AMIC– operation of most marine water pumps using a rubber vaned impeller is, the cavity between the vanes is what makes it pump water. As the off-center impeller rotates, the cavity enlarges, drawing in a gulp of water. As it continues to rotate, the cavity becomes smaller, and squeezes the water out and up the water tube to the powerhead.
These motors are equipped with a plastic/Nylon bodied water pump housing. There is also a stainless bottom plate, this plate has to be positioned with the suction hole in the proper location/relationship as indicated in the photos. If you are just replacing the impeller, you can tell which way is up by observing the wear of the impeller on this plate. The impeller fits in this stainless cup liner which gives a very long lasting situation. In most cases you will never need to replace the water pump assembly, just the impeller. Sometimes the rubber impeller becomes un-bonded to the metal hub making for an inoperative pump.
Shown below on the left is the water pump with the impeller on the driveshaft. During functioning the water is drawn into the housing in a slot in the lower plate of the water pump, (not shown here) but this slot is in the lower plate at the the center of the RH photo. The vanes are not as compressed allowing the water to be sucked into the cavity. As the impeller turns with the driveshaft, the vanes compress as the cavity is not centered , but offset or eccentric as shown in the LH photo. This offset forces the water up into the rotating impeller and out the gap seen on the top of the cavity and up into the copper tube which is located in the 1/2″ round hole in the top of the pump housing. Here the tube is secured and sealed by a rubber grommet in the top of the water pump.
When the motor is running, the water pump is submerged, the water in the water pump housing is pushed by the rubber vaned centrifugal impeller upward and into the 3/8″ copper supply tube up into the powerhead.
This is a centrifugal pump, this means that the impeller is a rubber vaned insert setting inside of a stainless steel housing. The impeller is offset to one side so that when the water comes into the pump housing at the widest part around the impeller and as the impeller turns the water is compressed and then forced out on the other side. This is illustrated by the LH photo below.
The outer edges of the vanes are not the only significant part of the water pump impeller as the circular rubber rings on both sides also act a seal, keeping the water trapped between the vanes.