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Pump Impeller

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I was taught to examine the fins on my raw water pump impeller and if there was no cracking at the base or any signs of wear cuts or pieces missing and those fins were still flexible and not distorted then that impeller could be returned to service.

However a brand new impeller could be ruined in a matter of minutes, regardless of how often it had been replaced if the intake or the weed filter had blocked and the impeller had run dry.

Was I trained properly or should it be changed anyway. 

Regards Marge and Parge 


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I think it would depend on your maintenance schedule and how long the impellor has been in-situ - If you know it has only been in for, say under 2 years, then 'on inspection' would be good - however, neoprene and other rubber derived items will degrade - especially if not flexed regularly, and I would change it. I would also change it if it had not been used for a very long time (ie a season out of the water) Otherwise, a good inspection will suffice, but take the opportunity to fit spin-off wing nut cover screws and carry a spare as it is easy to identify if things arent going well (ie housing not cold!!!) and rectify - well that is my plan anyhoo!

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12 hours ago, MY littleboat said:

I would also change it if it had not been used for a very long time (ie a season out of the water) Otherwise, a good inspection will suffice,

The most common cause of damage is leaving the impeller in place in winter, for several months.  In this case, the two or three vanes which are left bent over by the cam in the pump, will stay in that position, causing the pump not to prime properly when starting and not to give enough flow for cooling.  Quite soon these vanes will snap off as they have become brittle.

If the pump has been run dry, the tips of the vanes will be burnt, so they no longer make a proper seal with the pump housing.

If there is wear in the brass front and back plates - which are reversible - this has been caused by sandy silt in the river water causing abrasion.  Nothing much you can do about that, except stay in clear water!

Always take the impeller out when you winterise the boat, then inspect it in spring, putting it back with a liberal covering of special water pump grease.  This helps it to prime better when cold starting. I always leave the impeller on top of the engine in winter, to warn anyone else that the engine is winterised.  Some yards put their impellers in a bucket of water over winter as they believe this keeps them flexible.  Maybe it does!

When cold starting, if you have to rev the engine up for several seconds to get water out of the exhaust, then your pump is worn and needs attention.

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I always carry a spare for each engine, usually the last one taken out if they inspect as being ok, I don't change anually but every 2-3 years if no problems but as said a blockage can destroy one within minutes, if I've put anti-freeze through the system I fill the strainers first before starting and have some floating plastic balls in there so I can see water movement through the lids when they fire up, I also have thermocouples against the raw water pumps with a display at the helm as the get hot pretty quickly in case of intake blockage.

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