The Care And Feeding Of Sterndrives

Keeping your sterndrive in tip-top shape will add years to its life — and help to keep your boat afloat.

By Doug Alling
Published: July 2014

If your boat spends much of its time in saltwater, corrosion of the sterndrive becomes a concern. Manufacturers install sacrificial anodes that are designed for your boat's sterndrive and protect the aluminum housing from corrosion. Monitor them closely and replace them when they become half wasted. How often that is depends on the amount of time the boat spends in the water. Check your motor's manual for the location of all of the anodes because some are cleverly hidden, such as under the cavitation plate just above your propeller.

Photo of anode that needs replacement
Photo: Doug Alling
This anode is past due for replacement. Without proper anodes, an outdrive will quickly corrode.

Replacement with aluminum anodes is recommended. Magnesium anodes should only be used if your boat lives in clean fresh water all of the time. Remember that anodes should never be painted. Mercury Marine has gone the extra yard and in some sterndrives has installed the MerCathode system. This is an active or "impressed" system that actually delivers a small electric current to the sterndrive to counteract corrosion on the unit. The MerCathode derives its power from your boat's battery so, to be effective, your battery has to be charged, and all wires and connections have to be sound. Again, your mechanic should have a simple test to ensure that your Mercathode system is working properly.

Not only should you keep the paint brush away from your sterndrive's anodes, you should steer clear of getting too close to the sterndrive itself. Most bottom paints contain copper and most sterndrives are constructed of aluminum. These two dissimilar metals do not cohabitate well and underwater can turn into a battery of sorts that can lead to corrosion issues. It's important to keep the copper in your bottom paint away from the aluminum in your sterndrive. So, when painting the boat's transom, keep an unpainted area around your sterndrive. Most recommendations are for about one-and-a -half inches of unpainted surface. Use a copper-free, drive-specific paint, such as West Marine's Antifouling Outdrive Spray paint, if the boat is kept in the water full time.

Photo of cracked lower unit housing
Photo: Doug Alling
This lower unit cracked over winter when trapped water froze and expanded.
Check for milky oil in the unit before laying up for winter.

And finally, when it comes to storing your sterndrive, it's important to prevent water from entering the exhaust hub of the propeller. Openings that are designed to let the exhaust out can also allow rainwater and snowmelt in. Water will accumulate in the unit's housing if the drive is stored in the "up" position. In cold weather, any accumulated water in the sterndrive can freeze and, under the right conditions, ice may expand and crack the housing. When putting your boat away for the season, store the sterndrive in the "down" position or use a sturdy, waterproof cover over the prop to prevent water accumulation in the housing.End of story marker

Capt. Doug Alling is an American Boat and Yacht Council (ABYC) Master Technician, the principal surveyor for Rum Line Marine Consultants, LLC near Charleston, South Carolina, and a member of the Knox Marine Consultants team.

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