Raw Water Impellers

[mjk1040]

So per Viper's advice I waited until launch and tackled the raw water impellers. Port engine was running at 140 degrees, but I was getting steam at the 140 temp. Pulled out what appeared to be a Sherwood 10615 Impeller, and sure enough a few of the fins were broken, but all there. Put all back in and ran the engine up to the 140 temp again, this time no steam appeared. Hopefully problem solved. Even though I had no issue with the starboard side engine, figured it only took a hour to change out one impeller, why not do them both, then I know when they were both changed last. Well the starboard impeller was in tack, not a broken fin, but was showing signs of aged fatigue, cracks in the fins but in darn good shape. Now this impellers fins appeared to be twice the thickness as those of the Sherwood 10615. Since one of the previous owners had the spares already, I put the Sherwood 10615's in. Now I will try to find out what make the thicker finned impeller is. The other thing I noticed while looking for my next set of impellers, that several different number of fins per impeller. These were 12 fin impellers. Is more fins more water flow? Should it have 10's or 8's? What other brands of impellers are there besides Sierra, Sherwood, etc? Is paying more for a thicker fin impellers worth the extra expense or should one just plan on changing them out every X years and use the less expensive thinner fin impellers?

[g36]

never had a problem using the sherwood 10615k ,mallory .sierra or others replacements but i routinely change them every 2 yrs . piece of mind to me. we boat year round so they arent sitting in one place for months at a time as it might if only boating part of the year.

[waybomb]

Here's a thought - if there are less impellers, there will be more volume available for the water. Per revolution, less area taken up by the blades means more water moved.

Even though these pumps have flexible fins, they are essentially a positive displacement pump.

[mjk1040]

g36-so what shape would you say the impellers are in after 2 years and how many hours are on them. Just curious. Got another new set on order to have on hand.

Waybomb-my thoughts exactly! However the Sherwood pump when new comes with 12 fin impeller. Now I have open cooling, so it may be possible more water is not a better idea. I have never seen the dash temp gauge go over 140 degrees, even though that may not be the exact temp of the engines. However as I stated above temp was not reading high or above 140, yet I would start to get steam coming out of the exhaust when the bad impeller was still in. After replacement at 140 I did not see any steam after running at idle as before.

[km1125]

Qr Bbpost Here's a thought - if there are less impellers, there will be more volume available for the water. Per revolution, less area taken up by the blades means more water moved.

Not necessarily. It's the displacement of the fins that moves the water. If you have 12 "displacements" per revolution you'd get more water pumped then if you only had 10 "displacements"... the rest of the water is just there for the ride.

Nice video of a pump running: https://www.youtube.com/watch?v=ECv1VwW6RTo

[waybomb]

Beg to differ.
If the cavity is 5 cubic inches, and the impeller takes up 1 cubic inch, each revolution will move 4 cubic inches of water.
Same cavity, impeller takes up 4 cubic inches, there is only enough room for 1 cubic inch of water to be moved.

[km1125]

Qr Bbpost Beg to differ.
If the cavity is 5 cubic inches, and the impeller takes up 1 cubic inch, each revolution will move 4 cubic inches of water.
Same cavity, impeller takes up 4 cubic inches, there is only enough room for 1 cubic inch of water to be moved.

Don't think so!

By that logic, I could cut all the fins off my current impellers and improve my flow 500%!!

[waybomb]

Do you agree that the most water that could be moved by one rotation is the volume of water in between the vanes?

[Midnightsun]

Speaking of impellers, I bought the wrong ones for my Volvos and have a new set of Sherwood 17,000K impellers in the box new. This is the listing I used to buy them. http://www.ebay.com/itm/2-Each-Sea-Wate ... fX&vxp=mtr Any reasonable offer takes them.

[Viper]

More blades will decrease the volume in the pump cavity, there's no getting away from that. Given everything else remains the same, ie; rpm, hub diameter, cam size, and blade thickness, more blades occupying space will not allow the same amount of fluid into the cavity in one revolution since the mass of the extra blades is occupying more space. The pump flows however are well over rated for the cooling systems they are designed for so the loss of this amount of volume wouldn't make much of a difference under normal conditions.

For our applications, the number of blades is more about head pressures than volume. The more blades, the higher the head pressures the pump can deliver. The thinner the blades, the more they flex, this is desirable in some conditions as they are more forgiving but the pressures are reduced. The thicker blades offer higher pressures but are less flexible which is a drawback in some cases such as liquids that carry more debris.

To ensure you're pumping to pump design spec, find out what OEM impeller the pump manufacturer originally called for, then you can cross-reference to the same number of blades, thickness, etc., then stick to that if it works for you.