General Outboard Motor Water Pump Description and Operation

[cpostis]

I'm posting this to get some feedback, also not so into watching the Dolphins getting slaughtered by the Patriots.

Per Seloc Marine Repair Manual:

"The water pump uses an impeller driven by the driveshaft, sealing between an offset housing and lower plate to create a flexing of the impeller blades. The rubber impeller inside the pump maintains an equal volume of water flow at most operating speeds.
At low speeds, the pump acts like a full displacement pump with the longer impeller blades following the contour of the pump housing. As pump speed increases, and because of resistance to the flow of water, the impeller vanes bend back away from the pump housing and the pump acts like a centrifugal pump. If the impeller blades are short, they remain in contact throughout the RPM range, supplying full pressure."

Now, if this is true, and knowing something about pump dynamics, this makes sense to me. Also, the operation of the pressure control (relief) valve as well.

Positive comments please.

Thanks all!

[fairdeal]

great theoretical issue!

now be prepared for Rodbolt to check in and tell us that its a stupid simple mechanism and you just need to replace the impeller regularly.....



"maintains an equal volume of water flow at most operating speeds" is an interesting statement.

Seems that is saying less about the pump and more about it discharging into a (largely) fixed restriction.

I do marvel at the very idea of driving the same pump (and expecting satisfactory results) at 650 rpm AND and 6000 rpm

[robert graham]

I understand that the pump acts as a positive displacement pump at lower RPM's and a centrifugal pump at higher RPM's....seems like an interesting item...

[boscoe99]

It is a stupidly simple speed volume system. The faster the speed the greater the volume.

I been studying water flow since I was in kindergarten.

[99yam40]

why do they insert the statement
"If the impeller blades are short, they remain in contact throughout the RPM range, supplying full pressure."
if it is acting as a centrifugal at higher speeds by the vanes bending back away from the housing?

[boscoe99]

What Yamaha has to say:

Water Pump
The water pump is a rubber bladed impeller mounted in a housing and driven by the drive shaft. The top and bottom of the impeller create a seal against the surface of the housing and plate. The impeller blades also seal against the housing side and end surfaces. It is essential that no water leaks between these surfaces for efficient operation. The impeller is eccentrically aligned or off center in the pump body, and the impeller blades fluctuate as the impeller rotates. As the blades pass over the intake port, water is drawn into the pump. The blades are compressed during rotation due to the off set as they get closer to the housing wall. The water is discharged under pressure through the discharge opening.

Pressure Control Valve (PCV)
Since a higher degree of heat is generated in midsize and larger engines, the water pump facilitates a higher cooling system capability. Even when the thermostat is closed, the water pump operates and water is being pumped resulting in an increase in cooling water pressure. Due to this increase in pressure, water could start leaking through weak spots, such as gaskets. Should the water pressure rise beyond a certain point, a PCV valve has been incorporated which opens and dissipates the cooling water through a bypass passage to control the cooling water pressure.

[pstephens46]

Too busy refinishing an Evan Williams bottle. Btw, Blanton's single barrel whiskey is to die for if you can find it.

What is different with a Mercury water pump? I notice the water pressure to be higher, like a fire hose.....

[cpostis]

great theoretical issue!

now be prepared for Rodbolt to check in and tell us that its a stupid simple mechanism and you just need to replace the impeller regularly.....



"maintains an equal volume of water flow at most operating speeds" is an interesting statement.

Seems that is saying less about the pump and more about it discharging into a (largely) fixed restriction.

I do marvel at the very idea of driving the same pump (and expecting satisfactory results) at 650 rpm AND and 6000 rpm

Here is a pump curve diagram for somewhat like a pump we're talking about. however, nowhere near the rpm range. I would love to see an exact pump curve diagram for an outboard water pump.


Notice the 2 gpm vertical line. You can have the same flow at different rpm's. Depends on the pressure (Ft. Head). Now as rpm increase, the impeller blades bend more, thus the blades are less effective.

Now, I'll through in some theoretical questions..

What do you think happens to flow, pressure, resistance; all considering thermostat and pressure relief valve operation, bending of the blades, switch between positive displacement and centrifugal operation of the pump during:

1. Idle operation, thermostat closed
2. idle operation, thermostats open
3. 1,400 RPM, thermostats closed
4. 1,400 RPM, thermostats open
5. 4,000 RPM, thermostats closed
6. 4,000 RPM, thermostats open
7. WOT, thermostats closed
8. WOT, thermostats open
9. When do you think the PRV opens and closes in these cases above

Thanks Boscoe, didn't see your post while writing this post. I'll review.

[boscoe99]

Too busy refinishing an Evan Williams bottle. Btw, Blanton's single barrel whiskey is to die for if you can find it.

