How Often to Replace Boat Zincs and Anodes (and What It Costs)

By DiverHire

Replace a zinc, or any other sacrificial anode, when about half of it has wasted away, and at least once a year even if it still looks usable. Mercury, Martyr and Performance Metals all publish the half-gone rule, and Martyr and Performance Metals add the once-a-year minimum. On the BC coast, one dive service sees 4 to 10 months from a set on a boat plugged into shore power in a marina slip, against 10 to 18 months on a mooring (Danny's Diving, BC, 2026). Check them every 3 to 6 months in salt water (UMS Florida, 2025), and choose the metal by the water you keep the boat in.

Why do boats need zincs?

Galvanic corrosion needs three things, BoatUS explains: a second metal, an electrolyte such as seawater, and a conductive path such as a wire or direct contact. The more active metal then gives up electrons and slowly dissolves. Mercury gives the everyday example: an aluminium gearcase and a stainless steel propeller in the same water.

A sacrificial anode is a piece of even more active metal, fastened to the boat so that it corrodes instead of your propeller, shaft, rudder or drive. Boaters call them all "zincs", whatever the metal.

What do boat anodes look like?

On outboards and sterndrives, Mercury describes a small grey block on the gearcase, transom bracket or trim rams, usually held by a single bolt. On inboard boats you will find collars on the propeller shaft, a cap on the prop nut, and anodes on the rudder, trim tabs, transom, saildrive and hull (Danny's Diving).

Do I need an anode on my boat?

If you have metal running gear or fittings below the waterline, almost certainly: The Yacht Maintenance Hub says any boat with underwater metal needs them, fibreglass boats included, though Performance Metals notes that unbonded bronze fittings, especially on wooden hulls, can sometimes go without. Corrosion is slower in fresh water but it still happens, because fresh water carries enough dissolved material to conduct electricity (BoatUS, 2025).

When to replace boat zincs: the 50% rule

The published thresholds agree closely:

• Mercury: replace when 50% consumed (Mercury FAQ).
• Martyr: replace when 50% worn down, or at least once a year (Martyr FAQ).
• Performance Metals: replace at least annually, including in fresh water, or when corroded to half the original size (Performance Metals FAQ).
• AB Marine: a little earlier, at 45% wastage, and at least every year (AB Marine).
• Bottom Time Dive Service, southwest Florida: replaces at 50% wear, to avoid the protection gap that opens once an anode gets too small (btdive.com).

Why not run them down to nothing? Once the anode is gone, the corrosion moves to the next most active metal, usually the propeller and shaft. Danny's Diving warns that you can lose a propeller to dezincification in under a year with no zinc protection, and that through-hull fittings can fail.

Danny's Diving says most experienced divers work in a 40 to 60% window: if the zincs are checked only once a year, lean towards replacing at 40%; if they are checked every six months, 50 to 60% is fine.

Bottom Time checks every anode at each scheduled hull cleaning, so if a diver already cleans your hull on a schedule, that is your anode schedule too. See how often to clean your boat hull.

How long do zincs last on a boat?

Anywhere from two weeks to two years, according to AB Marine, which is why a calendar alone does not work. The electrical environment, above all shore power, drives the spread (AB Marine, Danny's Diving).

The sources disagree on what is normal. UMS Florida says a set is typically replaced every 6 to 12 months in most saltwater conditions. Boat Outfitters says anodes should last at least a full year, and that needing them sooner points to an undersized anode or stray current; it adds that warm water speeds up corrosion while cool water slows it.

Why does a slip matter so much? On shore power, every plugged-in boat on the dock is connected through the green grounding wire. BoatUS explains that your anodes then help protect the other boats on the dock, and a neighbour with too little anode drains the boats that have enough. Danny's Diving says one boat with a ground fault on your dock can make your zincs waste at three to four times the normal rate. AB Marine advises checking zincs after you move the boat, or when new boats arrive nearby.

Zinc vs aluminium vs magnesium: which anode for which water

Salt water

Zinc is the traditional choice, and Martyr says it gives stable, long-lasting protection in salt water. Aluminium also works in salt and brackish water and lasts longer: Performance Metals rates the relative capacity of zinc at 100, aluminium at 130 to 150 and magnesium at 30. Conventional zinc uses cadmium as an activator, which aluminium anodes do not release (Boating Magazine); Tecnoseal now sells a cadmium-free zinc activated with bismuth.

