In the picture on the left, we see the creases that begin to form on the blades over time. The next two pictures are of impellers with blades that have started to tear off. Considering the implications of a failure, an impeller should not be allowed to get to this point. Finally, the picture below is of an impeller after failure. You can see the remains of a couple of blades, but the rest have completely torn off and are now trapped in the raw water system.
Here we have different impellers from different model engines. Above on the left we see a new impeller. The middle impeller is a lightly worn unit with very little time on it. The impeller on the right is a worn impeller with a permanent "set" to the blades.
The impeller is the component of the raw water pump that actually moves the water through the pump. The impeller functions by rotating inside the pump housing. The vanes of the rotating impeller ride up and over a wedge like device called the cam. Each side of the cam has openings that allow water to pass through. The openings in the cam are lined up, on one side, with the raw water inlet, and on the other side, with the raw water discharge. Each time the vanes of the impeller ride over the cam, they are compressed between the body of the impeller and the cam. The water that was trapped between the vanes is now pushed through the openings in the cam and out of the discharge side of the pump. When the vanes continue to the other side of the cam, they stand back up, creating a low pressure zone, and subsequently draw water in through the pump inlet. The water stays trapped between the vanes until they rotate around to the other side of the housing and are compressed against the cam again. This process occurs over and over again creating raw water flow through the engine.
The impeller is made of rubber and it will eventually wear out. Over time impellers can lose their elasticity and the blades will not stand back up properly. Raw water flow will be diminished. After more time, the bases of the vanes that are continually being bent over will form creases. The vanes will begin to tear along the creases affecting flow even more. Finally, the vanes will tear off completely and the pump will stop pumping water. The impeller continues to spin inside the housing, but with no water flowing through it, the friction between the remaining portion of impeller and the housing creates heat very quickly. The heat rapidly destroys any remaining impeller vanes ripping them apart and pushing them out the discharge, where they are then trapped in the raw water system. At the same time, without water flowing through the pump, the raw water pump seal will be damaged as well. It is not uncommon for a pump, that has had an impeller fail, to also have a water seal fail shortly after.
The impeller maintenance is performed based on a time interval, not hours. Manufacturers recommend an annual impeller replacement. An annual replacement should all but guarantee against a failure. Also, during the impeller replacement, the raw water pump should be inspected for any leaks or worn parts.
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Above on the left is a view of the raw water pump from the back side. The crescent shaped piece, that the impeller blades are compressing against, is the cam. The picture on the right is of the cam after removal from the pump. The fingers you see are what the blades ride up on and the spaces between are where the water flows through.