Guide
Sacrificial Anodes - What Zincs Actually Do and Why Your Boat Needs Them
Updated · By Daniel Garcia
Every metal boat part in saltwater wants to corrode. Sacrificial anodes are the deal we make with electrochemistry - a small piece of cheap metal gets eaten instead of your expensive prop and shaft.
The Two-Sentence Version
Different metals in saltwater produce a tiny voltage against each other - the same battery effect that runs a lemon-and-zinc-strip science experiment. The less noble metal always loses electrons first, which means it corrodes first. A sacrificial anode is deliberately the least noble metal on your boat, so it gets sacrificed instead of the bronze, stainless, and aluminum components you actually care about.
Why Boats Especially Need This
Cars and bicycles don't use sacrificial anodes because they're not sitting in saltwater. Saltwater is extremely conductive - it amplifies galvanic corrosion hugely. A stainless shaft and a bronze prop that would corrode over decades in air can corrode in months in saltwater without protection.
Add shore power, onboard electrical systems, and stray currents from other boats at the marina, and you have a recipe for fast corrosion that sacrificial anodes are specifically designed to absorb.
How Anodes Work
Three conditions make galvanic corrosion happen: two different metals, an electrical connection between them, and an electrolyte (saltwater) between them. Remove any of the three and corrosion stops.
Anodes provide a fourth metal that's less noble than everything else on the boat. It has the same electrical connection (through the boat's bonding system or direct contact) and the same electrolyte exposure. Because it's more eager to lose electrons than the bronze or stainless, it loses them instead. Your prop and shaft stay protected. The anode slowly disappears.
Zinc, Aluminum, or Magnesium?
Three common anode metals, each for different water:
- Zinc - standard saltwater anode. Most BC boats have zinc. Works well in saltwater but is slowly being replaced by aluminum in many applications.
- Aluminum - newer saltwater standard. Protects a wider range of metals, lasts longer than zinc, works in slightly brackish water too. Good choice for BC saltwater.
- Magnesium - freshwater only. Never use magnesium in saltwater - it will corrode so fast it's gone in weeks and can overprotect and damage paint.
Where Anodes Go on Your Boat
Typical placements on a BC recreational boat:
- Prop shaft - collar-style zinc clamped to the shaft, protecting shaft and prop
- Prop nut or prop hub - threaded zinc on the propeller itself
- Rudder - plate zinc bolted to the rudder
- Trim tabs - small plate zincs on trim tab surfaces
- Saildrive - two dedicated saildrive zincs, one per side
- Transom plate - larger flat zinc bolted through the transom, provides bulk protection
- Engine - internal zincs for seawater-cooled engines (handled by engine service, not a diver)
- Through-hulls and seacocks - often have small anodes or are bonded into the main system
What Kills Anodes Faster
Normal zinc consumption is gradual and steady. Fast consumption usually means something is wrong:
- Ground fault on your boat or a neighbour's at the marina
- Shore power polarity or wiring issues
- Corroded or broken bonding wire between the anode and what it's protecting
- Stray current from onboard inverters, chargers, or bilge pumps
- Anode that's oversized and overprotecting, causing paint blistering
- Anode that's undersized for the metal surface area it's protecting
Common questions