How to change anodes on an outdrive boat

Our resident boating instructor Jon Mendez explains how to change the anodes on a boat with outdrives.

For owners of outdrive boats the annual lift-out is always greeted with some trepidation. Outdrives are complex pieces of machinery that offer excellent performance and economy but they can be costly to repair as they get older.

The trick is to maintain them properly – don’t skimp on servicing or anode changes or it could cost you a small fortune – and always take the opportunity to inspect and replace any items that show signs of wear whenever your boat has a lift out.

The bare minimum I would suggest doing every time your boat comes out of the water is as follows:

Change worn anodes

Your boat has sacrificial anodes to limit the corrosion on submerged metal fittings. My boat has them on the outdrive, trim tabs and bow thruster.

You can tell they are working as they will be pitted from corrosion. When 50% worn they should be replaced, especially if the boat is going to be afloat for longer than it has taken the first 50% to disappear.

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Some anodes come with pads that go between the anode and the surface of the trim tab to stop corrosion of the actual trim tab. Replace these as well.

Whilst you’re doing this, lower the trim tabs to their fullest extent so you can clean off any growth on the rams before trimming them all the way up again.

The outdrive anodes are a bit more involved. There is usually a bar anode set in front of the outdrive leg, which you can access by lifting the leg first.

There is also a pair of small triangular anodes in the corners of the outdrive frame, two larger round ones on the cavitation plate, and a ring anode just forward of the propellers.

To access this, you need to remove the propellers with a special spanner. While the props are off, it’s good practice to inspect the sealing rings on the propeller shafts.

If any fishing line gets caught here it can damage the seals and allow water into the gear oil.

Gear oil

To check the gear oil, trim the leg fully down, unscrew the dipstick on top of the leg and. dip the oil.

As well as checking the level, inspect the oil for signs of emulsification.

Replace the dipstick’s rubber sealing ring with a new one before retightening it.


Using a flexible clip drive, undo the drive shaft bellows (the top one) at the leg end rather than the transom end.

As you ease the bellows off at the base, check for water drips. Any rusty water in the bellows and the leg will need to be removed for further investigation.

When done, replace the bellows making sure it’s properly on all the way round and the orientation of the clip is correct.

Whilst out of the water, I’d also take the opportunity to polish the propellers and re-antifoul the outdrive leg using a special non-copper antifoul that doesn’t corrode the metal.


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