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Troubleshooting
Bow thruster anodes: the cheap part that saves your gear leg
The A block of zinc or aluminum bolted to the thruster that corrodes on purpose, so the seawater eats it instead of the gearbox. It is replaced when it is about half gone, usually once a year. is the cheapest part on your thruster, and the one most often found missing at a haul-out. It corrodes on purpose so the gear leg does not. This page shows which thrusters carry one, how often each maker says to check it, and the paint mistake the manuals warn about.
Where this comes from. The installation manuals of Lewmar, Quick, Max Power and Sleipner, VETUS’s maintenance manual and Sideshift’s warranty terms, read 20 September 2026, or earlier for the brand reviews, and one marine writer’s inspection list. How fast an anode goes is not a figure any of them publishes.
At the stern. A stern tunnel thruster is the same kind of unit as a bow one, and 61 of the 140 tunnel models this site lists are sold for either end. So it is covered by the tunnel anode rules below. The units made only for the stern are external, from Max Power, Sideshift, Sleipner Side-Power, Yacht Thruster and ezDrive, and the sections below on the other kinds cover the Sideshift and DockStar ones.
The short answer
Lewmar, Quick, VETUS, Sleipner and Sideshift all fit an anode. The exceptions are Max Power’s CT100 and CT125, whose composite drive leg carries none, and the DockStar pod, which lives out of the water between uses.
The makers that print an interval all say yearly. VETUS says every 12 months, and Quick says yearly or more often if needed.
Once it is all gone, the sea starts on the gear leg.
A painted anode is sealed from the water and protects nothing. Lewmar, Quick and Sleipner each print this rule.
Sideshift’s warranty excludes corrosion from failing to replace anodes on a regular basis.
Look after yours in four steps
- Find out whether your thruster has one. Every maker in the maker table fits one except Max Power’s composite leg. The table also shows how often each maker says to check it.
- Look at it a few months into a new install. Lewmar says to check it after about 3 to 4 months. Your own water sets the pace, as the section How fast it goes explains.
- Replace it when it is about half gone. Do it at the yearly service at the latest. The section What the anode is for explains why the gear leg is next once it is gone.
- Keep the paint off it. Antifouling goes on the gear leg and the propeller, never on the anode. The paint rule has each maker’s wording.
The anode is one job on a longer list. The maintenance page puts five makers’ yearly schedules side by side.
What the anode is for
A thruster puts metal into seawater: a bronze or aluminum gear leg, a stainless shaft and a propeller. They are all connected to each other and to the boat’s bonding wire, the wire that ties the underwater metal together.
Different metals in seawater make a battery, and the one that corrodes most easily (the least noble, in the trade’s word) gets eaten. That is Metal eaten away because two different metals sit in seawater connected to each other, or because stray current leaks into the water. It is the damage an anode exists to absorb. Most warranties exclude it..
The anode is a block of metal that corrodes more easily still. It is bolted onto the propeller shaft or the leg, so the sea eats it first.
An anode that is all gone protects nothing. The leg is next.
Who fits one, and how often to check it
Here is what each maker’s manual or warranty says about the anode, and what it says about paint.
| Maker | Has one? | How often | What the manual says |
|---|---|---|---|
| Lewmar | Yes: the anode kit goes on with the propeller (washer, propeller, anode, washer) | Checked at 3 to 4 months on a new install to gauge a schedule, then replaced at the annual service | DO NOT anti foul zinc anode |
| Quick | Yes: anode tips on the propeller nut, fastened with thread-locking adhesive | Replaced yearly, more often if needed | Do not paint the zinc anodes, the sealings and the gearlegs’ shafts where the propellers are lodged. |
| VETUS | Yes: a zinc anode comes with the unit, and every installation manual read says so | Every 12 months | Do not use copper based anti-fouling |
| Sleipner | Yes: its screw set with thread glue, and two of the eleven sign-off points are about it | No interval printed in the SE20 to SE40 guide | Do not apply anti-fouling to the anode(s) |
| Max Power | No, on the CT100 and CT125: the composite drive leg carries no anode and is sealed for life | None | Do not disassemble the leg. That voids the warranty. |
| Sideshift | Yes: sacrificial anodes on the motor housing, and one on the propeller shaft that comes off with the propeller | At least once a year (Rev 3.1 manual, maintenance section) | Never paint over the sacrificial anodes. The warranty excludes corrosion from failing to replace them. |
Sleipner’s installer signs off on two points about the anode. The first is about paint: “Antifouling has been applied to the gear leg and propeller, but not to the anodes, seals or propeller shaft.”
The second is about fitting it: “The anode is in position and its screw is tightened with thread glue.”
Max Power’s composite leg is the exception. There is nothing to check. There is nothing to open either, because opening the leg voids the warranty.