For outboards and sterndrives, follow the engine maker. MerCruiser sterndrives ship with aluminium anodes (Mercury FAQ), and Performance Metals warns that zinc anodes on sterndrives and outboards may invalidate the warranty.

Fresh water and the Great Lakes

Zinc forms an insulating film in fresh water and stops working (Boating Magazine). Performance Metals says this can happen within a few months, and Martyr says the same oxide build-up affects aluminium. Boating Magazine, Martyr and Mercury all point boats kept in fresh water to magnesium, which Mercury says is for fresh water only.

The sources do not agree on aluminium. Boating Magazine says it works well in fresh water, though magnesium is best, and Performance Metals recommends its own aluminium alloy over zinc there, citing American Boat & Yacht Council (ABYC) recommendations from 2008 to 2009 in which aluminium is the only anode type recommended for all water types. It says magnesium can be used in fresh water on fibreglass boats, but can overprotect aluminium drives and aluminium or wooden hulls. If your boat never leaves fresh water, the majority view is magnesium; if it moves between lakes and the coast, aluminium is the compromise. Either way, check your engine or drive manual first, and see our Toronto and Great Lakes guide for other freshwater issues.

Never take a magnesium-protected boat into salt water. Boating Magazine warns that a week in a coastal slip will leave you with "a catastrophe on your hands". And do not mix zinc and aluminium anodes on the same boat: Martyr says it causes uneven wear and weaker protection, and Performance Metals says the aluminium ends up protecting the zinc as well as the drive.

Anodes wearing out too fast? Stray current and galvanic isolators

If a new set disappears in weeks rather than months, look for an electrical cause before buying more metal. Martyr lists three causes of rapid wear: stray current from onboard wiring, harsh salt water, and anodes too small for the metal they protect. Performance Metals adds that a stainless propeller eats zinc faster than a bronze one, and Mercury advises having your protection checked whenever you add stainless props or trim tabs.

Shore power and galvanic isolators

BoatUS is blunt: "If your boat is plugged into shore power on a regular basis, you need a galvanic isolator", or an isolation transformer. The author recommends a UL marine-rated, fail-safe unit and says older isolators should be tested by a marine electrician. A sidebar in the same article calls shore power probably the most common cause of severe underwater metal corrosion after missing anodes. Mercury also recommends an approved isolator for any boat on shore power.

True stray-current corrosion

Stray-current corrosion, which BoatUS says is sometimes mistakenly called electrolysis, needs an active current, such as a live wire in bilge water. BoatUS says it can turn a healthy propeller or thru-hull into a crumbling mess in days, but it is not very common. Light pitting after a whole season is most likely ordinary galvanic corrosion.

Should you fit bigger zincs?

"Sometimes", says Danny's Diving, but oversized zincs can overprotect and cause problems such as paint blistering. Performance Metals disagrees for inboard boats with mainly bronze and stainless parts, saying zinc or aluminium cannot overprotect them; its warnings are for magnesium, as above, and for steel hulls, where the paint can lift. If the anode is simply too small for the metal it protects, one of Martyr's three causes, a larger one is the fix; if the cause is electrical, a bigger anode only hides it.

Engine makers also sell powered systems, such as Mercury's MerCathode and Volvo Penta Active Corrosion Protection for sterndrives, which keeps one back-up anode per drive. Electrical diagnosis is a marine electrician's job; a diver can give you a clear record of which anodes are wasting and how fast.

Signs of trouble, including anodes that last too long

An anode that still looks new after a season is not good news. Mercury calls it "a common misconception" that uncorroded anodes mean corrosion is under control, and says they may mean damage is happening on more critical parts. Martyr likewise says an anode that looks untouched may not be working. Common reasons:

• Painted over. Anodes must touch the water to work, so never paint them (Mercury, Performance Metals).
• Coated. A white, chalky crust or marine growth can stop an anode sacrificing itself (Mercury FAQ). A pitted, chalky surface is normal on a working zinc (Danny's Diving); the warning sign is a crust that has stopped it wasting.
• Wrong metal. Zinc in fresh water can go inactive within months (Performance Metals).
• Poor electrical contact. Anodes need clean, bare metal and good contact (Performance Metals FAQ). Performance Metals recommends shaft anodes because the path through the engine and transmission is a very poor connection. Uneven wear can be a clue (The Yacht Maintenance Hub).
• Sterndrive stored tilted up. Mercury says to leave it down when the boat is stored in the water, so the anodes stay submerged.

