Outboard water pump type?

[robert graham]

Is a rubber impeller type outboard water pump considered a centrifugal type or positive displacement type pump?......to settle an ongoing controversy.....

 

[garbageguy]

I would say centrifugal

A centrifugal pump produces a head and a flow by increasing the velocity of the liquid through the machine with the help of the rotating vane impeller. Centrifugal pumps include radial, axial and mixed flow units.

A positive displacement pump operates by alternating filling a cavity and then displacing a given volume of liquid. A positive displacement pump delivers a constant volume of liquid for each cycle independent of discharge head

 

[GA_Boater]

The water pump in your car or truck is centrifugal.

The impeller is displacement.

The difference is the fixed vanes for centrifugal and flexible vanes for displacement.

 

[garbageguy]

I stand corrected. Didn't think of it that way.

Learn something new every day - one of the reasons this forum is so good

 

[robert graham]

That's sorta what I thought....since the impeller operates off-center from it's stainless steel cup then the "volume" between the rubber vanes actually changes as it rotates?.....wonder if this type of pump is unique/mostly used for outboards only?....

 

[GA_Boater]

No, marine cooling is just one of it's uses. In fact here is another use;



Mmmm - wort, liquid sugar or yeast slurry - I like this pump. It's used in a brewery! :very_drunk:

 

[GA_Boater]

This has more uses for the same pump;

 

[robert graham]

Thanks guys!...this makes a lot on sense for outboards that may be operated in sandy, turbid water conditions....live and learn!

 

[Frank Acampora]

OK! Let's clear it up. The impeller is offset in the pump casing. At low RPM it acts like a displacement pump due to the different volumes between the impeller blades. Water lubricates the vanes against the pump wall. At higher RPM, the water pressure forces the vanes backward away from the pump wall and the pump becomes centrifugal. That is why an impeller will last relatively long if not abused by running it dry. Unless you really run it hard into a sand bar, at high RPM sand will pass through without doing much harm. At low RPM there will be some wear associated with sand ingestion.

 

[hardwater fisherman]

Agree^^^ My service manual confirms that by saying thus at low speed the impeller works as a displacement pump. At higher speeds, water resistance keeps the blades from contact with the housing, and the pump acts as a centrifugal pump.

 

[robert graham]

OK! Let's clear it up. The impeller is offset in the pump casing. At low RPM it acts like a displacement pump due to the different volumes between the impeller blades. Water lubricates the vanes against the pump wall. At higher RPM, the water pressure forces the vanes backward away from the pump wall and the pump becomes centrifugal. That is why an impeller will last relatively long if not abused by running it dry. Unless you really run it hard into a sand bar, at high RPM sand will pass through without doing much harm. At low RPM there will be some wear associated with sand ingestion.

...
Frank, that is really interesting!...no wonder the rubber vanes on my 5+ year old impeller show almost no wear and the stainless cup was almost like new.....I run mostly in clear, fresh river water(almost no mud or sand)......two types of pumps in one unit!....pretty darn slick!...

 

[Fed]

So the faster you drive the longer the water pump lasts, my Missus won't believe that.

 

[metriccrescentwrench]

So the faster you drive the longer the water pump lasts, my Missus won't believe that.

Yep, absolutely correct. Yet another reason not to be shy with the throttle.!!! :lol:

 

[Frank Acampora]

No, not really. Assuming it is not abused or run dry: The bulk of water pump impeller wear comes at start-up and idle and also due to deteriorating of the rubber. I don't know the exact RPM, but, for example, at cruise RPM the wear would be negligible Any RPM that forces the vanes away from the pump wall will reduce wear and going faster will not improve life over the minimum RPM needed to move the vanes away from the wall. Good argument to use on the admiral though! Gee Honey, I'm only trying to prolong water pump life! LOL

 

[metriccrescentwrench]

Don't confuse our fun with facts. I'll take any excuse to go fast that I can find.

 

[garbageguy]

OK! Let's clear it up. The impeller is offset in the pump casing. At low RPM it acts like a displacement pump due to the different volumes between the impeller blades. Water lubricates the vanes against the pump wall. At higher RPM, the water pressure forces the vanes backward away from the pump wall and the pump becomes centrifugal. That is why an impeller will last relatively long if not abused by running it dry. Unless you really run it hard into a sand bar, at high RPM sand will pass through without doing much harm. At low RPM there will be some wear associated with sand ingestion.

Well then, I was half(?) right. Thanks for the info Frank. And yes, 2 pumps types in one. Yet another example of the design and engineering that goes into complex machines that almost anyone can operate.