What is different with a Mercury water pump? I notice the water pressure to be higher, like a fire hose.....

Different brands, different designs.

In my Mercury EFI the pressure control valve was a completely different design. It opened to let more water flow through the block when at higher RPM's. The engine would run much cooler at speed than it would at idle. The Mercury was a hot pisser. Only pissed when the thermostat opened.

Now some two stroke Yams will piss like race horses from the get go. Some Yams that send water through a fuel cooler and a rectifier/regulator cooler before said water makes its way to the tell tale port may dribble like an old man. Yams piss from the get go. The tell tale water does not flow through the thermostat.

[cpostis]

What Yamaha has to say:

Water Pump
The water pump is a rubber bladed impeller mounted in a housing and driven by the drive shaft. The top and bottom of the impeller create a seal against the surface of the housing and plate. The impeller blades also seal against the housing side and end surfaces. It is essential that no water leaks between these surfaces for efficient operation. The impeller is eccentrically aligned or off center in the pump body, and the impeller blades fluctuate as the impeller rotates. As the blades pass over the intake port, water is drawn into the pump. The blades are compressed during rotation due to the off set as they get closer to the housing wall. The water is discharged under pressure through the discharge opening.

Pressure Control Valve (PCV)
Since a higher degree of heat is generated in midsize and larger engines, the water pump facilitates a higher cooling system capability. Even when the thermostat is closed, the water pump operates and water is being pumped resulting in an increase in cooling water pressure. Due to this increase in pressure, water could start leaking through weak spots, such as gaskets. Should the water pressure rise beyond a certain point, a PCV valve has been incorporated which opens and dissipates the cooling water through a bypass passage to control the cooling water pressure.

So, what Yamaha says is that their pump never acts as a centrifugal pump (there is never a gap between the impeller and the housing (insert). It is a positive displacement pump (sorry Townsends). So my possible theories goes in the trash pile?

By the way, The Seloc manual I refer to in my OP is for Yamaha. Thanks Seloc, for nothing!

[boscoe99]

By the way, The Seloc manual I refer to in my OP is for Yamaha. Thanks Seloc, for nothing!

Yamaha can't keep up with all the different variables amongst all of their different models. I suspect that Seloc is using very generalized information trying to cover all brands and all models of a brand with one book.

Don't centrifugal type pumps normally have their outlets oriented radially to the impeller vanes as opposed to the way that the outlet in the Yamaha pump is oriented?

What Seloc is good at (better than Yamaha) is providing some words as to how a procedure is to be performed. Yamaha provides damn near nothing.

[cpostis]

Yamaha can't keep up with all the different variables amongst all of their different models. I suspect that Seloc is using very generalized information trying to cover all brands and all models of a brand with one book.

Don't centrifugal type pumps normally have their outlets oriented radially to the impeller vanes as opposed to the way that the outlet in the Yamaha pump is oriented?

What Seloc is good at (better than Yamaha) is providing some words as to how a procedure is to be performed. Yamaha provides damn near nothing.

Well, if you look at a Yamaha housing and insert, the outlet is oriented radially. Then it is directed upward in the housing guts.



Agreed, I have both manuals, Seloc procedures are excellent!

Still would love to see a Yamaha pump curve chart!!! I guess only Yam engineers will have that?

[boscoe99]

Well, if you look at a Yamaha housing and insert, the outlet is oriented radially. Then it is directed upward in the housing guts.



Agreed, I have both manuals, Seloc procedures are excellent!

Still would love to see a Yamaha pump curve chart!!! I guess only Yam engineers will have that?

Yep, and the Yam engineers all be in Japan. I doubt they can speak much English.

Problem with publishing curves (for all models for all times) is that some yahoo some where will do a test and find that his model does not meet the curve. Is flow rate is a tenth of a gallon less and his pressure is 1.75 psi below specification. Motor runs and cools just fine. He wants a new replacement motor because his is not to specification.

[boscoe99]

http://www.tuthillpump.com/dam/2525.pdf

I would consider the Yamaha water pump to be a positive displacement pump and not a centrifugal pump. I guess others will see it differently.

[fairdeal]

what caught my eye about the highly-stylized Seloc pump illustration

are the skinny vanes and enormous open spaces between them.

If there was that much room to install the impeller in my F225,

my hands wouldn't be crippled with arthritis for weeks whenever I do a pump service.



In the SM for the F225 Merc-aha is this illustration of pressure expected in the telltale tubing
just after it leaves the block (therefore downstream of the action of the pressure control valve)
but before it goes to the fuel cooler / rectifier:

[url=https://postimg.org/image/dycuerka1/]