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.
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. Fuss budgets 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.
You'll find at least one submersible centrifugal bilge pump like this Rule aboard almost every yacht. 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.
Unlike centrifugal pumps, impeller pumps often get cranky and need some tending, especially if they've been run dry for more than a couple of minutes. We put up with them because of their self-priming ability: Impeller pumps can draw fluids into themselves from a remote supply, such as a through-hull fitting. Virtually every inboard engine depends on an impeller pump to feed raw water into its cooling system. They're also commonly used as washdown and baitwell pumps, in oil-change systems, as cooling pumps for air-conditioning systems-anywhere fluid is drawn from a reservoir by a remote pump and moved somewhere else.
An impeller pump works by creating suction with its flexible impeller, which is generally made of neoprene and turns inside a dose-fitting housing. (That's why lubrication is important.) A cam in the housing deflects the vanes of the impeller as it spins; this creates the suction that draws fluid into the housing. The function of an impeller pump depends on the vanes being intact and not excessively worn. Also, the housing and cover plate must fit snugly against the impeller to preserve the suction.
Impellers can be as fragile as actors' egos, especially if allowed to run without fluid to lubricate and cool them. If your engine overheats because a plastic bag in the seawater strainer has cut off the supply of water, it's a good idea to check the impeller before you start up again. Remove the faceplate from the pump housing-it's usually held on by four or six screws-and open the pump carefully (sometimes you have to slip a knife blade under the cover to persuade it to pop oft). The impeller inside will look like a paddlewheel, with the vanes pushed out of shape on one side by the cam. Even if the impeller appears okay, inspect each vane carefully to ensure it's healthy and not torn across its base. When in doubt, take it out: If a vane comes loose and is pumped into the cooling system, it can block the cooling-water flow and cook a manifold.
Replacing an impeller is easy. If you are lucky, it fits onto a splined shaft with no setscrew holding it in place, and you can pull it out with a pair of slip-joint pliers; their widely adjustable, offset jaws are ideal for reaching into the pump. If the impeller won't come out, there is probably a setscrew through the body and shaft. Remove one of the hoses, whichever one is easier, and bump the starter until you can see the setscrew through the unoccupied nipple. Remove the screw with a screwdriver, and the impeller will slide right out.
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 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.
Eventually your pumps will need maintenance or repair. An exploded view, included in most repair kits, will make that job easier.
This article was in Power & Motoryacht, April 2002, by Mike Smith. Mike Smith is a licensed yacht- and commercial-boat captain living in Stamford, Connecticut.
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