Water Pump Leak

[NickWilson1972]

If the pump won't prime after a (typically) brisk sail and you don't have a blockage then you either have an airleak (strainer lid, jubilee clips, raw water pump) or the pump is worn and has lost some of its suck. If you fill the pipework manually by topping up the strainer it will probably work (I'm assuming you hadn't drained the pump when on the hard which is why it was working before) but when sailing quickly the venturi effect can pull air into the system and if the pump is worn and/or you have an airleak it will no longer prime. Often this is because the inside of the cover plate has worn in which case you may be able to simply reverse it provided its flat and not stamped with the part number etc. Alternatively you can buy one as part of a refurb kit. A worn seal and/or shaft in the pump can also allow air in or the body of the pump can wear. The former are easy to repair and the latter probably means a new pump. Other reasons for a raw water pump not working include a blocked heat exchanger / exhaust elbow or a failed impeller where the rubber has separated from the central boss but this sounds unlikely in your case.

I hope that helps. I did have a quick look for a YouTube video but you may have better luck if you search the manufacturer. If the cover plate has 3 screws I think its a Jabsco. If it has 6 it should be a Johnson and if you can identify the relevant rebuild kit it's not complicated even without a video.

Many thanks for your reply.

I know there are no blockages as in the post mortem I managed pour water in at various points and watch it flow out at various points so I know the whole system from intake to exhaust outlet is clear enough for water under gravity to flow

Air leak sounds like a possibility. I hope its that rather than a pump rebuild which is well outside my field of experience

The cover plate has 6 screws so thanks for the tip its a Johnson

This would appear to be the appropriate rebuild kit


Beta 10, 13.5, 14hp Water Pump Rebuild (6 hole) Kit - D

 

[NickWilson1972]

Following previous advice I spent the afternoon looking for air leaks - even went to the extent of using drain trace fluid to try and see if any of the pipes leaking. Tried different gaskets and o rings on the impeller housing. Ran the engine with the impeller cover off. Its turning nicely so that rules out the sheared drive shaft . Confirmed again no blockages from inlet to exhaust - orange drain trace is most helpful.

Still no flow.

On the way home thinking about this I realised the impeller is 2-3 inches above the water line. The operators manual is silent on this but is the impeller self priming or does the system need to be filled with water? The most practical way to do this would be to disconnect the hose going to the anti siphon point (highest point in the cooling system and then backfill from there. Kicking myself I didn't try this earlier...

 

[vyv_cox]

On the way home thinking about this I realised the impeller is 2-3 inches above the water line. The operators manual is silent on this but is the impeller self priming or does the system need to be filled with water? The most practical way to do this would be to disconnect the hose going to the anti siphon point (highest point in the cooling system and then backfill from there. Kicking myself I didn't try this earlier...

In theory the pump will self prime almost 10.metres above the supply level. The reality is considerably less but easily 1 metre. If it does not prime there is an air leak. The best way to overcome this problem is with a strainer above the pump inlet. Filling this with water primes the pump.

 

[Pye_End]

Following previous advice I spent the afternoon looking for air leaks - even went to the extent of using drain trace fluid to try and see if any of the pipes leaking. Tried different gaskets and o rings on the impeller housing. Ran the engine with the impeller cover off. Its turning nicely so that rules out the sheared drive shaft . Confirmed again no blockages from inlet to exhaust - orange drain trace is most helpful.

Still no flow.

On the way home thinking about this I realised the impeller is 2-3 inches above the water line. The operators manual is silent on this but is the impeller self priming or does the system need to be filled with water? The most practical way to do this would be to disconnect the hose going to the anti siphon point (highest point in the cooling system and then backfill from there. Kicking myself I didn't try this earlier...

The 'classic' is air getting in the lid of the strainer, which is why I suggested running the engine and filling the strainer manually (with the seacock off). It confirms the 'suck' of the pump is ok.

Putting water into the anti syphon has the possibility of being an unwise move.