Weird impeller problem - Sherwood pump

[Garth]

Hi All - any ideas or suggestions on this one would be appreciated. It's driving me nuts because it doesn't make any sense.

I replaced the impeller and the new one didn't work - no water. I replaced the old impeller, it worked. I tried the new impeller, it didn't work. I did this 6 times with the same result. The next day I bought a new impeller and had the same thing happen - went back and forth with the new impeller/old impeller several times with the same result.

I shared this story with an experienced diesel mechanic and he looked at me like I was an alien, an alien that was obviously doing something really stupid. But I don't think I am doing something stupid - I have followed the exact same steps with 3 different impellers multiple times. The new impellers won't work.

What the hell is going on???

Thanks,

Garth

[pablosgirl]

I would check the thickness of the new impaled verses the old.  I bet that the new ones are not as thick as the old andhence you are not getting a good seal between the impeller and the face plate of the pump.

[Stu Jackson]

Garth,  where did you get your new impellers?

[Garth]

Hi Stu - the first one I tried was on the boat from the PO's spares. The second (guaranteed new one) was from one of the local marine engine shops - it is a Sherwood impeller in the packaging. FYI - I have an '89 C34 with a 25XP. Is there anything special about the impellers with the 25XP that I need to know.

Thank  you,

Garth

[Stu Jackson]

I have an '89 C34 with a 25XP. Is there anything special about the impellers with the 25XP that I need to know.

No, the M25XP engine has little to do with the impellers in the Sherwood pump.  It's the pump, not the engine.

This is a new one on me, since I agree it's perplexing that the old ones would work and the new one wouldn't.  What I would try is to personally buy a new one yourself from another store or from Depco pumps.  Do a search on Depco here and you'll find their contact info.  Only other thing I can think of is that the impeller may be for a Sherwood pump, but not THE specific model pump that you have installed on your boat.

[Mike and Joanne Stimmler]

You may have already tried this but put the two impellers side by side and see if you can see any visual differences between the two.

[Ron Hill]

Garth : Look at the inside of the impellers.  Sounds like the problem is that the keyway on the shaft is not engaging in the slot on the impeller - so the shaft is spinning with the impeller is standing still !?! and/or the keyway is staying stuck on the old impeller!?!
A thought

[Garth]

Hi Ron - thank you for the suggestion (which makes sense) but the key is properly in place.

Hi Paul - I have checked them carefully and the part no. is the correct part no. But I will pull the old one out and do it again. I was also thinking of ordering one of the Globe impellers and trying it - I figure it can't hurt. Next step will be a diesel mechanic, but as I mentioned they don't believe me.

Thanks for your suggestions.

Cheers,

Garth

The key is properly in place.

[Stu Jackson]

Garth, I suggest you be careful with the Globe impellers.  If you do a simple search on "Globe" you'll find a few discussions about it.  If it was my boat, to follow Ron's concept, I'd wait for the engine to cool down (or do it first thing in the morning) and run the engine without the pump cover on and see what's going on.  All you'd need is like 15 seconds to see if the impeller is indeed spinning.  How old is your pump?  Many times the answer is the "cog" inside the pump.  There's a proper name for it, but I can't recall it right now.  If the pump guts are worn down, that may lead you to an answer but I still can't figure out how the old impeller still works, unless somehow it got "swollen.".  A true mystery. 

[Garth]

Thanks Stu. That will be my next check - good idea!

Cheers,

Garth

[SeaFever]

Garth,

In situations like this I have found that the root cause is simpler than we expect. However, yours may be a true unique case, we will find out...

I am with Mike on this one. If you have not already done so, keep the two impellers you tried (old one that works and new one that does not work) side by side on a flat surface and compare them carefully. Most important is the height of the impeller (width on its axis) when laid flat - circular section laid on a flat surface.

Somehow my feeling is that your new impeller is a fraction smaller than the older one. Given that the key is ok and the impeller is spinning, if the height (width of the impeller on its axis) is slightly smaller, it will allow the water to flow freely around in the impeller, rendering the impeller completely useless. That is only explanation my mind is able to come up with.

Let us know what you find out...!

[Garth]

Thanks all. Stay tuned and I will provide an update, but it will be a couple of weekends because we're heading skiing for a week this friday.

Cheers,

Garth

[Stu Jackson]

Here's the Globe issue:  http://forums.catalina.sailboatowners.com/showthread.php?t=113348

[Ron Hill]

Garth : FYI, The Sherwood G903 and the later G908 pumps both use the Globe #610 impeller. 

[Garth]

Weird impeller problem solved!!!!!

I had a mechanic by the boat on Friday and he was stumped initially as well but he figured it out.

The impeller must have been in the pump for a while and had worn very slight grooves in the cam. This prevented a tight seal and a prime with the blades of a new impeller. He removed the cam and smoothed the surface at his shop.

I didn't know what a cam was on Friday morning but I do now. It is the part of the pump that causes the impeller blades to bend before they hit the intake. And it is removeable which I also learned on Friday.

The good news is that the mystery is solved and there is lots of water pumping through.

Cheers,

Garth