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

Photo of marine growth on bellows
Photo: Doug Alling
Marine growth can be razor sharp and slice open bellows, allowing water into
the boat. Inspect bellows frequently throughout the season.

That's what to look for outside of the boat. Inside the boat, look for any water tracks at the transom pass-throughs and standing water in the bilge. If your bilge pump has been cycling on and off, it's time to inspect the bellows. OK, we've done our inspection and things look good, but keep in mind that all manufacturers have a recommended replacement schedule for your sterndrive bellows. You will most likely find it in your owner's manual and you should adhere to it. If you are unsure or can't find the information, a call to the manufacturer's customer service center is in order. I have dealt with both Mercury and Volvo in this regard and both manufacturers have been very responsive. Even if your bellows inspection shows good results, the manufacturer knows best when it comes to keeping them in tip-top shape and how often they should be changed. Most manufacturers agree that a 5-year-old bellows is living on borrowed time. Not replacing it is inviting water into your boat.

If you poke around the Web, you'll undoubtedly find videos posted on how to change out your exhaust bellows while the sterndrive is still attached to the boat. It can be done — with really small hands and the right tools. But the exhaust bellows is the only one that can be changed while the sterndrive is attached. The other two bellows contain the drive shaft and shift cable, and they can only be changed by removing the sterndrive from the boat and disconnecting those systems. Changing out the exhaust bellows alone goes against one of the first rules that a marine surveyor learns: If something breaks or wears out underwater (a bad cutless bearing, a bent rudder) and there are two or more of them, you must always check them ALL. If you are going to the trouble of changing the exhaust bellows, replace the other two, period.

Beyond Bellows

While you are inspecting or replacing the 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.

Photo of fishing line
Photo: Doug Alling
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.

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.

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