Why we have impellar pumps.

E

earthmover17

New Member
Jan 3, 2008
706
land o lakes florida
Boat Info
560 sedan bridge
Engines
cat 3196
I have often wondered why our boats have Impellar pumps. It seems to me that as the pump is below the water line you could use a different pump like a centrifical. Thus taking out the flimsy rubber impellar that likes to occasionally commit suicide. Just a thought.
What do you think Frank W.
Jack
 
1. All pumps have impellers.
2. These are positive displacement pumps, a centrifugal pump wouldn't pull the water up and force it through the system near as well.
3. Your circulating pump is a centrifugal pump.
 
The impeller is only a part of the design issue..........the drive mechanism for sea water flow must be almost bulletproof . Some gas pumps are belt driven, but for more expensive diesels the pumps are usually gear driven off an accessory drive in the engines timing cover. That constraint tens to limit the use of some other types of pumps for sea water.

I really don't have much of an opinion on the rubber impeller type pump other than to offer that they are cheap and easy to repair with common hand tools and the parts are small and inexpensive enough to carry along several spares. I guess, for me, this one is an "if it ain't broke; don't fix it".
 
Good question. Maybe it has something to do with the fact that the pump will self destruct or flex if you suck up some nasty sand or mud, since it is the first thing incoming water goes through. but it seems like a steel impeller like the circ. pump on the front of the engine would maybe work better. But then you would need some sort of shear pin. I dunno :smt101
 
I really had thought that rubber impellar pumps were suction pumps as they had to pull the water up to themselves to send it past the discharge. If the water is already there then I wouldn't think you would need the rubber impellar. When my seacock is left open and I pull the hose off the pump I have all kinds of seawater. I would think a bearing pump with a bronze impellar would be a better application. Those type pumps don't tend to have catastrophic failure as a rubber one does.
As for how it's driven gearcase or belt once again it's just a way to drive it so that shouldn't matter.
I obviously have too much time on my hands if I'm wondering about all of this.
Jack
 
I'm going to guess three reasons they used a positive displacement pump.
  • Self priming
  • Reliable and predictable volume at all speeds
  • Reliable pressure at all speeds required for wet exhaust systems that may have some back pressure.

Best regards,
Frank C
 
Maybe mine is different because it's in an outdrive, but it's not positive displacement. It's just a rubber wheel with fingers on it, works the same as any other centrifugal pump.... i thought.
 
berth control said:
Maybe mine is different because it's in an outdrive, but it's not positive displacement. It's just a rubber wheel with fingers on it, works the same as any other centrifugal pump.... i thought.
Click to expand...

If you have a "rubber wheel with fingers" then you have a positive displacement pump. The rubber impellers are basically squeezed on one side of the cam and expanded on the other side of the cam. The action of expanding pulls water in and the action of squeezing pushes the water out. That's positive displacement. It's not sensitive to head pressures so if you have to push X-amount of fluid through a system with significant potential head pressure, you have to use a displacement pump.

Because of all the inline heat exchangers and the significant sea water flow needed to remove the heat coupled with the potential of "slightly clogged" strainers or "slightly clogged" heat exchangers, a centrifugal pump is not practical.

A centrifugal pump acts, like an internal water pump in an engine or the AC pump, by adding energy to the water... basically the water enters the center of a plastic/metal impeller and it is slung to the outer portion of the pump by centrifugal force (or is it centripetal?) which increases the pressure and moves the water... they don't have any rubbing parts lie a positive displacement pump but they are very susceptible to blockages and head pressure changes. In a closed system like the internal coolant on an engine, they can be designed to handle the correct water flow. On a system than is more susceptible to crap in the line or clogging or needing high water flow with high head pressure, you need the displacement pump.
 
That appears to explain it pretty well. I am always looking to reengineer something that works but it looks like I'll have to stick to the suicidal rubber impellars.
Jack
 
Four Suns said:
If you have a "rubber wheel with fingers" then you have a positive displacement pump. The rubber impellers are basically squeezed on one side of the cam and expanded on the other side of the cam. The action of expanding pulls water in and the action of squeezing pushes the water out. That's positive displacement. It's not sensitive to head pressures so if you have to push X-amount of fluid through a system with significant potential head pressure, you have to use a displacement pump.

Because of all the inline heat exchangers and the significant sea water flow needed to remove the heat coupled with the potential of "slightly clogged" strainers or "slightly clogged" heat exchangers, a centrifugal pump is not practical.

A centrifugal pump acts, like an internal water pump in an engine or the AC pump, by adding energy to the water... basically the water enters the center of a plastic/metal impeller and it is slung to the outer portion of the pump by centrifugal force (or is it centripetal?) which increases the pressure and moves the water... they don't have any rubbing parts lie a positive displacement pump but they are very susceptible to blockages and head pressure changes. In a closed system like the internal coolant on an engine, they can be designed to handle the correct water flow. On a system than is more susceptible to crap in the line or clogging or needing high water flow with high head pressure, you need the displacement pump.
Click to expand...

Thanks for clearing this up. I've always wondered why this is- now it makes complete sense.

When you read "Keel Cooled" (or similar) what does that mean? You see that sometimes on Nordhavn's. I've often wondered if they have some kind of a different set up on their engine cooling designs.
 
Chris R said:
Ok, I found the answer to my own question (above) in the link below about 3-4 paragraphs in. Very interesting. It must be nice to NOT have to worry about impellars.

http://www.nordhavn.com/46/overview_engine.php4
Click to expand...

Chris,
Thanks for posting that link. One of my very next questions was going to be why do we even use salt water inside the boat. I see many of the larger yachts down here that have exhaust pipes just like a semi truck. I just couldn't figure out how they did it. Now I know. I'm Off to engineer my own Keel cooler.
 
Interesting read, especially the Nordhaven link. But, Jack, I am not too sure on adding smoke stacks to the clean lines on the 560! :wow:
 
earthmover17 said:
Chris,
Thanks for posting that link. One of my very next questions was going to be why do we even use salt water inside the boat. I see many of the larger yachts down here that have exhaust pipes just like a semi truck. I just couldn't figure out how they did it. Now I know. I'm Off to engineer my own Keel cooler.
Click to expand...

Let me know when you figure it out! :)
 
You must log in or register to reply here.

Forum statistics

Threads
113,248
Messages
1,429,275
Members
61,128
Latest member
greenworld
Back
Top