More of this Feature • Part 1: Centrifugal Pumps • Part 2: Impeller Pumps • Part 3: Diaphragm Pumps
Related Resources • Maintenance Index

Smear the new impeller with liquid dish soap or something equally slippery and water-soluble, and push it onto the shaft. (The lubrication is important to keep the impeller healthy until the pump primes, so use plenty.) Don’t worry too much about deflecting the vanes in the proper direction; they’ll sort themselves out on the first rotation. Replace the setscrew and hose, if necessary, then replace the paper gasket under the faceplate with the one that came with the new impeller. Don’t use gasket compound. When you start the engine, watch for water to appear in the exhaust before getting underway.

Diaphragm Pumps
Diaphragm pumps are less common onboard than centrifugal and impeller pumps but frequently found in pressurized freshwater systems and serving as fuel-lift pumps (bringing fuel from the tank to the engine) on diesel engines. Manual bilge pumps–the kind with handles that rock back and forth–are diaphragm pumps, too. Unlike either centrifugal or impeller pumps, diaphragm pumps rely on one-way valves to control fluid intake and exhaust as the diaphragm flexes in and out like a bellows. When the pump misbehaves, chances are either the intake or exhaust valve is clogged with debris. (That’s why small, electric diaphragm pumps make poor bilge and sump pumps but are great for moving clean water and fuel.) Eventually the diaphragm will split after being flexed a few million times, but in my experience one diaphragm will outlive several sets of valves.

Again, maintenance and repair are simple, but read the directions first, as some diaphragm pumps are more complex internally than others. Basically you open the pump body, clean or replace the valves (making sure to install them so they operate in the correct direction), check the diaphragm and replace it if necessary, and reassemble. You don’t have to lubricate anything, and the pump will prime itself quickly. Diaphragm pumps run dry without harm. Diesel-fuel lift pumps of this type will live for literally thousands of hours of operation, freshwater pumps for almost as long.

It’s daunting the first time you have to repair any pump, just like the first time you do anything involving tools and expensive bits of gear. But most folks with opposable thumbs can handle the job easily by taking their time and reading the directions. Knowing how to keep your pumps pumping will save you lots of money, downtime, and aggravation, so it’s worth learning how ahead of time…before you’re high and dry.

Mike Smith is a licensed yacht- and commercial-boat captain living in Stamford, Connecticut.

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