Impeller failures

[Tony Walker]

Is this normal?

I have about 800 hours on my Volvo engines and yesterday I suffered my 12th impeller failure.  (approximate number but it IS close, one or two either way)  We were motoring away from home dock and bingo off goes the hight temp alarm.  This time it was the port engine.  It seems to be random as to which engine it is.

Since we were going out for a pleasant night on the hook, we changed our choice and picked the nearest.  We spent a nice 24 hours on the boat.  If I had a single engine boat, this would have been 12 times that would have been a much bigger deal.  Maybe only 6 failures since I would have had only one engine.  That would have been reason to change the failed unit on the spot.  But depending on where the spot is, it could still be a BIG problem.

But, are there different grades of impellers?  Is the failure rate that I have experienced anything like one would expect????

Thanx for any input.

Tony
Salt Shaker 342

[dazeoff]

Hello again.... We are down from the Canadian snow visiting Florida again ...I just received a new impellor from Quicksilver.. merc. parts  . It  is way better quality than the the one i am replacing ..I do it every 2 years .. no quesion ....I cannot talk for Volvo but for Merc. i always pay the extra , even tho i dont believe the parts are always better ..  however the Impellor was .....At the rate of failure you are experiencing I would question the housing , or the unit itself .....Good luck , hope u find a good resolve .... ..

[pepmyster]

Has to be the housing like Dazeoff said or, it is starving for water which will burn the impellar.

[Dream 'Inn]

Tony, it seem you are averaging 1 per year per engine.  But, you are also averaging 125 hrs/yr (I'm sure one year was a lot more than the rest though ).  I've heard 100 hrs or 2 years.  But, as others have said, it could be something in the housing that is wearing them out as well.  have you looked in the pump?  Are there any rough surfaces inside the pump that could cause the wear?  I know I replace both of my entire pumps about 2 years ago, partially cause they were leaking, also because they had excessive wear inside.

[frodo13056]

I know your home port is in Florida - any chance that you routinely find yourself in water with a lot of sand suspended in the water? If you routinely get gritty water in your water pumps, not only will your impellers fail but your housing should be pretty scuffed up. Personally, I change my impellers every 3rd season or 100 hours - which ever comes first and I don't boat in gritty water. If you were using my philosophy on when to change impellers, you'd have gone through 16 by now - so maybe you aren't doing so bad with  changing out 12.....

[Tony Walker]

Thank you all for your input.

All of the suggested as causes have been factors in some of the failures but most of them have been complete mysteries to me.

I never had a failure on plane, they were always at slow speed.  Usually from a cold start up.

I have replaced water pumps because of wear.

The three I can identify probable causes were on the loop trip.

First was going into Trenton, Ontario where we were traveling thru weeds, dense weeds like I have not seen prior.  When we got the boat to the dock, we dropped a pile of them at the dock.  My solution for this one became back the boat up periodically.

Second one happened when we went thru the lock into the Chicago River.  It was loaded with plastic trash bags and other garbage.  We may have picked one of these things up.  Again, I should have recognized the possibility of a blockage and backed the boat up once we were clear of that lock.

The third one was while traveling down the Mississippi.  This river is unbelievably full of suspended mud.  The water looked like chocolate milk and was gritty to the feel.  This was the one body of water I questioned whether the boat had any business even being  there.  This was the only event that took enough time to develop that I could monitor the temperature rising on the gage.  I shut the engine down before the alarm went off.  Another minute and it would have.

I motored on for about 3 hours more on the good engine.  Naturally the one that failed was the starboard engine so I had to armstrong the boat around the tow boats plus the logs and planks that were everywhere.  We wound up in the little diversion canal which was a quality anchorage.  We were able to raft off of our new best friends 55 footer.  Although he had practically an entire machine shop aboard, I had all the tools required for this problem aboard tthe Salt Shaker.

It was difficult to diagnose because it was not a blade failure but the hub had separated from the blade assembly.  You could turn the impeller shaft by hand and the impeller would follow.  But with it buttoned up, the starter torque would cause the hub to spin and the impeller proper would not follow.  More good training.

I think I learned another lesson in hind sight as a result of this failure.  I allowed the failed engine to free spin and I think 3 hours of this caused excessive heat on the out drive oil and excessive wear on the gears.  Now with an impeller failure, I do not allow the prop to free wheel more than a few minutes.  I put that transmission in reverse to prevent the prop from spinning.  I could use some advice on this topic.  I am only using my experience to dictate this procedure.

For all the other failures, I have no clue.  Any additional ideas would be welcome.  I will post more as I learn more.

Tony
Salt Shaker 342