How to Install an Inspection Port (Deck Plate) in a Deck or Bulkhead

BoatUS stresses a perfectly flat mounting surface, because a plate on a curved deck is hard to seal. Don Casey adds that few decks are truly flat: lay a straightedge across the spot to check. Then figure out what is underneath. Look for stringers, bulkheads, tanks, fuel lines, wire runs and rigging tubes. Ask the builder for a drawing, look from below with a camera or mirror, and tap with a small hammer to find solid and hollow areas. Stay clear of other fittings and edges. If the only spot that reaches your target means cutting structure, call a pro.

For a deck with a cabin or liner below, Casey's trick is to measure from two features that pass through both surfaces (a mast, a bolt, an existing hatch corner) and transfer the outline to the underside. That way you know the hole will not land on a bulkhead, light fixture or wire. If a liner hides the space, you can drill a small hole in the center of the planned cutout and peek through it with a camera first.

Cover the area with blue painter's tape; BoatUS says this helps stop the brittle gelcoat from chipping. Mark the outline on the tape using the maker's template or measurements, square to the boat's centerline so the plate looks straight.

• Round opening: use a hole saw or circle cutter. BoatUS recommends starting with the drill in reverse, which chips the gelcoat less, holding the drill square to the surface and going slowly so the saw does not wander. Stop as soon as you are through.
• Non-round opening: drill a hole in each corner so the hole forms the corner radius, then connect them with a jigsaw. Casey suggests checking first that the saw body will clear nearby fittings all the way around.

Keep the vacuum running to catch dust. If you will seal the core in step 6, BoatUS notes you may want the hole slightly oversized to leave room for the epoxy.

Look at the piece you removed and the edge of the hole. You will see either solid fiberglass, or two fiberglass skins with a core between them (balsa, plywood or foam). Dry, light-colored core is good news. Dark, wet, crumbly or smelly core means water has been getting in. Note it, take photos, and see "When to call a pro" before you go further.

Casey is blunt: the sealant under the plate's flange is not enough to protect a cored deck. Water that gets past it will run straight into the core. To seal it:

• Rake out the core between the skins all the way around the hole, about half an inch deep, with a bent nail in a drill or a pick. Vacuum the debris.
• Lightly sand the inside surfaces of both skins.
• Brush or syringe unthickened epoxy into the cavity so it soaks into the exposed core.
• Thicken more epoxy with colloidal silica to about peanut butter consistency and pack the cavity full, smoothing it flush with the cut edge.
• Let it cure fully, then sand the edge smooth.

On solid fiberglass or a thin liner, a coat of epoxy on the raw edge is still a good idea. On a plywood bulkhead, seal the cut wood edge the same way.

Set the plate in the hole and mark the screw holes. Put tape over the spots, then drill pilot holes. BoatUS recommends a bit just slightly smaller than the screw, because fiberglass is hard and a too-tight screw can crack it; try one screw first and step up a bit size if it binds. A light countersink on the gelcoat at each hole helps prevent cracks. If the screws land in the epoxy ring from step 6, they never touch the core.

Wipe the deck around the hole and the back of the plate's flange with acetone or isopropyl alcohol to remove wax and contamination, as BoatUS suggests. Casey masks the deck right around the plate and the flange edge so squeeze-out lands on tape instead of gelcoat.

Most deck plates are plastic, and some sealants attack or will not stick to certain plastics. Casey notes that polysulfide is for metal frames, and plastic hatch and deck plate frames call for silicone or a silicone/polyurethane hybrid such as BoatLIFE Life Seal. BoatUS also names 3M 4200 or Life Seal as semi-permanent choices that still let you remove the plate later. Check the plate maker's instructions; some name a specific sealant type and will not cover damage from others. Avoid permanent adhesives like 5200 on a part you may want to replace.

Lay a continuous bead of sealant around the back of the flange and around each screw hole. BoatUS warns against overdoing it; you want enough to seal the plate and screw holes without a huge mess. Set the plate in place and wiggle it gently to spread the sealant.

Use 316 stainless screws long enough to grip but not long enough to poke through the underside, where they would leave an unsealed hole and a sharp point for your arm. Drive them with a hand screwdriver so you can feel binding, in a criss-cross pattern, getting all of them started before snugging any. Stop when the sealant squeezes out evenly; overtightening squeezes out all the sealant and leaves a leaky joint. On a thin liner where screws will not hold, BoatUS suggests through-bolting with machine screws and nuts (you will need a plate that opens to reach them).

Casey lets the sealant cure for two or three days, then runs a razor around the edge of the flange and peels the tape, taking the squeeze-out with it. Follow the cure time on your sealant's label.

After full cure, soak the plate with a hose, then open it and look for water inside. If water got in, BoatUS says to check that the O-ring or gasket is in good shape and seated, and reseal the plate if needed. Log the location and what is under it in BoatMend.

Years later, when the port starts to drip, it is usually the seal, not the bedding. On most deck plates the seal is a simple O-ring in a groove. BoatUS (Mark Corke) suggests this sequence: have a helper below while you hose the plate, so you know whether water comes through the lid or around the flange. If it is the lid, check the O-ring: it should be soft and pliable, with no cracks or flat spots. Pry the old one out with a flat screwdriver (less likely to gouge the groove than a pick), clean the groove and the underside of the lid, and seat the new ring with a couple of drops of dish soap. Water around the outside of the flange means the plate needs rebedding (steps 8 to 12).