There are two types of pumps which are used in bilge pump applications. The first, and most common type of pump, is the centrifugal pump, all of the Rule, Johnson, and Atwood pumps are centrifugal type pumps. The second, and far less common type, is the positive displacement, or diaphragm pump. Though they have their place in the various applications they do have some drawbacks which we’ll look at. Centrifugal pumps have a couple things going for them which makes them well suited for your bilge. The first is volume. They are able to move significantly more water than the diaphragm pump. They also have only one moving part making them virtually maintenance free.
Centrifugal pumps operate with a single spinning impeller mounted directly onto the motor shaft in a sealed, concentric housing. In the case of bilge pumps with vertically mounted motors it’s called a tubular housing. The vanes of the impeller angle back in the opposite direction from the pump rotation. If the pump rotates clockwise, the vanes will fan out in a counter clockwise pattern when viewed from the face of the pump. As the impeller is rotated this design forces water to the outside of the housing as well as developing a rotational water flow, both of which create a rotational positive pressure at the fin ends of the impeller. This in turn creates a negative pressure at the center of the impeller which pulls water into the pump. Due to the constant motion, when compared to the diaphragm pump action, the centrifugal will move more water. Also, due to the simplicity of the design, this type of pump will move a certain amount of sediment and bilge debris. That said large objects in a dirty bilge can, and do, jamb the impeller and stop the pump. It’s best to keep a clean bilge.
The downsides to this type of pump is that they need to be submerged in water, or primed, to function. Without water rotating in the pump head there is negligible negative pressure developed and it won’t pick up water. Another down side is that bilge pumps, with the impeller at the bottom of a vertically mounted motor, will always leave close to an inch of water in the bilge. This can be dealt with by installing the pump in a small sump, or depression, in the bilge to minimize the amount of bilge water sloshing around.
Diaphragm pumps operate in a reciprocal motion much like the piston in a car. The piston, plunger, or in the case of bilge pumps, the diaphragm is cycled up and down by a drive belt. There is an intake side and a discharge side of the diaphragm. On the intake stroke the intake check, or joker valve, opens up and allows the chamber to fill with fluid. On the discharge stroke the intake check closes and the discharge check opens allowing the fluid in the chamber to be pushed out and into the discharge hose.
The main benefit of this type of pump is that it can be mounted remotely. Given the vacuum created on the intake stroke diaphragm pumps will self-prime. They also, with a low profile intake foot, can remove virtually all the water from the bilge.
Downside of the diaphragm type pump is that, unlike the constant action of the centrifugal, the back and forth motion contributes to significantly less volume being moved. Also, due to the more complex design with check or joker valves, they tend to not do well with bilge debris. If one chooses a diaphragm pump as the primary nuisance bilge pump it is certainly advisable to have a high water back up pump of the centrifugal type.
Both these pumps have their applications in your bilge so let’s look at what those are next.