STERN DRIVE MAINTENANCE
While you are inspecting your bellows, take a little time to look over your prop as well.
Any visible damage?
Any signs of fishing line down there?
Fishing line can damage the seals that hold the gear oil in the sterndrive housing. Monofilament line can even reach the melting point when wrapped around a revolving propeller shaft.
Fishing line wrapped around the propshaft can cause the lower unit seal to fail, allowing water to contaminate the oil, which can destroy the unit. (Photo: Doug Alling)
If line is discovered, removal of the prop may be necessary to disentangle it. Take a look at the seal that is just forward of the prop and look for any signs of leaking oil. Then, take a look at the gear oil itself.
Is the level OK? If the oil has a milky white appearance, water has entered the sterndrive and the oil will have to be replaced.
Remember, refilling your sterndrive oil is counterintuitive.
New oil must be pumped UP into the sterndrive. Never refill from the top oil port or vent hole.
If you are concerned about the integrity of your oil seals, your mechanic has a simple method of checking them by vacuum or pressure.
As you run down the list of recommendations your sterndrive manufacturer has outlined for annual maintenance, you'll note that you should be lubricating some important moving parts. Don't overlook the prop shaft, U-joint shaft splines, and steering system cables. One grease gun may not cover the needs of these different applications. Check your owner's manual for recommendations.
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.
This anode is past due for replacement. Without proper anodes, an outdrive will quickly corrode. (Photo: Doug Alling)
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.
This lower unit cracked over winter when trapped water froze and expanded.
Check for milky oil in the unit before laying up for winter. (Photo: Doug Alling)
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.