Signs the damage has moved to other metal:

• A pink or copper tinge on the propeller or shaft (Danny's Diving). BoatUS explains that bronze containing zinc turns pink as the zinc corrodes out.
• Pitting on running gear or thru-hulls (Danny's Diving).
• Paint blistering on sharp edges below the waterline, or white powder on exposed metal (Mercury).

Danny's Diving adds signs that a set is simply due: zincs worn smooth and rounded rather than pitted and chalky, a last change more than a year ago, or new electrics (inverter, charger, shore power set-up) with no check since.

Which anodes can a diver change in the water?

Most of them. Danny's Diving changes shaft, prop nut or hub, rudder, trim tab, transom plate, saildrive and keel or hull anodes in the water and documents each job on video. The exception is engine zincs inside the cooling system, which are a job for your mechanic, not a diver.

For external anodes, a haul-out is mostly about timing and convenience. The Yacht Maintenance Hub calls haul-out the best opportunity to check anodes, and some owners do their zincs then (TrawlerForum, 2017). A New Jersey owner on the Hatteras owners forum (2017) preferred a short haul in the slings, finding zinc changes easier there for little extra cost. But on Danny's BC figures a powerboat in a shore-power slip can use up a set in 4 to 8 months, so in-water changes are what keep it protected between yearly haul-outs.

Do not go under the boat yourself unless you are trained for it, and never swim in a marina. BoatUS says electric shock drowning is far more likely in fresh water than salt, warns that salt water can turn brackish or fresh with tides, wind and rain, and advises people never to enter the water within 150 feet of any electrical equipment or wiring. A diver working in a marina is inside that zone, so ask how they deal with shore power on your boat and the boats around it. Our guide to hiring a diver covers the other questions to ask.

What do divers charge to change zincs?

Both published rate cards we found charge per anode. Bottom Time adds the anode at list price; DS Diving in Jacksonville adds a USD 50 special trip charge and does not say whether the anode is included. We found no published Canadian or UK rate for zinc changes; Danny's Diving in BC, for example, quotes per boat.

Table sources: DS Diving, Bottom Time rates, TrawlerForum, May 2025 and the Hatteras owners forum thread linked above.

Some cleaners fold zinc labour into a monthly service, charging only for the parts (a Southern California owner on TrawlerForum, 2017). Get the price basis in writing. In a May 2025 TrawlerForum thread, the owner of a 30 ft boat took a USD 350 quote for a cleaning and zinc change to be a flat fee. It was an hourly rate, and the bill came to almost USD 1,200. Before the diver goes in, ask whether the price is per anode or per hour, whether parts are included, and whether a trip charge applies. For cleaning prices by region, see boat hull cleaning cost, and for what a cleaning visit usually covers, see boat and hull cleaning divers.

What a good diver report shows

Whoever you hire, ask for:

• A photo or short video of each anode before and after.
• An estimate of how much of each anode remains, and which ones were replaced.
• Anything odd: an anode that looks untouched, one wasting much faster than the rest, pink metal, pitting or blistered paint.
• Which metal was fitted, so you never end up with zinc and aluminium mixed.
• The date. A few dated reports will show your real interval at your berth better than any general rule.

Hiring a diver through DiverHire

DiverHire is a new beta marketplace, so there may not be a diver listed near you yet. You can post a zinc-change job free: list the anodes you know about (shaft, rudder, trim tabs, drive), their metal, the boat's length, where it is berthed and whether it is on shore power, and divers can bid. DiverHire takes no fee on jobs, and you arrange payment directly with the diver you choose. Check the diver's certifications and insurance yourself before they go in the water. City pages such as Jacksonville, Tampa, San Diego and Toronto show who is listed locally.

