Guide
When to replace zincs: the 50% rule
Zinc anodes corrode on purpose so your shafts, props and rudders do not. Here is what they do and why we replace them at about 50%.
What a sacrificial anode does
- Put two different metals in salt water and they act like a small battery; a zinc is bolted on so it gets eaten away instead of the prop and shaft.
- Zinc and aluminum both work in salt water; aluminum also covers brackish water, and magnesium is for fresh water only.
Why 50%
- A partly consumed anode still protects; a mostly consumed one may lose contact or surface area.
- 6 Feet Under replaces zincs at approximately 50% of their lifespan and reports zinc condition after each cleaning (old site, awaiting Jake, A1/A9).
Where zincs live on a boat
- Shaft collars, prop nuts, rudders, trim tabs, transom plates, outdrives and some through-hulls.
- Painted anodes do not work; never paint over a zinc.
Sources
- Martyr Anodes: About corrosion — https://martyranodes.com/about-corrosion/
- Martyr Anodes: Marine anodes explained — https://martyranodes.com/marine-anodes-explained/
- Performance Metals: Sacrificial anodes FAQs — https://performancemetals.com/pages/sacrificial-anodes-faqs
- Steve D'Antonio Marine Consulting: Bonding systems and corrosion prevention — https://stevedmarineconsulting.com/bonding-systems-and-corrosion-prevention/
- Fisheries Supply: Sacrificial anodes and replacement zincs — https://www.fisheriessupply.com/zincs-anodes
Related service
Zinc Replacement — Zincs protect your boat’s critical metal components from corrosion. We replace zincs before failure to keep your vessel safe and electrically balanced. We provide a zinc condition report after each cleaning so you’re fully informed about what’s happening below the waterline