Ezloc threaded anchors

Has anyone installed a threaded anchor such as ez loc anchors into a fiberglass deck? I’m assuming it’s solid fiberglass or thick layer of glass on plywood. It’s the foredeck next to the anchor locker on a97 MS 350 trawler.
I need to make a secure strong attachment. There is no access to the backside of the deck for backing plate or thru bolting.
Only other option are short 1/2 “dia screws.
Thoughts?

I'd suggest drilling a small pilot hole where one of these anchor might be placed. Use say a 1/8" bit. Examine the cuttings as you drill to work out how deep the GRP extends, and what is below it, possibly wood. Such a small hole will be a fairly inconspicuous repair if you later don't need to use that location.

If you find you have a thick GRP layup then it is quite feasible to tap a thread into the GRP just as you would into some metal. But if your intended use isa cleat or bollard for a snubber , then it may be marginal in terms of shear load strength.

If you find perhaps 1/4" GRP and then wood, try and determine how thick the wood is. If quite thick then a screw might be OK for your purpose. But I would probe a bit deeper and if you are lucky then you may find a void below GRP or GRP+wood. Even though you don't have access to it, it may still enable you to insert a spring-toggle style anchor sometimes used for drywall situations where you don't have rear access but need to support a heavy load eg wall mounted TV's.

If you do find a void, then perhaps enlarge your initial hole just enough to insert a borescope camera. You should try and verify that your intended locations for all of the full-size fasteners are free of hidden obstructions. You may find that you will need to reposition your planned fastener locations a little bit.

The person who set up the mast and boom rigging and some other hardware on our boat drilled and tapped right into the fiberglass then used machine screws. It has endured some significant loads without budging. All depends on your application but extra bits of hardware might not be needed.

Thanks for the replies. The tailings on the 1/8” drill bit were white powdery. To work with I have deck thickness of approximately 3/4”, a gap of approximately 1/2” then the fiberglass shell of the cabin . I accidentally punched thru. Now I have an 1/8” hole over my pillow.
I purchased a Mainship factory Sampson post. It is ment to be bolted to the back of the anchor locker with six 3/8@ dia. Bolts and a backing plate. And 4 bolts at each corner thru the foot at the deck. These are 1/2” dia.
I’m assuming screws were used since there is no access to the backside at that point of the deck.
I’ve seen a couple MS350/350 trawlers with these and their owners do not know how it was mounted.
Factory install while it was apart?

I’d think a samson post would have to be thru-bolted with a substantial backing plate.

Can’t be done unless there was accsss. About 1/2” gap

If you intend to use the samson post you need a backing plate. Cut a large hole in the liner for a 6" access plate. Cut a circle of stainless plate just thin enough to fit between deck and liner, drill and tap for bolts. Install then cover hole in liner with access plate.

I've mounted a ton of hardware, even heavily loaded, by tapping G10 at about 1.5 - 2x thickness/bolt diameter, up to about 3/8 fasteners. But I would not mount a cleat or samson post with 1/2" bolts that way, especially in a production boat layup which is likely chopper gun or heavy with resin.

I would cut an access hole for a small access hatch. Then use a S/S backing plate or 2 if you need to fit them through the hole. Butter the top side of the backing plate with thickened epoxy and stick it to the bottom of the deck. After the epoxy cures then drill the holes for the sampson post and through bolt it. Then install the access hatch. The thickened epoxy will make the backing plate be in close contact with the deck. The plate will not be in solid contact otherwise since the bottom of the deck will not be completely smooth. Don’t caulk the bottom or seal it in any way. If you get a leak in the bedding you want the water to leak out so you will know it is leaking. I would use butyl tape to bed this. Howtomarine.com has really good butyl and instructions on how to bed the fittings. Be sure to chamfer the top of the bolt holes.

Not that they need my endorsement, but since you are kind of tallying up what people have to say before you decide how to proceed: I heartily agree with DDW, etc. I've tapped fiberglass for numerous things (or tapped my own structurally filled epoxy), but... not for a Samson post, IMO.

There's not only the mess of "torn up holes" if it fails, but also ..... did the boat just come untied? Did the Samson post just fly into your eye? That sort of thing.

Once you have an access hole in the liner, you can install, maintain, re-bed, replace or whatever (plus install solidly). Hopefully you can do that in your location?

If it’s got to be really strong, I like Comodave’s idea. Do it right with a backing plate. Install an access plate on the inside to cover the hole.

If you don’t want to do that I’ve had pretty good luck using Wellnuts for blind fastening. The photo shows them being used in thin material. They also work well in material that is thicker than they are long.
https://www.boltdepot.com/Threaded_...uts_EPDM_rubber_with_brass_insert_3_8-16.aspx

Things like sampson posts and cleats must be through bolted and backed up. Everyone will assume that they are a trusted and secure fitting. Think about in an emergency and a towboat hooks up to the sampson post and assumes it is an adequate and proper fitting. Then pulls it off the boat. Hopefully no one gets injured and there is only damage to the boat...

I’ll use an endoscope to properly view the space between decks before I do anything. If I cut a hole in the fiberglass ceiling I can use a flat light fixture to cover the hole.