Guide
When to Replace Boat Zincs - A Commercial Diver's Guide for BC Saltwater
Updated · By Daniel Garcia
Zincs are supposed to disappear - that's literally their job. But how far is too far, and how do you know before your prop starts dissolving instead?
The 50% Rule
Industry rule of thumb: replace zincs when they're 50% consumed. Below that and you're wasting protection. Above that and you risk them failing before your next inspection.
In practice, most experienced divers push this a bit - 40-60% is the working range depending on how confident we are about inspection timing. If you're having zincs checked annually, err toward 40%. If you're inspecting every 6 months, 50-60% is fine.
Typical Replacement Intervals in BC
What we see on boats we dive regularly:
- Shore-power-connected sailboat at a marina: 6-10 months
- Shore-power-connected power boat: 4-8 months
- Boat on a mooring (no shore power): 10-18 months
- Boat with an electrical fault: 2-4 months or faster
- Boat with heavy onboard electrical load (inverters, chargers, stray currents): 4-6 months
Signs Your Zincs Need Replacing
Most accurate way to know is to have a diver inspect them. Short of that:
- Last change was over a year ago - almost certainly time
- Prop or shaft showing pink/copper tinge (dezincification starting on bronze)
- Pitting on running gear or through-hulls
- Zincs appear smooth and rounded rather than pitted and chalky - could be depleted
- You changed electrical systems (inverter, new charger, shore power setup) and haven't reinspected
Why Shore Power Changes Everything
Shore power plugs your boat into a shared electrical system with every other boat at the marina. If even one boat on your dock has a ground fault, you can end up sinking zincs at 3-4x the normal rate to protect against stray current.
This is why boats on moorings typically go 12-18 months between zinc changes while boats in shore-power slips can need replacements at 4-6 months. Same boat, same water, different electrical environment.
Common Zinc Types on BC Boats
Most BC boats have a combination of:
- Shaft zincs - cylindrical, clamped to the prop shaft
- Prop nut or prop hub zincs - threaded onto or built into the propeller
- Rudder zincs - plate-style, bolted to the rudder
- Trim tab zincs - small plates on trim tabs
- Engine zincs - internal to the engine cooling system (we don't handle these - your mechanic does)
- Transom plate zincs - flat plate bolted through the transom
- Saildrive zincs - specific to Volvo and Yanmar saildrives, usually two per saildrive
- Keel or hull zincs on sailboats - bolted through the hull
Aluminum vs Zinc vs Magnesium
Traditional saltwater sacrificial anodes are zinc. Aluminum anodes are a newer option - they last longer and are recommended by some manufacturers. Magnesium is for freshwater only - never use magnesium in saltwater.
For Vancouver Island saltwater: zinc or aluminum both work. Aluminum generally protects a wider range of metals and lasts longer. Zinc is what most boats came with and what most replacements are. We can install either.
Common questions