Ever stop to think how dependent your boat is on a little rubber impeller? Or how many of them you have aboard? Depending on the size of your boat, it could be from one to as many as seven or more. They reside in your outdrive, electric head, pressure water system, washdown pump and oil change system, for example. Failure of any of them could result in consequences that range from a smelly mess to a possible sinking or expensive damage to your engine and drive train.
Why Use an Impeller Pump
Flexible impeller pumps are in wide use for several reasons. They are simple pumps, having only one moving part. They are versatile, being able to pump anything from water, oil or the “stuff” in your holding tank. They handle small debris well.
They are self-priming and use the fluid being pumped to lubricate the rubber impeller. A well-sealed pump can push fluids 15 to 25 feet. Impeller pumps are also scalable, meaning they can be built large or small, depending on the application.
Impeller Pump Applications
Flexible impeller pumps are common in outboard engines, used to pump the cooling water through the engine. They are also in many outdrive units for the same purposes. Inboards of all sizes also use impeller pumps to drive coolant through the cooling system.
Applications that are more mundane include things like macerator pumps. These usually have chopper blades as part of the design, for obvious reasons. Engine oil change system, especially portable ones, often use impeller pumps. Impeller pumps are also common in washdown pumps and live well aerators.
Impeller Pump Theory
Impeller pumps are quite simple in operation. The impeller runs in a cavity that, at first glance, looks cylindrical. On closer inspection, the cylindrical cavity or housing either is offset from the centerline of the pump shaft or is indented to form a cam on one side of the cylinder.
This cam provides the pumping action. As the blades of the impeller pass the cam, the volume between the blades expands, sucking or drawing in the fluid being pumped. As the blades hit the cam on the other side, the volume between the blades contracts, expelling the pumped fluid. The continuous rotation of the impeller provides a smooth, even flow.
Pump Materials
The heart of the impeller pump is obviously the impeller and they come in several different materials. The most common is neoprene rubber. This material is used when pumping water-based fluids. Nitrile rubber is used when pumping petroleum-based fluids.
A variety of materials is used for the pump housings. Most outboard pumps use a plastic housing with a stamped steel liner in the cavity. A wear plate and gasket seal the open end on the pump. The pump shaft is also the prop drive shaft and passes completely through the pump.
Inboard and large I/O pumps are often made of cast bronze. These units have a separate arc-shaped cam that bolts into the pump cavity. A bronze cover plate seals the open end of the pump and allows access to the impeller.
Macerator pumps usually have plastic housings. They can be built into the base of the toilet or operate as a stand-alone pump.
Pump shafts are usually steel and come equipped with one of more seals to prevent leakage. Belt or engine driven cooling pumps also have bearings and bearing supports as part of the design. Keys, gaskets and cover screws complete the pump bill of materials.
Pump Maintenance
Here is where opinions and recommendations differ. Some manufacturers recommend replacing the impeller every year. Some recommend removing the impeller from its housing during winter lay-up. Most of us don’t do either.
Impeller pumps generally fail in one of two ways. The first is a gradual degradation in pump performance and flow. This is usually due to impeller or housing wear or impeller age.
The second failure mode is sudden, catastrophic failure, with the impeller usually loosing some or all of its blades. This is usually caused by running the impeller pump dry. Remember, the fluid being pumped provides the cooling and lubrication for the pump. This is often caused by a water inlet seacock not being opened or a plastic bag covering the water inlet.
The impeller often sheds its blades in such a failure. The errant blades must be tracked down and removed from the system, especially if it is an engine cooling application. Failing to do so will usually lead to further problems and blockages later on.
Outboard engine pumps are probably the most difficult to maintain. Changing the impeller requires separating the lower gear case from the engine. This also may require disconnecting the shift linkage. Consult your engine manual for the specific steps required the get to the pump.
When the lower unit is free, the pump will usually be located on top of the gear case. The housing can then be unbolted and the impeller replaced. Carefully inspect the housing and the stamped steel liner for signs of wear or overheating. Rebuild kits are available that range from just the impeller and gasket, to kits that include upper and lower housing as well as the wear plate. If in doubt, replace. You don’t want to have to drop the gear case any more than necessary.
Most engine cooling pumps have a cover plate held on with several screws. Removing the screws and cover plate exposes the impeller. If the impeller has been changed regularly, it should just slide off the shaft. In many case, it will need to be pried out with a couple of screwdrivers. Take great care not to damage the housing, these can be expensive to replace.
Many manufacturers make impeller puller, much like gear puller, for extreme cases but they are pricey. Some manufacturers have threaded the inside of the exposed end of the impeller. Removing this type of impeller is a simple matter of threading the right size bolt in the open end. As the bolt is tightened, it bears on the pump shaft and backs the impeller out. More manufacturers should consider this design feature.
Freestanding impeller pumps are usually easy to disassemble and service, if the pump itself is accessible. A hint when servicing a macerator pump: place plenty of adult diapers around and under the pump to soak up any “stuff” that may dribble out of the pump.
Before you decide to perform maintenance on your impeller pump, take a few preliminary steps. Make sure you have identified the exact pump you have and have the appropriate rebuild kits or replacement impellers on hand. There are several pump and impeller manufacturers and many make interchangeable impellers. Have all those numbers available when you shop for spare parts. Your store may carry only one brand or the other (you DID document those numbers in your onboard maintenance manual, didn’t you?).
Make sure the right tools are available. Do a dry run to make sure the tools fit the jib. Special wrenches or short screwdrivers might be required. Trying to replace an impeller at night on a rolling sea isn’t the time to develop your repair strategy.
Conclusion
Impeller pump maintenance isn’t rocket science and is well within most boat owner’s capabilities. Make sure you have the parts and tools before starting and don’t be afraid to look at the service manual or rebuild kit directions.
Capt’n Pauley
Drawings:
How and Impeller Pump Works
Typical Impeller Pump Housings
Impeller Types
Photos:
Typical outboard motor water pump housing with stamped steel liner
Impeller Pump rebuild kit (Typ.)
Engine driven cooling water impeller pump
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