How fast it goes
Nobody publishes a rate, because there is no one rate. Your water and your berth set the pace:
- Salt water eats an anode faster than fresh.
- A marina with stray current eats one faster than a mooring.
- A boat that lives in the water goes through them faster than one on a trailer.
Lewmar prints the plainest method for finding out on your own boat. On a new install, look at the anode after about 3 to 4 months “to gauge an appropriate replacement schedule”. Then replace it at the annual service.
Quick says the same thing with a caveat: yearly, “more often if needed”. Frank Lanier’s inspection list in Southern Boating (October 2017) has them renewed annually at a minimum.
Read what the old anode tells you. An anode that is gone in a season points to something on the boat: a bonding fault, a neighbor’s shore power, or a leg left unpainted. An anode that looks new after a year is worse news. It means the anode is not connected to anything, or somebody painted it.
Never paint the anode
Keep antifouling off the anode. Lewmar prints this in capitals beside the propeller drawing. Quick’s manual puts it this way: “do not paint the zinc anodes, the sealings and the gearlegs’ shafts where the propellers are lodged.”
Sleipner colors the gear leg and propeller on a drawing and says to paint those and nothing else.
The reason goes back to what the anode does. A painted anode is sealed from the water and protects nothing, while the leg beside it keeps corroding.
VETUS adds a second paint rule: no copper-based antifouling on the leg. Copper paint on an aluminum leg is a battery of its own. The barnacles page is built around that rule.
Zinc, aluminum or magnesium: which anode for your water
Thruster anodes come in more than one metal, and the right one depends on the water the boat lives in. VETUS and Quick both name zinc in their manuals, and Sleipner sells an aluminum version of its thruster anodes beside the zinc ones.
| Water | What to fit | What the guide says |
|---|---|---|
| Salt | Zinc or aluminum | “Use zinc or aluminum anodes in saltwater.” |
| Brackish (part salt, part fresh, as in an estuary) | Aluminum | “Go with aluminum anodes in brackish waters.” |
| Fresh | Magnesium, or aluminum | “zinc is not recommended for freshwater use.” |
Magnesium belongs in true fresh water only. In saltier water, the same guide says, it “overprotects, and corrodes too quickly”, which can damage paint and nearby metal. A boat that moves between waters, on a trailer or up a river, is safest on aluminum. Defender’s listing for Sleipner’s aluminum thruster anode says it is “Safe for salt, fresh, and brackish water”.
On a steel or aluminum hull, the hull has anodes of its own, and the thruster sits on the same boat, so ask whoever looks after the hull which metal it uses before you buy. VETUS adds one instruction for a metal tunnel: “Corrosion of a steel or aluminium thrust tunnel can be reduced by ensuring that the tail piece is completely insulated from the thrust-tunnel.” Its gaskets already insulate; the bolts and the shaft need nylon bushes. More is on the aluminum and steel boats page.
From the VETUS BOW6012D and BOW6024D installation manual, page 19.
External and retractable units
Sideshift fits anodes to the motor housing and one on the propeller shaft, and its manual says once a year at least, with pliers, counterclockwise off, hand tight and a half turn on. It also says what a fast-wasting anode means: water temperature, salt against fresh, stray current from another boat or a faulty shore-power ground, or a short somewhere in the thruster system. Look every few weeks while the boat is in use.
DockStar names no anode at all. Its user manual (v.260305, linked from the DockStar review) protects the metal a different way: the unit lives out of the water on its track between uses, gets a freshwater rinse and a silicone wipe after each one, and a silicone spray on all metal parts before storage.
Retractables carry the same anode a tunnel unit does. Lewmar’s RT manual says look at three to four months on a new install to learn how fast yours goes, then replace at the annual service. Quick’s BTR manual says yearly, more often if needed, and never paint the anodes, the seals or the pivot shafts.
What it costs when it is ignored
A neglected anode can cost you a gear leg, and the warranty with it.
Sideshift. The warranty does not cover “Corrosion or paint failure caused by neglect or lack of maintenance, including failure to replace anodes on a regular basis, or by electrolysis caused by faulty connections or stray electrical currents”.
Sleipner. Its terms say a failure to follow the manual’s maintenance voids the warranty on every part it affects. Its answer to a failed gear leg is a replacement leg rather than a repair.
The anode itself is a few dollars and a trip up the ladder at the haul-out. The maintenance page has the rest of that list. The survey page says what a missing or painted anode tells you when you are buying a boat.
Sources: Lewmar’s Gen 2 manual, Quick’s BTQ250 and BTQ300 manual, Max Power’s CT100 and CT125 manual, Sleipner’s SE20 to SE40 guide and VETUS’s maintenance manual. Also Sideshift’s warranty as the Sideshift review reads it, and Southern Boating’s Bow Thruster Maintenance.