text: Some impellers are similar to small propellers but without the large blades. Among other uses, they are used in water jets to power high speed boats.
  Because impellers do not have large blades to turn, they can spin at much higher speeds than propellers. The water forced through the impeller is channeled by the housing, creating a water jet that propels the vessel forward. The housing is normally tapered into a nozzle to increase the speed of the water, which also creates a Venturi effect in which low pressure behind the impeller pulls more water towards the blades, tending to increase the speed.
  To work efficiently, there must be a close fit between the impeller and the housing. The housing is normally fitted with a replaceable wear ring which tends to wear as sand or other particles are thrown against the housing side by the impeller.
  Vessels using impellers are normally steered by changing the direction of the water jet.
  Compare to propeller and jet aircraft engines.