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 Volvo DP-3 sterndrive
Photo: Volvo Penta
Giving your sterndrive some attention now can prevent major problems later.

Sterndrives were first commercially introduced in the early 1960s, and came to dominate the boat propulsion market in short order. The biggest names in the sterndrive game today are Mercury and Volvo with thousands of older Outboard Marine Corporation (OMC) drives still in use but no longer being produced.

When you power up your engine and pull away from the dock, you set in motion a lot of activity at the stern of your boat. You push the throttle, engaging the prop. Power from your engine is transferred to the sterndrive via the drive shaft. Exhaust is pushed from the engine and exits at your prop. All of these functions start inside your boat and end at the sterndrive and require holes through the transom of your boat, at or below the waterline. Three to be exact: One for the throttle cable, one for the drive shaft, and one for the exhaust. And there's only one thing that keeps the water outside the boat from entering those openings — the sterndrive's bellows, those black corrugated rubber things that are clamped to the boat's transom and sterndrive in a place that makes them really hard to see. Perhaps the old saying, "out of sight, out of mind," is why bellows become such a big issue when it comes to sinkings. Take away the biggest and most obvious cause of boat sinkings at the dock — hurricanes — what do you think comes in second place? If you guessed sterndrive bellows failures, take a bow.

Photo of a corroded sterndrive
Ignoring your sterndrive now can cost you money and down time this season.

Take a moment to think about the environment that bellows operate in and the work they are tasked to do. They are exposed to hot and cold, wet and dry, vibration, UV, and marine life (sharp barnacles), and through all that they are expected to stretch, expand, and contract as you tilt and steer your sterndrive. Bellows will wear out — and when they do, they can easily let enough water into the boat to sink it. To keep your sterndrive-powered boat afloat, bellows require inspection and maintenance on a regular basis.

Bellows Inspection And Maintenance

Inspection of the bellows can be accomplished while the boat is out of the water — on a lift, a work stand at the marina, or on its trailer. Keep safety in mind while doing this! Make sure the boat is properly supported and if on the trailer, chock the trailer tires. It is good practice to make sure that there's no way the engine can be started while you are performing your inspection by disconnecting the batteries. The inspection area is tight and will require moving the sterndrive from one position to another — port and starboard, up and down — to access as much of the bellows as possible. The inspection is as simple as taking a close look (a flashlight is required) at the condition of the rubber bellows and their clamps. Are there any obvious signs of cracking? Is the rubber becoming brittle? Is there any rust showing on the bellows? Do the stainless steel clamps show any signs of corrosion? If you spot any marine growth (barnacles, mussels, or the like), they have got to go. Barnacles and broken shells can be razor sharp and your rubber bellows won't stand a chance against them.

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10 Sterndrive To Dos

1. If you keep your boat on a trailer, inspect your sterndrive's bellows several times each season. Haul out boats kept in the water at least once a year for a complete sterndrive inspection.

2. If one bellows needs to be replaced, replace them all.

3. Replace the bellows per the manufacturer's recommendations, even if you see no damage.

4. Lubricate moving parts at least annually.

5. If you get fishing line caught on your prop, inspect the seal in front of the prop for damage.

6. If gear oil is milky, replace the seals, as well as the oils.

7. Remember to pump gear oil UP when refilling.

8. Replace anodes when they are half their original size. Use aluminum for sterndrive anodes.

9. When applying copper bottom paint, leave a margin around the sterndrive to reduce the potential for corrosion.

10. To keep water from entering the exhaust hub when storing your sterndrive, leave it in the "down" position or use a waterproof cover.