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

Boats and yachts live or die by the pump. There are freshwater pumps, saltwater pumps, washdown pumps, bilge pumps, sump pumps, baitwell pumps, fuel pumps, oil pumps, engine-cooling pumps, air-conditioner pumps–well, you get the idea. And with so many pumps doing so many important tasks, knowing how they work and how to maintain and repair them is an important part of a skipper’s expertise.

Pumps are basically simple machines, so it’s pretty easy to keep them working, or to revive them when they die. Knowing how a pump works takes you a long way to understanding how to treat it when it’s sick. Although pumps serve many different functions onboard, there are only three basic designs.

Centrifugal Pumps
A centrifugal pump has a solid impeller similar to a turbine rotor; when the impeller is rotated at high speed, it discharges fluid through an exhaust port. Because the rotor is not tightly enclosed in a housing, the pump is not "self-priming" and cannot create suction and draw fluid into itself, so an outside force, usually gravity, has to supply it. This type of pump is most often found sitting in the fluid it’s meant to move, usually water (centrifugal pumps make perfect submersible bilge or shower sump pumps).

Centrifugal pumps demand virtually no maintenance, rarely break down, are difficult to clog, and can even run dry, sometimes for days, before their bearings seize up. Their major shortcoming is the inability to self-prime, which limits their use onboard. So how do you maintain a centrifugal pump? Mostly by keeping them clear of debris that can clog them or get inside and break the impeller vanes. Usually, submersible pumps are fitted with float switches to turn them on automatically. In my experience you’re more likely to have trouble with the switch than with the pump. Fussbudgets like to install their submersible pumps and float switches inside "strum boxes," wire-mesh or perforated-fiberglass boxes that keep gurry away from both components.

Most centrifugal pumps are inexpensive, and it’ll usually cost more to fix a broken one than to install a new one. Some builders install expensive, rebuildable centrifugal pumps; these can be repaired using kits supplied by the pump manufacturer. The job is simple, demands only basic tools and skills, and is seldom necessary.

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