In short: Choose the anode metal for the water your boat spends most of the season in.
- Salt water (North Sea, Skagerrak): zinc or aluminium.
- Brackish water (the Belts, the Sound, the Baltic and many Danish fjords): aluminium.
- Fresh water (lakes, rivers, canals): magnesium. Never use magnesium in salt or brackish water.
- Replace an anode when about half of it has gone, and never paint over it.
For most boats cruising Danish and Baltic waters, aluminium anodes are the safe all-round choice. They work in the salt water of the North Sea and in the brackish Baltic, where zinc can stop working. Zinc is fine if you stay in full-strength sea water, and magnesium belongs only in fresh water. A sacrificial anode, often just called "the zinc", corrodes so that your propeller, shaft, drive and engine do not. If your boat's or engine's manual specifies an anode type, follow it.
What does a sacrificial anode do?
A sacrificial anode protects underwater metal from galvanic corrosion by corroding first. When different metals are electrically connected in water, a small current flows between them and the less noble metal dissolves. The anode is less noble than bronze, stainless steel and aluminium drive legs, so it takes the damage.
Without one, a brass propeller can turn porous and pinkish, a stainless shaft can pit, and an aluminium sterndrive can blister under the paint and eventually corrode through. An anode only protects parts it is electrically connected to, which is why a boat often carries several.
How salty are Danish and Baltic waters?
Salinity drops from near-ocean levels off Jutland's west coast to single figures in the Baltic, and Danish waters sit right across that gradient. Salty North Sea water flows in along the bottom while fresher Baltic water flows out at the surface, so readings in the Kattegat, the Belts and the Sound change with wind and current.
| Area | Approx. surface salinity (psu) | Counts as |
|---|---|---|
| North Sea off the Danish west coast | 30–35 | Salt water |
| Skagerrak | 25–32 | Salt water |
| Kattegat | 20–27 | Mostly salt, varies |
| The Belts, the Sound and the western Baltic | 10–20 | Brackish |
| Baltic Proper, e.g. around Bornholm | 7–8 | Brackish |
| Bothnian Bay and eastern Gulf of Finland | about 2 or less | Almost fresh |
The figures are approximate (psu is roughly grams of salt per kilo of water). For anodes, the practical point is that a summer cruise from the Kattegat through the Great Belt to Kiel or Bornholm takes the boat from salt into brackish water.
Zinc or aluminium: which anode suits your waters?
Choose the metal for the water the boat lies in most of the time: zinc or aluminium in salt water, aluminium in brackish water and magnesium in fresh water.
| Water | Recommended metal | Good to know |
|---|---|---|
| Salt water, e.g. North Sea and Skagerrak | Zinc or aluminium | Aluminium delivers more protection per kilo and often lasts longer. |
| Brackish water, e.g. the Baltic, the Belts and many fjords | Aluminium | Zinc can build up a hard white crust and stop working. |
| Fresh water, e.g. lakes, rivers and canals | Magnesium | Zinc and aluminium work poorly in fresh water. |
| Alternating salt and brackish water | Aluminium | Works in both. |
If you move between fresh and salt water, choose for the water the boat spends longest in, and follow the engine or drive maker's advice.
Published capacity figures put aluminium anode alloys at around 2,500 ampere-hours per kilo and zinc at around 800, so per kilo aluminium delivers roughly three times the charge. Aluminium also weighs well under half as much as zinc, so an anode of the same shape holds only about a fifth more charge. A like-for-like swap therefore gives a modest gain in anode life.
Magnesium is less noble than zinc and aluminium and drives a stronger protective current, which fresh water needs. In salt and brackish water it corrodes very fast and can overprotect, which harms aluminium drives in particular and makes paint lift. A Dutch boat from the IJsselmeer or a German boat from inland lakes and rivers may arrive with magnesium anodes. Replace them before a season in Danish or Baltic waters. If you sail in fresh water and cannot find magnesium anodes for your boat in our range, get in touch.
Can you mix zinc and aluminium anodes?
As a rule, no. Use the same metal for all anodes that are electrically connected, so they protect evenly and one does not wear out much faster than the other. If you switch from zinc to aluminium, change every anode at the same time. Follow the engine and drive maker's advice if it says otherwise.
