How to Mount Rod Holders, Fish Finders, Cleats and Antennas Without Causing Leaks

Hold the accessory in place and check reach, clearance and how it will be used (where a rod will swing, where you will see a screen from the helm). Look and feel underneath with a flashlight and mirror. Confirm there is a flat, solid surface for the backing plate and nothing behind the panel you could hit.

Use the template or the accessory base to mark holes on painter's tape. BoatUS suggests drilling a tiny pilot hole (about 1/16 inch) first. That lets you check from below that the bolt and nut will clear everything, and if you need to move it, a tiny hole is easy to fill with epoxy.

Keep the tape on to protect the gelcoat. Run the drill in reverse until you are through the gelcoat to reduce chipping, then switch to forward. Look at the shavings: if you hit wood, balsa or foam, the panel is cored. Wet or black shavings mean the core is already wet; stop and get advice.

For a cored deck or panel, do not just bolt through the bare core:

• Tape the bottom of the hole.
• Remove the core around the hole (a bent nail in a drill, or drill oversize through the top skin and core only), and vacuum out the dust.
• Flood the cavity with unthickened epoxy for a few minutes so it soaks into the core, then let the excess drain.
• Fill flush with epoxy thickened to about mayonnaise with colloidal silica.
• After it cures, drill the bolt hole through the center of the plug.

BoatUS notes that if the core is marine plywood, coating the raw edges with a few coats of epoxy is usually enough, while soft cores like balsa should be removed and replaced with an epoxy plug. The plug also keeps the deck from crushing when you tighten the bolts.

Put a slight bevel on the top edge of each hole. BoatUS says this helps prevent gelcoat cracking around the fastener and leaves a small pocket for sealant to seal around the bolt.

A backing plate spreads the load so the bolts do not pull through or crack the deck. Good materials (per BoatUS):

• G10 fiberglass board: strong, stiff and cannot corrode or rot. Often the best choice.
• Aluminum plate: easy to cut and drill.
• Stainless plate: strongest and stiffest, harder to cut.
• Marine plywood: works if it is sealed with epoxy on all faces and edges.

Avoid plastic lumber like StarBoard as a backing plate: BoatUS warns it slowly squishes under bolt pressure and the fitting loosens. If the underside is rough, bed the plate in thickened epoxy so it sits flat (put petroleum jelly on the bolts so they do not get glued in). Large stainless fender washers are the minimum for light items.

Put the accessory, bolts, backing plate, washers and nuts together without sealant. Check that everything lines up, the bolts are long enough for a full nut, and the plate sits flat. Fix problems now; it is much harder once sealant is out.

Mask around the base. Coat the whole base of the accessory with a marine bedding sealant and put a ring under each bolt head, or use butyl tape on bolted fittings. Choose a sealant you can remove later (polysulfide or butyl) for anything you may want to move or replace; avoid permanent adhesives like 5200. For sun-exposed topside fittings, 3M sells its 4000 UV as a medium-strength, color-stable sealant that resists UV and still lets parts come apart later; whatever you pick, check the label says it suits your surfaces (some sealants attack certain plastics). BoatUS advises not sealing the nuts, washers or backing plate underneath, so if the top seal ever leaks you will see a drip inside rather than trapping water in the core.

Tighten in a criss-cross pattern until sealant squeezes out evenly around the base. A common method (used in the BoatUS cleat article) is to snug the nuts, let the sealant set overnight, then tighten to the final torque the next day. Do not overtighten: you can crush the laminate or bend the accessory. Use the torque in the accessory's instructions if it gives one. Trim the squeeze-out with a blade once it is firm and peel the tape.

For fish finders, antennas, lights and cameras, use a proper deck fitting instead of a bare hole stuffed with sealant:

• Cable gland: a fitting with a rubber seal sized to the cable. BoatUS notes the hole in the seal must match the cable diameter; split glands let you install cables without cutting off the plugs.
• Clam-shell or gooseneck cover: a hood that faces away from the weather and takes several cables. Seal around the wires with a removable sealant. Seal and epoxy-protect the core around the pass-through hole like any other hole (step 4). Below deck, leave a drip loop (a low U in the cable) before any connection so water runs off the bottom of the loop instead of into the plug.

Disconnect the battery before connecting anything. Use marine tinned wire, a fuse or breaker sized for the device close to the power source, sealed connectors, and support the wires every so often so they do not chafe. Follow the electronics maker's instructions (see the electrical corrosion and connections guide).

Stainless bolts in an aluminum hull or deck can cause galvanic corrosion where the metals touch in salt or brackish water. Coat the fasteners with an isolating or anti-seize compound made for stainless in aluminum, and use isolating washers or pads where the accessory maker recommends them. Avoid copper or brass hardware against aluminum. When you take old hardware off an aluminum boat or pontoon, note any plastic or fiber washers between the stainless and the aluminum; they are there on purpose as isolators, so put new ones back rather than leaving them out.

After the sealant has cured, give the accessory a firm tug, check there are no gaps, and hose the area while someone looks underneath. Recheck the nuts after the first few trips. Record what you installed, where and with which sealant in BoatMend.