Sources

• Danny's Diving: When to replace your boat zincs — zinc life by berth (mooring 10–18 months, slip 4–10 months, fault 2–4 months), 40–60% window, 3–4x wear from a ground fault on the dock, in-water anode list, engine zincs for mechanics, warning signs, bigger-zinc answer, video documentation; BC salt water (updated April 2026)
• Mercury Marine: Frequently asked questions — replace at 50% consumed, chalky coating stops anodes, aluminium anodes on MerCruiser sterndrives, magnesium for fresh water only, galvanic isolator for shore power, sterndrive stored down (accessed September 2026)
• Mercury Marine: Preventing corrosion — galvanic corrosion example, what anodes look like, uncorroded anodes as a warning, never paint anodes, paint blistering and white powder on exposed metal, MerCathode (accessed September 2026)
• Martyr Anodes: FAQs — replace at 50% or yearly; zinc or aluminium in salt, aluminium in brackish, magnesium in fresh; causes of rapid wear; untouched anodes; do not mix metals (accessed September 2026)
• Martyr Anodes: Choosing the right anode — zinc for salt, oxide build-up on zinc and aluminium in fresh water, check anodes when changing water type (accessed September 2026)
• Performance Metals: Sacrificial anode FAQs — replace yearly or at half size, relative capacity 100/130–150/30, ABYC 2008–2009 recommendations with aluminium for all waters, zinc inactive in fresh water within months, zinc on sterndrives and outboards and warranty, magnesium and steel-hull overprotection, good contact, no painting, no mixing, stainless props (accessed September 2026)
• Performance Metals: Why bond metal parts on a boat? — unbonded bronze fittings sometimes need no anode, poor connection through engine and transmission, shaft anodes (accessed September 2026)
• Tecnoseal: Zinc sacrificial anodes, new Bi-TEC alloy — cadmium activator in conventional zinc, bismuth-based cadmium-free alloy (accessed September 2026)
• Volvo Penta: Active Corrosion Protection — powered system for sterndrives in salt and brackish water, one back-up anode per drive (accessed September 2026)
• Boating Magazine: Choosing the right sacrificial anode — zinc for salt, magnesium best for fresh with aluminium also working, zinc film in fresh water, magnesium in salt, aluminium and cadmium (Kevin Falvey, October 2023)
• Boat Outfitters: Aluminum vs zinc anodes for boats — aluminium for salt and brackish, at least a year of life, shorter life points to undersized anodes or stray current, warm water speeds wear (September 2023)
• The Yacht Maintenance Hub: Zinc anodes — any boat with underwater metal needs anodes, inspection 3–6 months in marinas and 6–12 months on moorings, metal by water type, uneven wear, haul-out inspection (March 2026)
• UMS Florida: How frequently should you replace boat zinc anodes? — inspect every 3–6 months in salt and 6–12 months in fresh or brackish water, replace every 6–12 months in most salt water (August 2025)
• AB Marine: How long should my zincs last? — two weeks to two years depending on electrical current, 45% wastage, replace at least yearly, check after moving the boat (updated February 2025)
• BoatUS: Galvanic isolators explained — shared green wire, isolators for boats regularly on shore power, fail-safe units, testing older isolators, shore power as a leading cause of severe corrosion (Ed Sherman, October 2019)
• BoatUS: 6 boat corrosion myths — three ingredients of galvanic corrosion, corrosion in fresh water, stray-current corrosion speed and rarity, pink bronze (Kelsey Bonham Bailey, August 2025)
• BoatUS: Avoid electric shock drowning — ESD far more likely in fresh water, salt water can turn brackish or fresh, never enter the water within 150 feet of electrical equipment or wiring (April 2017)
• DS Diving Service: Pricing — USD 20 per anode plus USD 50 special trip charge, Jacksonville FL (2026)
• Bottom Time Dive Service: Service rates — USD 20 per anode installed plus anodes at MSRP (Zimar, Camp, Tecnoseal), 2026 rates sheet; southwest Florida (accessed September 2026)
• Bottom Time Dive Service: Anode replacement — anode checks at each scheduled cleaning, replacement at 50% wear to avoid a protection gap; southwest Florida (accessed September 2026)
• TrawlerForum: Bottom cleaning charges — owner-reported zinc labour of USD 10 per anode (San Diego; a 65 ft owner) and USD 20 per shaft (St. Petersburg FL), and the hourly-rate billing dispute (May 2025)
• TrawlerForum: What do you pay for hull cleaning? — zinc labour included with monthly cleaning (Southern California), annual haul-out with zinc replacement (October 2017)
• Sam's Marine Hatteras Owners Forum: Diver bottom cleaning rates FL — USD 20 labour per zinc in New Jersey, preference for a short haul for zinc changes (August 2017)

Last updated: September 2026. Prices and rules change; confirm with local providers.