Boat Bonding System Explained: How to Inspect Bonding Wires, Through-Hulls and Anodes

Open the bilge and engine space with a flashlight. Look at each bronze through-hull, seacock, strut bolt and the rudder post: a green or green/yellow wire bolted to a tab or stud means it's bonded. Follow those wires; they usually run to a heavy bonding wire or copper strip along the keel or a stringer, which in turn connects to the engine or DC negative and, through the hull, to an anode. Your owner's manual or the builder may also have a bonding diagram.

Unplug shore power, turn off the generator and inverter, and turn the battery switch OFF before touching terminals. If the boat is in the water, look but don't loosen any fitting below the waterline.

Draw a simple map: each fitting, the wire from it, where it lands, and how the main bonding conductor reaches the engine, DC negative bus and anode. Some builders run a separate wire from each fitting to a central strip; others "daisy-chain" one wire from fitting to fitting. In a daisy chain, one bad joint cuts off every fitting beyond it, so mark where the links are.

At each fitting and junction, look for:

• Green or white crust on terminals and studs
• Wire strands that are black, broken or "hairy" where they enter the terminal
• Loose nuts, or terminals you can twist by hand
• Wires lying in bilge water or pulled tight across a hatch opening
• Wires held by a loose or rusty hose clamp, or just wrapped with tape (see step 7)
• Plain (untinned) copper that has turned dark and brittle

Photograph anything suspect for your records.

Bronze should look like dull brown metal. BoatUS describes stray-current damage turning bronze fittings pink and crumbly in days in severe cases. Other warning signs: a fitting with a crumbly rim outside the hull, a seacock handle that has become very stiff, white powder or blistered paint around aluminum parts, or bonding studs eaten away. Any of these is a reason to call a surveyor before your next launch.

With power off and shore power unplugged:

1. Set the meter to its lowest ohms range. Touch the two leads together and note the reading; that is your lead resistance, to subtract.
2. Clip your long test lead to the engine block or the DC negative bus.
3. Touch the other meter lead to bright metal on each bonded fitting (scratch a clean spot on the fitting body or its bonding stud, not just the wire terminal).
4. Practical Sailor cites ABYC guidance that the resistance between any two parts of the bonding/DC negative/AC ground system shouldn't exceed 1 ohm. A healthy connection usually reads well below that. A higher or jumping reading points to a bad joint between that fitting and the engine.

Also test from the engine to the anode's internal stud or bolt. If the anode isn't connected, it's protecting nothing.

For each high-reading or corroded joint:

• Remove the terminal and clean both the terminal and the stud or tab to bright metal.
• If the wire end is black or green, cut it back to clean copper and crimp on a new heat-shrink ring terminal with a ratchet crimper.
• Reinstall with a stainless nut and lock washer, tighten firmly, then spray the finished joint with corrosion inhibitor.
• Retest with the meter.

Hose-clamped bonding wires. Many builders strap the bonding wire to a seacock or rudder stock with a stainless hose clamp. Boating Magazine's electrical columnist says that works acceptably as long as the clamp stays tight and the wire end is clean, and prefers it to copper "bonding clamps," which he says can themselves cause corrosion on bronze. His best-practice fix is a tinned ring terminal held by a stainless screw in a hole drilled and tapped into the fitting's flange, sealed with a corrosion inhibitor. Drilling into a seacock flange risks going through the hull, so leave that upgrade to a yard; as an owner, just keep any hose clamps tight and the wire clean.

Use tinned, stranded green (or green/yellow) marine wire at least as large as the original. Practical Sailor's experts describe tinned 8-gauge wire as typical for bonding conductors, and heavier wire is used when the system doubles as lightning protection; match what your boat has unless a professional specifies otherwise. Route it above normal bilge water, support it so it can't chafe, and keep it clear of hatch openings. Wire attached to the engine block must be heavy enough to carry engine starting current if it is also the DC negative path, which is one reason major rework belongs to a pro.

• Hull anodes, shaft collars and rudder anodes should be replaced when about half worn (see our anodes guide), and never painted.
• Make sure each anode's mounting surface and bolts are clean and tight. BoatUS stresses that an anode needs a good electrical connection to do anything.
• On bonded boats, BoatUS says to be extra vigilant: when the anode fails, the through-hulls start corroding.

Anodes tell you a lot about the system:

• Disappearing much faster than usual: BoatUS advises having a specialist check for stray current, either on your boat or from a neighbor at the marina.
• Hardly worn after a season: the anode may not be connected, may be the wrong alloy or may be coated with an oxide film. Check the connection and continuity first.
• Different anode metals mixed on one boat: BoatUS warns that the less noble anode will end up protecting the other. Use one type throughout, chosen for your water (salt, brackish or fresh).

BoatUS points out that internal stray current often comes from a poorly insulated wire in the bilge, a badly wired accessory or a wire with damaged insulation that gets wet now and then. BoatUS's surveyor Ed Sherman describes a new boat whose bronze thruster leg was consumed in about a week after gear stuffed in the compartment damaged the thruster's positive cable. Look for:

• Splices, terminals or wires lying in bilge water
• A bilge pump or float switch with cracked wiring
• Battery cables that could touch the hull or a fitting
• Anything on the boat that runs warm when it should be off

When you plug into shore power, the green AC ground connects your bonding system to every other boat on the dock. BoatUS explains that this can turn your anodes and underwater metal into part of a dock-wide galvanic cell, and a galvanic isolator (or isolation transformer) is the safe way to block that low-voltage DC current while keeping the safety ground intact. If anodes are vanishing at the dock, see our shore power guide and have a technician check the system.

The two devices are different. Ed Sherman explains in BoatUS that an isolation transformer passes shore power to the boat by magnetic induction, with no wire connecting the boat's AC system to the dock. That breaks the galvanic link to other boats, makes the boat's AC side self-polarizing (a dock with reversed hot and neutral no longer matters), and sends any fault current back to the transformer aboard instead of into the water. The catch: transformers are heavy (roughly 80 to 200 pounds), bulky, run hot, cost real money, and a boat with two shore inlets needs one per inlet. Sherman sees them as worthwhile on mid-size and larger boats that live on shore power. A galvanic isolator is smaller and cheaper and blocks only the low-voltage galvanic current in the green wire. Either is a professional installation.

Write down each meter reading, the date and the anode condition in your BoatMend boat profile. Re-inspect the bonding connections at every haul-out, and any time you replace a through-hull, an anode or an engine component.