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Raw water pump not priming?

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mattplowman View Drop Down
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    Posted: 25 January 2019 at 08:13
We struck a problem today that I am still trying to fix while at anchor. We had a great sail last night and used the engine when getting under way this morning, no issue and water streamed from the exhaust as usual. Then sailed for approx 20 nm and when we went to drop the sails and anchor I noticed no water coming from the exhaust. Sailed into bay dropped anchor and spent the next 6 hours doing the following:

First step was check the impeller , no issues but replaced it as per usual service.
Took a snorkel under boat to have a look at sail drive inlet and no issues.
Dismantled sea water strainer and appears to be no issues , inlet and out let all free flowing.
Checked all pipes for blockages, no issues

Here is where is gets frustrating. I took the strainer outlet hose off and put in bucket of water and water streamed out of exhaust. I then did the same with the inlet hose and again water streamed out as it should when engine run at idle. I then manually opened the sea cock from the sail drive with the hose still disconnect and water flowed freely. So it appears the pump has enough pressure at idle to prime from a bucket at bilge level but has suddenly lost the ability to prime from the underwater sail drive inlet level, Iím baffled by this.

Any ideas , solutions?
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Fortuna View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fortuna Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2019 at 08:53
Good Morning.
That sounds like a leak in the supply line - a small crack or thread defect in the seawater filter or a leak on the hose clamps.
The low water pressure at engine standstill is not enough for a leak over the immense negative pressure of the impeller pump uses every possibility to draw air.

A lot of success
Joachim
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mattplowman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mattplowman Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2019 at 14:51
Thank you for your reply, the bit I am confused by is what that would be an issue only when the inlet hose is connected to the sail drive inlet sea cock but not an issue when I use the same hose in a bucket, is it because of the extra depth of the sail drive inlets and hense more pressure needed? Thanks again
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Martin&Rene View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Martin&Rene Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2019 at 17:05
At the beginning of a season when we put the yacht back in the water, we sometimes had a similar problem.  I felt that what was happening is that on our yacht the pipe from the strainer to the water pump went under the engine and then back up to the pump, so when I reconnected the hose, there was actually an air bubble present at the pump inlet and so it would not prime.  

My solution has been to; 
Ensure the pump impeller has a lot of the lubricant on it or vaseline
pour water into the outlet pipe of the pump by the heat exchanger to fill the pump with water
close the saildrive seawater inlet and top up the strainer, refit the strainer cap and then reopen the seawater inlet to cut down any air in the system
when we start the engine, take it up to high revs quite quickly.
  Once primed like that, we do not have a problem. 

Have you tried starting the engine and taking it up to 2000rpm plus immediately, to see if you can generate enough suction to prime the pump.  

Otherwise, it sounds like a leak in one of the inlet hoses. 

When our yacht is out of the water, I also push some flexible wire up the saildrive water inlet slots, as you can occasionally get some growth in there.
Martin&Rene Hanse 341 Dipper Wheel steering, 3 cabin layout & shallow keel, normally based in Scotland
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plattgatt View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote plattgatt Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2019 at 17:16
Klatschen


370#641 "aqua d`or"
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Mark_J1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mark_J1 Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2019 at 23:06
Iíll second the comment from Martin&Rene. Doesnít always need that approach but have to do that when we relaunch.

Mark
Hanse 400e "Grey Goose" Hull #31
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mattplowman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mattplowman Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2019 at 00:26
Problem solved. Rechecked the seacock inlet and was in fact blocked. We ended up using the dinghy pump to clear the blockage with some high pressure air. Thank you all for the replies.
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Mark_J1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mark_J1 Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2019 at 10:04
Excellent! I have found a boiled sweet wrapper in the seawater inlet galleries inside the sail drive before now.

Mark
Hanse 400e "Grey Goose" Hull #31
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