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Old 14 September 2020, 13:31   #1
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Priming a new water pump impeller.

Hi, i wonder if any one has any tips regarding the above.
I put a suitable container under the engine and lowered it into the container, the water just covered the water intakes where the mufflers would normally be, but there is no water coming out of the tell tail. Ive read somewhere that this may not be deep enough to prime the pump!
Any suggestions please?
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Old 14 September 2020, 14:43   #2
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Quite possibly.
Only ever done mine by
A. Muffs - the water is under pressure so will go everywhere.
or
B. Immersing the leg in water to the depth it would normally be on the boat & having done impeller changes I know the pump is well under water.
Not had a problem.
You might want to try it on muffs or a deeper container with more water.
As ever I stand to be corrected!
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Old 14 September 2020, 15:21   #3
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You need deeper water in the bucket... this is a known issue that pumps are reluctant to self-prime when the water is that much lower than the impellor itself.
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Old 14 September 2020, 15:56   #4
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If there's not enough head of water the impeller carnt pull enough water through
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Old 15 September 2020, 09:08   #5
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When I'm running at any kind of speed the water only just covers the intake grilles so I would have thought as long as they are covered the impellor should draw water ok. I presume you haven't forgotten the woodruff key?
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Old 15 September 2020, 09:20   #6
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Quote:
Originally Posted by Erin View Post
When I'm running at any kind of speed the water only just covers the intake grilles so I would have thought as long as they are covered the impellor should draw water ok. I presume you haven't forgotten the woodruff key?
at speed water is forced into the intake not so in a bucket
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Old 15 September 2020, 10:12   #7
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Quote:
Originally Posted by jeffstevens763@g View Post
at speed water is forced into the intake not so in a bucket
Yes - plus the impeller has primed itself when lower in the water.

Also - poor sealing in the waterpump assy can result in failure to prime.
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Old 15 September 2020, 13:41   #8
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The impeller spins and creates a wall of water that needs extra pressure to overcome on the intake side when it's not deep enough once it has flow raising the engine (on the plane) creates enough force for it to pump efficiently
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Old 23 September 2020, 06:04   #9
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Originally Posted by Erin View Post
When I'm running at any kind of speed the water only just covers the intake grilles so I would have thought as long as they are covered the impellor should draw water ok. I presume you haven't forgotten the woodruff key?
To get to speed you would have started from a stop or slow speed, the water pump will already have been primed - so can continue to draw in water at speed.
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Old 23 September 2020, 11:32   #10
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In a nerdy, don't really need to know way, just correcting what seems to be a general misconception, though there are exceptions...

It is likely that there is no ram effect at the water intakes on the side of the gearcase when the gearcase is moving, it is likely that there is a pressure reduction at the water intakes on the side of the gearcase.

When a fluid is accelerated the pressure within it falls. The water around the gearcase is accelerated because the gearcase takes up the space where water was just a moment earlier. If there is to be no vacuum behind the gearcase the water has to get there quickly, the distance around the gearcase is further than the direct line so the water needs to speed up to get to the rear of the case. Hence a pressure drop within that water.

It's the principle used to provide forward movement from your propeller, to provide flight, to draw fuel from a carburettor etc.

Worth noting is that if you bring your boat close to an object while moving; another boat, jetty etc., the water between the boat and object will be accelerated so your boat will be drawn closer to the object because of the reduction in pressure in the water in-between. Sharp folk will notice that when coming close to a stationary object the water does not initially appear to be moving but it will be forced to by the presence of your moving hull. An object moving through the water has the same effect as water flowing past a stationary object.

Two boats coming alongside each other when moving will be drawn together quite powerfully.

Does the impeller need to be primed? Not in my experience but that is limited. It should be a tight fit inside the housing and capable of pumping air. And, to link with a previous thread, a drop of lubricant is a good idea on a new instal to prevent the grippy rubber running dry.

All IMHO, of course...
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Old 23 September 2020, 11:42   #11
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>>>Does the impeller need to be primed? Not in my experience...

No and not in the way I'd think of priming a water pump that needed it such as pouring a bucket of water into it... but I think the ability of an OB pump to self prime pulling air for the first instant is limited to a very small lift distance hence the need to get them in a normal depth of water rather than just covering the intakes.
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