Mounting Hardware Where You Can't Reach Behind: Blind Fasteners

Ask what happens if someone grabs it, steps on it, or a wave slams it. If the answer is "a person could fall" or "the boat could break loose", stop: use through bolts and a backing plate (see our through-bolting and cleat installation guides). Blind fasteners are for gear that only has to hold its own weight plus boat motion.

Tap-test with a small hammer to find hollow areas, stringers and bulkheads. Use an inspection camera through an existing opening. Look at where wiring and fuel lines run on similar boats or ask the builder. BoatUS's rule for cutting holes applies to every drill bit: if you are not sure what is behind it, don't cut.

A round screw-out deck or inspection plate near the mounting spot lets you reach in, install a backing plate and real nuts, and check the area later. Pick a flat spot, clear of structure, and seal the cut edge, especially in cored panels (see our inspection port guide). This turns a blind job into a proper through-bolted one.

Measure at an existing hole or opening. Is it solid fiberglass, cored fiberglass (a sandwich with balsa or foam), plywood, plastic or aluminum? The fastener maker gives a grip range for each size; a fastener outside its range won't set properly.

• Thin aluminum or metal: sealed blind rivets of a compatible alloy, or rivet nuts.
• Solid fiberglass or plywood, light load: well nuts, rivet nuts with a wide flange, or toggles if there is room behind.
• Cored panels: be careful. Crushing the core lets water in and rots it. Prefer a port and through bolt, or bonded studs, and seal every hole edge.
• Liner panels with stripped wood screws: threaded inserts bedded in epoxy (Practical Sailor used these to reattach ceiling liner panels).

If you choose bonded studs (no hole at all), the bond is only as good as the prep. Good Old Boat's technical editor and Panbo readers who use them describe the same routine:

• Degrease, then sand the spot with medium paper (Good Old Boat uses 150 grit) and wipe with a dewaxer. Interior gelcoat that came out of a mold can still carry mold-release wax, and paint should be sanded off the spot.
• Use the adhesive the stud maker specifies. Fast methacrylate "plastic welder" types hold a stud in place even on vertical or overhead surfaces; slow epoxy needs tape or a clamp until it cures.
• Press the stud on and give it a slight twist until a small bead squeezes out all around. Wait the full cure time on the label before loading it.
• A handy trick: hang the studs through the item's mounting holes and set the whole thing in place while the glue is wet, so everything lines up.
• Light loads only. On a cored panel, a hard pull can peel the skin off the core before the stud lets go. Some adhesives also have a short shelf life once opened, so check the date.

Hold the item in place, mark the holes, and lay painter's tape over the gelcoat to reduce chipping. Double-check spacing against the item's base.

Use the exact hole size the fastener maker specifies; too big and it spins or pulls out, too small and it won't seat. On gelcoat, a light countersink at the hole edge helps prevent spider cracks.

In cored decks or panels, drill oversize, dig out a little core around the hole, fill with thickened epoxy, let it cure, and redrill to size through the epoxy. This keeps water out of the core for good.

Put sealant in the hole and under the flange, then set the fastener with the right tool, keeping it square. Don't over-squeeze rivet nuts or well nuts: overdoing it crushes fiberglass or spins the insert. With toggles, hold the toggle against the back of the panel by pulling the bolt toward you while you start the thread.

Apply sealant under the item's base and on the screw threads. Use marine-grade screws of the right length: too long can hit wiring or bottom out, too short won't engage enough thread. Tighten snugly by hand, not with an impact driver. Clean off the squeeze-out.

Give the item a firm, sensible push in the direction it will be loaded. Any movement, cracking sound or spinning fastener means it isn't holding. Recheck after the first few trips and again at season's end.

Note the fastener type, size and location in BoatMend so you, or a buyer, knows what is holding that item and what not to hang from it.