Mercury Water Pump

Stern-Drive Impeller Maintenance

By Vincent Daniello

When it's time to change raw-water pump impellers, boaters who are reasonably handy with a wrench can save some money by doing it themselves. The procedure is similar for most makes and models of outboards. While inboard engines have a few more variables, in the end they're all pretty similar as well. Stern-drive engines, on the other hand, use at least three completely different types of raw-water pump systems. Here's an overview of the differences and how to service each type.

Within the Lower Unit
Mercury-powered boats built before 1991 or newer boats with no more than 300 hp per engine most likely have Mercury Alpha stern drives. The raw-water pump fits inside the stern-drive lower unit. This is roughly similar to most outboard engine raw-water pumps.

The stern-drive lower unit has to be removed to maintain these pumps. This can't be done as an emergency repair, so regular maintenance is essential. Fortunately, these impellers are easy to change when the drive is being serviced. Most mechanics recommend stern-drive service every two years in the North where boats are only used in the summer or once a year for boats in the South that are used year-round. While it may seem odd to engineer the raw-water impeller into the middle of the stern-drive, by putting the impeller here, the pump is underwater when the boat is idling, so even if the boat is trailered, the impeller primes itself as soon as the boat is in the water.

Belt-Driven Raw-Water Pump Inside the Boat
The Mercury Bravo stern drives on larger boats, as well as most other stern-drive brands, cool engines with a raw-water pump inside the boat. In fact, raw-water pumps are often nearly identical on stern-drive and inboard engines. The main difference is inboard engine pumps draw water through the hull and then a sea strainer. Stern drives typically draw water through intakes on the lower unit and typically don't employ a strainer.

Gear-Driven Pumps
Many engines, both inboards and stern-drives, drive the raw-water pump directly off the crankshaft or a camshaft. With no belts to deal with, pumps face forward for easy access. Changing the impeller couldn't be much easier, assuming the boatbuilder provided room.

On OMC stern-drives — built for just a few years in the 1980s — the raw-water pump is at the top of the stern-drive. They're quite easy to change. It can even be done while the boat is in the water, but given the location, it's easy to drop parts or tools overboard.

Pt.3 Mercury 50HP Outboard Water Pump Replacement | All Used ...

Wow, thank you…I just bought a boat with a 2002 50hp Merc that had been in storage for 2 years. I fired it up in the driveway and it was pumping water like a 90 year-old with a bad prostate. I picked up a pump from the local shop and thought I’d search youtube for a “how-to” video. Amazing what you can find on the internet…you saved me several hours of frustration. I got the pump changed quickly and now it pees like a race horse. Thank you!

Philosophical magazine

The London, Edinburgh and Dublin philosophical magazine and journal of science

The new international encyclopaedia

Physical review

The experimental study of gases, an account of the experimental methods involved in the determination of the properties of gases, and of the more important researches connected with the subject