Sea Water Pump -- I don't understand...

[generator12]

After a couple of months roaming these forums to acquaint myself with my new (to me) 170-Alpha, I'm impressed with both the depth of knowledge and the breadth of lore contained within. Most of what I see, I buy into. But some of it I don't understand...!

For example:

"A couple of seconds run-time without water will totally trash your impeller..."

I don't get this. I've dealt with centrifugal pumps in the industrial environment for a couple of decades and understand the threat to seals with dry running, and the build-up of heat as well.

But damage to the impeller??? What is it about the marine engines that makes the impeller itself vulnerable to damage if run without water? I mean for a few seconds...!

Can somebody explain this to me?

Thanks all.

 

[boat127]

Re: Sea Water Pump -- I don't understand...

it will burn out quickly because the water is the lubicant for impeller its a tight fit rubber on metal and lots of friction. you do that a few times then while out in the middle of no where you have an over heat because of a failed impeller

 

[paultjohnson]

Re: Sea Water Pump -- I don't understand...

it will burn out quickly because the water is the lubicant for impeller its a tight fit rubber on metal and lots of friction. you do that a few times then while out in the middle of no where you have an over heat because of a failed impeller

Good question and good answer .. If you ever take one apart or change one u will understand

 

[thumpar]

Re: Sea Water Pump -- I don't understand...

Good question and good answer .. If you ever take one apart or change one u will understand

Yep. Once you see it apart you will know.

 

[JustJason]

Re: Sea Water Pump -- I don't understand...

I'm a mechanic and get this question asked often. What I tell people is this.

Take a brand new tire and put it on the back of a muscle car. Now spin it until your smoking up a parking lot for the excact amount of time that your running an impellar dry.

30 second burn out, 2 minute burn out....
The tire and impellar will both come back looking in sort of similar condition. Except that impellars are more fragile than tires.

Running an impellar without water is like doing a tire burnout, but tires usually handle it better.

 

[Don S]

Re: Sea Water Pump -- I don't understand...

"A couple of seconds run-time without water will totally trash your impeller..."

I have seen this written often, however, it is just not ture. A couple of seconds at starting rpm (usually 1500rpm or less) will just begin to heat the impeller.
Here is a service bulletin that shows the damage to the rubber impellers when running dry.

http://www.boatfix.com/merc/Bullet/87/87_14.pdf

There was a better picture of those impellers, actual photo's instead of drawings, but I can't find them right now. But you will get the idea.

 

[Aloysius]

Re: Sea Water Pump -- I don't understand...

A raw water pump is NOT a centrifugal pump, for one thing. It's a rubber impeller positive displacement type that creates both suction and pressure.

Don is correct about short term damage...won't happen. I just did it yesterday....................

 

[generator12]

Re: Sea Water Pump -- I don't understand...

It makes sense to me now.

Thanks everyone.

 

[Lyle29464]

Re: Sea Water Pump -- I don't understand...

Next time you see a stainless table put you finger on it, press down a bit and slide you finger about 2 feet fast. ( you will never start one dry again)