How do I repair stripped trolling motor mount holes in a fibreglass hull?
My bow-mounted trolling motor has ripped out of the fibreglass deck on my boat. The mounting holes are completely stripped and enlarged. How should I go about repairing the deck and making sure it doesn't just pull out again next time I hit some chop?
Answer
To fix stripped trolling motor mount holes permanently, you must drill out the damaged fibreglass, fill the voids with high-density thickened epoxy, and redrill the fresh plug. Whenever you can reach the underside of the deck, you should ditch the original self-tapping screws and upgrade to through-bolted 316 stainless steel machine screws with heavy-duty backing plates.
Clearing damaged fibreglass
Remove the trolling motor and inspect the deck. Stripped holes leave fractured gelcoat and crushed core material. Drill out the existing holes using an 8mm or 10mm bit to remove the compromised fibreglass and expose solid laminate. If your deck has a wood or foam core and feels damp, bend a small nail into an L shape. Chuck this nail into your drill and spin it inside the hole to rout out the wet core material by about 12mm in all directions. Vacuum the dust. Let the core dry completely before proceeding. Wear a dust mask and safety glasses when drilling.
Casting new epoxy plugs
You cannot simply squirt marine sealant into the holes and drive larger screws. The motor will inevitably rip out again under load. You need a structural repair.
- Brush the inside of the drilled holes with unthickened epoxy to prime the exposed fibreglass and core.
- Mix a batch of marine epoxy, such as West System 105 resin with 205 fast hardener. Thicken this with 404 High-Density filler until it reaches the consistency of peanut butter.
- Tape off the underside of the holes if they penetrate the deck completely. This prevents epoxy dripping into the cabin.
- Use a plastic syringe to inject the thickened epoxy into each hole from the bottom up to avoid trapping air pockets.
- Overfill the holes slightly to account for shrinkage. Leave the epoxy to cure for 24 hours.
- Once cured hard, sand the epoxy plugs flush with the surrounding gelcoat.
Fitting backing plates
Trolling motors endure high torque and vibration. This movement explains why self-tapping screws eventually fail in fibreglass. You must spread the load over a wider area. Redrill your mounting holes through the centres of the new epoxy plugs using a bit sized exactly for your new bolts.