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AmazonPut two different metals in salt water with an electrical connection between them and the more reactive one corrodes away. An outboard is mostly aluminium, sitting in the water with a stainless or bronze propeller, steel fasteners and the boat's other metalwork, so something is always being eaten. An anode is a piece of an even more reactive metal, bolted on so it's eaten instead, and replaced before it's gone.
- Zinc for salt water, aluminium for salt or brackish water, and magnesium only for fresh water. Fit the kind your motor's maker sells for where you use the boat.
- Replace them once a third to a half has worn away: the service manual says when. An anode that isn't wearing at all usually isn't connected properly, so it isn't protecting anything.
- Never paint them, and fit them on clean, bare metal, so they have a good electrical connection to what they protect.
Most outboards have the trim tab anode above the propeller, others on the transom bracket, the bottom of the gearcase and the trim and tilt unit, and many have internal anodes in the water jackets of the block and head, behind a small cover. Those are the ones that get forgotten.
A broken anode blocking the cooling
Dave's Yamaha 60HP was overheating. With the access plugs out we found a piece of internal anode that had broken off and lodged in the tiny hole the cooling water goes through. Once it was out, it ran under load with no more overheating.
Four internal anodes, all broken off
With the head off a Yamaha 50HP four-stroke, not one of its four internal anodes was where it should have been: they'd all broken off and were rattling around inside the water jackets. They're easy to check and change from outside, by the spark plugs (a 10mm cover, and an 8mm screw holding each anode), so there's no reason to leave them until they fall apart.
Getting out what's left
The holders were still in the head with the ends of the old anodes snapped off inside them. Threading a fastener into the remains pulls most of each one out. With the head off this was easy; with it on, a snapped anode is much harder to get out, which is one more reason to replace them before they get that far.
When the manual says to replace it
Changing the prop shaft seal on an Evinrude 150, the anode by the bearing carrier was about a third worn, which is when the service manual says to replace it.
Anodes on a steel hull
On Renko, my steel trawler, I tried three ways of fitting the hull and rudder anodes: bolted through with conductive carbon grease behind the strap, which is easiest but has the least reliable connection; a stainless bolt with its head welded to the hull, which is my preferred way, as they can still be changed; and the strap welded straight on, which can't be changed in the water. Don't paint over them.
Four or five years later
Back on the hardstand, the hull anodes were well eaten away, which means they were working, and well due for replacement. The bolted ones had worn much more than the welded ones, which raises the question of whether the stainless bolts were adding to it.