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post #11 of 15 Old 12-14-2016
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Re: Outboard question

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The impeller on an outboard is not a flexible vane impeller. It is a solid piece of metal and can be run dry just like a submersible pump impeller.
In fact I own a 1926 Johnson with a metal piston coolant pump, although every single newer outboard I have ever owned used the flexible vane pump design. I have had rubber impellers fail either by breaking vanes off or by losing the rubber's bond to the hub. An impeller is too cheap to leave an unknown one in place.


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post #12 of 15 Old 12-14-2016
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Re: Outboard question

Every OB I've ever worked on had a centrifugal water pump. Perhaps they have changed to plastic impellers recently, but as it is mounted below the water, a flexible impeller is not needed.
My 15 hp Johnson has been running daily for 9 years without a flush, impeller change or a water circulation problem, which I sincerely doubt would be possible with a flexible impeller.
I hope you all are wrong for that would mean that a perfectly flawless, reliable, maintenance free water circulation system has been supplanted by one that is none of those.

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post #13 of 15 Old 12-14-2016
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Re: Outboard question

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Originally Posted by capta View Post
Every OB I've ever worked on had a centrifugal water pump. Perhaps they have changed to plastic impellers recently, but as it is mounted below the water, a flexible impeller is not needed.
My 15 hp Johnson has been running daily for 9 years without a flush, impeller change or a water circulation problem, which I sincerely doubt would be possible with a flexible impeller.
I hope you all are wrong for that would mean that a perfectly flawless, reliable, maintenance free water circulation system has been supplanted by one that is none of those.
If you mean by "recently" the last 30 years you may be right, I've never worked on one older than that.

All I get out of the above quote is that you've lucky and you've never actually seen your own impeller so don't know if it's flexible vane type.
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Last edited by boatpoker; 12-14-2016 at 06:46 PM.
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post #14 of 15 Old 12-15-2016
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Re: Outboard question

Quote:
Originally Posted by capta View Post
Every OB I've ever worked on had a centrifugal water pump. Perhaps they have changed to plastic impellers recently, but as it is mounted below the water, a flexible impeller is not needed.
My 15 hp Johnson has been running daily for 9 years without a flush, impeller change or a water circulation problem, which I sincerely doubt would be possible with a flexible impeller.
I hope you all are wrong for that would mean that a perfectly flawless, reliable, maintenance free water circulation system has been supplanted by one that is none of those.
If you have 15 hp Johnson with a centrifugal water pump then you may have the only one ever made. Johnson only ever made two different water pumps and they are both rubber vane. the older ones in the 50's use the long vane impeller and when they get hard they look a little like a centrifugal impeller and the '74 and up models all use the same rubber vane impeller. The reason they use rubber vane impellers in outboards is because they are mounted on the drive shaft and rpm at idle would be to slow for a centrifugal pump which require at least 1500 rpm to pump properly. Being mounted below the water line has nothing to do with which type of pump is used. if yours is 9 years old it might be time to check it. just changed the one in the clubs 15 hp Johnson on the stake boat. getting it ready for the race season which starts end of January

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post #15 of 15 Old 12-16-2016
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Re: Outboard question

40 years ago I was repairing Johnson 9.9 and 6 hp engines for a US Naval station's recreational services... (temp assignment awaiting orders)
They used "rubber" (rubber-like compound) flexible vane impellers.

I've never seen a water cooled outboard that used anything other than a flexible vane impeller for the water pump.

The impellers can fail from age even if you never installed the impeller. I had to check the Rec Service's old stock and toss over half because they were cracking in the boxes.

Without water to act as a coolant and lubricant the flexible vane impellers get hot and will come apart in minutes.

Any operation dry will cut potential lifespan of the impeller some, but if you are replacing yearly (to be safe you should...) an occasional few seconds run dry, to check that the engine starts, then immediately shut it down, won't matter.
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