Article by Capt. Steve Katz
Impellers -
What, Where and When
We hear lots of talk about impellers on all types of boats. What are impellers? Where are they located and when should we worry about them?
An impeller is the rotating component of a centrifugal pump. For this column, we are going to review the soft rubber-type flexible impeller used in many marine water pump applications. Other types are rigid impellers, sometimes referred to as rotors that are located inside centrifugal pumps, such as bilge pumps, high flow marine air conditioning pumps and the impellers that power Jet skis and the like.
The flexible impeller marine pump is a positive displacement pump. A positive displacement pump traps a fixed amount of water and forces it into a discharge pipe. The rubber impeller blades deform as it rotates inside of housing, making for a tight seal, allowing the pump to be self-priming, and where applicable, allows pumping in either direction. The size and design of the housing and impeller, along with rotational speed determines the pumps volume and pressure output.
The design of the impeller pump with multiple blades results in a smooth and consistent output in both volume and pressure making them very useful for marine pumping needs. Raw water engine cooling pumps increase volume with engine speed, supplying more water at higher engine speeds matched for the higher heat loads.
In the preceeding paragraphs we mentioned a few key points that are directly related to a pumps performance.
Flexibility – the rubber impeller needs to be flexible. That means the rubber vanes should spring back when bent in either direction. As the impeller rotates, it is forced around an elliptical housing, causing the blades to deflect and extend with every revolution. This motion will eventually cause the impeller to lose it’s resilience. Once this happens, the pump output will be reduced since the vanes will no longer form a tight seal against the housing.
Housing – The housing of the impeller pump is often made of soft metal, such as bronze. These metals are designed for saltwater use and polished smooth so they do not prematurely wear the rubber impeller blades, but these metal housings can become worn too. When worn evenly, the wear might not be immediately noticeable, but the diameter has become enlarged. Once enlarged or scored, even a new impeller may not make a tight seal against the worn housing, resulting in decreased pump output or the decreased ability to self prime. Most marine impeller pumps have a replaceable insert or housing, allowing you to return an aging worn pump back to top performance.
The face plate and seals are the other two areas of maintenance and performance concerns. On many of these pumps there is a face plate (wear plate) that holds the impeller inside of the pump. This face surface needs to be smooth and fit tight against the impeller. If it is worn or scored, the rubber impeller will not form a tight seal and reduced performance will be noticed. On the opposite end of the housing is a seal designed to keep water from the pumps bearings, drive shaft and motor or drive device. While these seals are hard to inspect without disassembly, many pumps have a weep hole on the back side of the bearing. If you see water dripping, or pouring, from this hole, it is a sign that the seal is bad and replacement of the seal and most likely the bearing is necessary.
Does your boat have an impeller pump?
Outboard engines have a flexible, rubber impeller pump for the seawater cooling system. Most outboard engines have the pump and water pick-up located in the lower unit. The inlet to this pump is located below the water line, allowing fast priming upon startup. To inspect an outboard’s impeller, the lower unit needs to be removed from the engine and the pumps housing needs to be removed to gain access to the impeller.
The outboard engine centrifugal water pump has the same components as described above, rubber impeller, metal housing and metal wear plate.
Often times a boater will only replace the rubber impeller, since the housing and wear plate “look good”. While they might look okay, there may be wear that is not immediately visible and since the effort is the same to inspect versus replace, the wear plate and housing should be replaced when an impeller is replaced. Most engine manufactures and aftermarket suppliers offer a kit for each engine model with all the necessary components to replace the impeller, housing, wear plate and seals – allowing you to properly restore your water pump back to new performance and eliminate a surprise failure due to a worn out pump.
How often should you inspect or repair your outboard engine seawater pump? Well that depends on how you use the boat. An impeller pump that sits with little use can fail faster than one that is used regularly. The impeller blades can take a “set” and lose their flexibility if not used often and can also dry rot. This most often occurs when the boat is stored out of the water with little usage.
Engines operated in silty or sandy areas will wear the water pump at a faster rate and even a brief grounding in shallow water may cause excessive sand or mud to wear or clog the impeller.
It is a good idea to inspect or replace the outboard engine impeller and related components annually, usually as part of the spring commissioning or when the engine oil and or gear lube is changed. Yamaha recommends impeller/water pump maintenance annually or every 100 hours for their outboard engines.
If you have an inboard engine, the seawater (raw water) pump is often mounted on the engine and driven by gears or a belt. The housing is often a large, bronze housing with an accessible cover plate that can be easily removed for inspection. The removal and re-installation effort can vary depending on the pumps location and engine manufacturer and take from few minutes to a few hours to replace the impeller. If the housing needs to be changed or seals need to be replaced, the pump usually needs to be removed from the engine and taken to a workbench for service. This could be time consuming and expensive depending on the make and model of the engine. Similar to outboards, the pump manufacturers offer kits to repair or rebuild these water pumps.
This article would not be complete without mentioning that if a rubber impeller pump runs dry (with an adequate water supply) the rubber will quickly heat up and become damaged. It can also damage the housing and other components of the pump.
To avoid unexpected failures, perform regular maintenance of your impeller pump. Use it regularly, keep a clean and adequate supply of water to the pump and replace the impeller and components at the first sign of a weep hole leak or when the pump is subjected to mud, sand or a low water supply.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers
