Yacht Bonding Systems Explained: The Electrical Safety Net Below the Waterline

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Ever wonder why your boat has a bunch of green wires running to things like the shaft, struts, and seacocks? That’s your bonding system—and while it may not be as flashy as your navigation gear or electronics, it plays a critical role in protecting your boat from corrosion, electrical faults, and even fire.

In this post, I’ll break it down simply so you understand what a bonding system is, how it works, and why you need to care—especially if you own a fibreglass yacht with valuable underwater metals.

What Is a Bonding System?

A bonding system is a network of green wires that electrically connect all the major metal components on your boat—like:

• Engine blocks
• Prop shafts and struts
• Rudders
• Metal thru-hulls and seacocks
• Fuel tanks
• And sometimes the AC and DC grounds

These wires "bond" everything together to equalise electrical potential and give electricity a safe path to flow if something goes wrong.

Simple Analogy:

Think of a bonding system like a group safety rope while hiking on a glacier. If one person (metal part) falls into a crevasse (gets energized or corroded), the rope (bonding wire) prevents them from falling too far and helps everyone recover safely.

Why Do Boats Have Bonding Systems?

Great question! Your bonding system has 3 main jobs:

1. Safety – Fault Protection

If a live wire touches a metal part (like a fuel tank or strut), the bonding wire provides a low-resistance path for the fault current to safely flow back to ground, which trips the breaker or fuse. This prevents shock hazards and fires.

2. Corrosion Control – Cathodic Protection

When all your underwater metals are bonded together and connected to sacrificial anodes (zincs), the anodes do their job better. They protect all connected parts evenly, rather than letting one item (like a seacock) rot away while the anode sits there doing nothing.

3. Electrolysis and Stray Current Protection

Bonding helps to reduce electrolysis risks—the damage that comes from stray electrical current flowing through water. It helps contain and redirect these currents before they attack your expensive parts like propellers or keel bolts.

What Happens When Bonding Goes Bad?

Here’s the truth: A broken or poorly designed bonding system is worse than none at all.

If your bonding wires are:

• Corroded
• Loosely connected
• Missing connections
• Overloaded with added devices

…you could be unintentionally creating paths that encourage corrosion or block fault current—putting your boat and crew at risk.

Red Flags to Watch For:

• Green wire connected to only some underwater parts
• Unconnected thru-hulls or shaft logs
• Zincs disappearing rapidly
• Unexpected corrosion on bronze parts
• Crusty or oxidised bonding terminals

How to Maintain a Healthy Bonding System

Keeping your bonding system in good shape is easier than you think:

Inspect Regularly

Check all bonding wires and connections once or twice a season. Look for:

• Green corrosion
• Loose ring terminals
• Broken wires or heat damage

Test Continuity

Use a multimeter to check that all bonded parts show low resistance (less than 1 ohm) to each other. If you find an “island” (something isolated), it won’t be protected.

Keep It Simple

Don’t go overboard connecting everything if it isn’t necessary. Over-bonding can create ground loops, which increase galvanic activity and defeat the purpose.

Check with a Pro

Have a marine electrician inspect your bonding and grounding setup, especially if you’re adding new equipment, changing shaft seals, or installing new thru-hulls.

Bonding vs. Grounding vs. Galvanic Protection – What’s the Difference?

Let’s clear this up:

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Bonding

Connecting all metal parts together

Equalizes voltage + aids corrosion control

Grounding

Connecting to electrical ground (safety)

Trips breakers during faults

Galvanic Isolation

Blocking DC voltage from shore power ground

Prevents stray corrosion from dock

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They all work together—but they’re not the same thing.

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