Article by Capt. Steve Katz
Pumps
Every boat has at least one pump and may even have a dozen or more. Whether it is a flexible rubber impeller pump to cool an outboard engine or a specialized Vaccuflush diaphragm pump to pump waste, each system should be equipped with the proper pump that meets the demands of the task.
Engine cooling is usually accomplished with a flexible rubber impeller pump. From outboards to large diesels, the time proven flexible rubber impeller pump is designed to move water with high volume and medium pressure. These pumps are usually self priming, meaning they can pull water into the pump from a short distance upon start-up. The rubber impeller is cooled by the water it is pumping and as long as it does not run dry, it will usually last for at least a few seasons, depending on usage. These types of pumps are low maintenance, consisting of a rubber impeller, metal housing (sometime plastic) and seals on the pump shaft. These pumps work best when operated regularly, keeping the rubber in shape and lubricated. When these impellers sit too long without use, they can take a “set” and not be flexible enough to pump the required volume.
After sitting unused for awhile, the rubber can dry-rot and fail upon start-up. It doesn’t take much for the impeller to fail when run dry or a bag, mud or any restriction in the pick-up supply to these pumps can cause an impeller to overheat and fail in a few minutes or less. If you do have a failure with an impeller pump, the rubber blades often disintegrate and work their way deeper into the cooling system, requiring a clean out that may require disassembly to remove the rubber pieces blocking the cooling system.
Another similar style pump used on most boats is the centrifugal pump. This pump uses a spinning, rigid impeller and centrifugal force to move the water or liquid. The rotating impeller speeds the liquid flow and moves it to the outside of the casing, towards the discharge port. The most common centrifugal pump on a boat is the submersible bilge pump. These pumps are not self priming; they need to be at or below the water level to allow gravity to push the water to get the it pumping water. If the pump is above the water level, it must be initially primed.
A centrifugal pump is the highest volume style of pump used on a boat. The most common bilge pump is rated at 1,500 gallons per hour or 25 GPM at a low pressure. When used as a bilge pump, the spinning impeller needs to be submersed in the water before it will begin pumping.
Another popular use of the centrifugal pump is for live wells and air conditioning cooling water, where both have the pump mounted below the water line and are designed to run continuously. In a larger format, centrifugal pumps are often used as fresh water pumps on larger boats, similar to a residential shallow well pump. Centrifugal pumps should not run dry since the motor is often cooled by the liquid passing through the pump.
The most popular general purpose pump used on boats is the diaphragm pump. This is often used in general purpose pumping needs, such as washdown pumps, bilge pumps, freshwater pumps, head pumps and general utility pumps. Diaphragm pumps use a variable volume pump chamber and check valves to move fluid. As the diaphragm rises, liquid is pulled into the pump chamber while the outlet valve is pulled close. When the diaphragm is lowered, liquid is forced out the outlet valve while the intake valve is pressed shut. More chambers ensure smoother operation and higher volume. These pumps are self-priming and can usually be run dry without immediate damage. They are generally quiet and draw little current, so they are ideal for many marine applications.
Since these pumps often comprise of many small diaphragms (the size of a dime), they are sensitive to debris and can easily clog. When debris gets inside the pump it gets into the valves and keeps them from closing properly. A proper filter or strainer is required on all diaphragm pumps. Diaphragm pumps are capable of the highest pressures of the pumps we have reviewed, up to 70PSI with a lower volume than most other styles of pumps, typically 3-5 GPM.
When installing or replacing a pump, review your needs and the types of pump to be sure you get the right pump for the job and be sure to have a spare impeller and seal kit for your flexible rubber impeller pumps.
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.
