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Impeller/water pump off center?

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Gareth B

New member
[COLOR=rgba(0, 0, 0, 0.701961)]I am replacing the water pump on a 85 Johnson 15 hp outboard. On the replacement part, The hole in the impeller housing cup for the drive shaft appears to be off center thus making the impeller off center in the housing cup. Is it supposed to be off centered a bit or should everything be centered in the cup as I would expect? Thanks for the help. -Gareth[/COLOR]
 
Gareth, I believe you have a centrifugal pump in mind as you are trying to reason how your pump works. A centrifugal pump impeller would be centered and works by throwing water outward as it rotates.
However, your Johnson has a displacement pump. With the impeller in place, slowly rotate the shaft and watch the cavities between the individual impeller blades. Due to the off-center condition, the cavities are larger on one side than when rotated to the other side. It is due to that change that the cavity draws in water as they get larger and expel (displace) water as they get smaller. Displacement pumps excel over centrifugal pumps at slow speeds.

Before, somebody calls me on this, yes the Johnson pump also acts as a centrifugal pump at high speeds, where centrifugal pumps excel.
 
Thanks for the information fdrgator. I had to ask because the impeller was centered in the old housing and in the replacement I recieved its off centered. So just wanted to get that clarification. Again thanks for your help. -Gareth
 
Gareth B said:
Thanks for the information fdrgator. I had to ask because the impeller was centered in the old housing and in the replacement I recieved its off centered. So just wanted to get that clarification. Again thanks for your help. -Gareth
Click to expand...

:confused: :confused: Some photos called for to explain this
 
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