping butyl,plugging holes with white fairing compound,gelcoat,sand & buff with 3M cleaner buffing wax-or similar.
Cheers/Len
https://marinehowto.com/bed-it-tape/

I agree with the course thread, but if you drive a screw through without drilling and countersinking, there will be big chips of gelcoat flaking off.

No, on thin fiberglass without access to the back, gluing is the best option and there is every level of glue available, from temporary to permanent, but even that can be ground off.

[deblen]

Originally Posted by s/v Jedi

I agree with the course thread, but if you drive a screw through without drilling and countersinking, there will be big chips of gelcoat flaking off.

No, on thin fiberglass without access to the back, gluing is the best option and there is every level of glue available, from temporary to permanent, but even that can be ground off.

I agree on the (usually) minor chipping caused by not pre-drilling.
I remove the screw & countersink after(for sealing compound) so chipping is not a concern. Also,you can eliminate chipping by drilling or driving in reverse a few turns first. I am suggesting self drilling screws-with built in drill tip-not tapered wood screws.


My other suggestions were to minimize possible damage to original gel coat,because he said he may remove & restore area at later date.
My personal experiences with any glues/resins is that the ones that work, have caused damage and/or much work when removed.
Glad you have found otherwise.
Len

[catsketcher]

There is another method to bolt onto things when you can't get to the back. I used to use it in my youth to bolt saddles onto my Laser boom.

Put a very thin piece of cotton in the hole with a small weight at the end that will fit through the hole, even a needle is usually enough weight. Then keep feeding the thread until the needle and thread appears. (I am hoping the OP will be able to have the thread fall down somewhere he can access).

Then tie on a long flat head machine screw to the thread with a washer and pull it back up to the hole. Some jiggling can get the the screw to pull through the hole. Then put the saddle through the bolt and then lightly hold the screw with pliers (put some fabric on them if needed) and use a spanner to spin a nyloc nut and washer on.

Eventually you will come to the pliers and can't spin the spanner anymore. BUt your machine screw is long enough to then put the pliers on the end of the screw thread. Then you can hold the end and tighten the nut hard down.

I have a 30 year old laser boom out front with the bolts done this way. It is as strong as normal bolting - just the reverse way around. At the end, cut the excess off.

Easy, cheap and really strong.

[CarlF]

Self tapping screws are the best way in thin decks.

But in thicker decks West Gflex epoxy works very well. Gflex bonds well to dissimilar materials like stainless and fiberglass. It is also somewhat flexible so it doesn’t crack.

Glue both the fasteners and the base to the deck. It can be removed by heating it with a hair dryer or hot air gun

https://www.westsystem.com/instructi...-and-hardware/

[s/v Jedi]

So how about pop rivets?

[deblen]

Originally Posted by s/v Jedi

So how about pop rivets?

Should work fine.
Punch the remains of the nail mandrel out & filler the center hole.

[Bloodhound]

Since the back is completely hidden, what about toggle bolts in this application , or even closed pop rivets?

[Cheechako]

Originally Posted by jbinbi

I want to install a small pad eye in the cockpit to hold my locker open. Where I want to install it, I can't get to the underside that I could secure it with a bolt. I know that that my dodger was installed without nuts on the backside of the bracket mounts, so this is always done. Just don't know how..

What type of screw should I be using? I am assuming I drill a pilot hole, caulk, then screw? Sorry for this simple question.

Drill and tap just like metal, then use a machine screw is how I usually do it. Clean dust from everything then caulk and mount.
I never allowed a dodger or bimini to be mounted any other way. Those guys will just drive a screw in chipping gel coat etc.

I have done this hundreds of times with the drill and tap method.
There are various forms of blind nuts etc available in SS also, including simple drywall types of expanding nuts in SS if the glass is real thin. But 3/16" thick and up the tap works fine for many uses.

[iansan5653]

I like banana peel rivets for low-medium load applications. I feel like fiberglass isn't hard enough for pop rivets - they will expand and crack the hole. But banana peel rivets split open instead of expanding so they are great for this application.

My tiller holddown line runs across the cockpit between two clamcleats that are held down with banana peel rivets and it's worked perfectly fine. And it was easy to install. (https://imgur.com/a/mHMKvii)

This is what they look like when they are installed: https://imgur.com/a/ek1W8PB

[parkstone bay]

As I read the OP, the question is less on the lines of how to get round/fudge the inaccessibility issue, because there are plenty of ways of doing that, but more of how to make the best possible job. (Ideally - as probably most responders have at the backs of their minds - you would in fact be able to access the back of the location to add an appropriate load bearer).
In the circumstances, and it's only a suggestion, would some form of cavity wall fixing work - assuming you can find one in suitable stainless?