Where are the anodes on a boat?
Anodes sit next to the underwater metal they protect:
- Shaft and propeller: a shaft anode clamped around the shaft, or an anode on the propeller nut.
- Outboard and sterndrive: on the gearcase, the transom bracket and under the anti-ventilation plate, often shaped like a trim tab.
- Bow thruster: a small anode by the propeller in the tunnel.
- Hull and rudder: a hull anode plate wired inside the boat to its underwater metal, sometimes with anodes on the rudder and trim tabs.
- Engine: some inboards have a small anode in the cooling system, for example in the heat exchanger.
See our anodes for propellers, drives and engines and bow thruster anodes. For many drives and outboards there is an anode kit with every anode for one specific model.
How do you find the right anode for your boat?
- Drives and outboards: Note the make, model and year or serial number from the ID plate. Anodes for Volvo Penta, Mercury, MerCruiser, Yamaha and Suzuki are made for specific models, and the maker's part number is often in the manual or stamped on the old anode.
- Shaft: Measure the diameter with callipers. Boats built in Britain or North America may have imperial shafts rather than metric ones. The shaft anode must fit tightly so it cannot turn or slide.
- Hull anode: Start with the same size as the old one.
Choose the same metal as the boat's other anodes. Browse all zinc and aluminium anodes.
When should you replace an anode?
As a rule of thumb, replace an anode when about half of it has corroded away, unless the manufacturer says otherwise.
- Check the anodes when the boat comes out and again before it goes back in. In Denmark that usually means the autumn haul-out and the spring launch.
- On an outboard, tilt the engine up and look during the season.
- An anode that is almost half gone in spring rarely lasts the season. Replace it while the boat is ashore.
Keep spares for the shaft, drive and bow thruster, which are hard to reach afloat, and check the anodes when you lay up the engine; see Winterising an outboard engine.
Why is my anode wearing too fast, or not at all?
Untouched after a full season: it is probably not protecting anything. Typical causes are paint over the anode, a corroded contact surface, or a metal that does not suit the water, such as zinc in brackish water with a hard white crust.
Gone within a few months: the anode may be too small or be magnesium in salt water, or stray current may be flowing through the shore power, the boat's wiring or neighbouring boats. On shore power, check the anodes more often. If they keep disappearing, have a marine electrician test the installation and assess whether you need a galvanic isolator.
How do you replace a sacrificial anode?
- Unscrew the old anode.
- Clean the contact surface on the shaft, drive or hull back to bright metal with fine abrasive paper or a wire brush.
- Fit the new anode, with new screws if supplied, and tighten to the maker's instructions. Tighten a shaft anode's screws alternately so it sits straight.
- Check the connection. With a multimeter, measure the resistance between the anode and the part it protects. It should be very low, close to zero.
- Never paint the anode. Mask it off when you antifoul; see Antifouling in Denmark.
If a new hull anode means drilling through the hull, or the wiring needs changing, leave the work to a professional.
Frequently asked questions
Is a zinc anode the same as a sacrificial anode?
In everyday speech, yes. Many sailors call every sacrificial anode "a zinc", but it can be zinc, aluminium or magnesium. Check the metal in the product description and choose the one that suits your waters.
Can I switch from zinc to aluminium?
Yes, and it is a common change in Danish waters, especially in brackish water. Replace every anode on the boat at once, and check that the engine or drive maker approves aluminium for your model.
Do I need different anodes for the Baltic than for the North Sea?
Not if you fit aluminium, which works in both. Zinc suits full-strength salt water and can stop working in the brackish Belts and Baltic. If your boat arrives from the North Sea or the English Channel with zinc anodes, aluminium is the safer choice for a Baltic season.
Can I paint over an anode?
No. An anode must be in direct contact with the water to work. Paint, including antifouling, insulates it and stops the protection.
Any questions?
Not sure which anode fits your shaft, drive or engine? Write to us via the contact page with the boat type, engine make, model and year, and where you sail, and we will help you find it. The full range is under sacrificial anodes.
