Marine Maintenance at it's Best! Est. 1977


Changing the Raw Water Pump Impeller.   


Changing an impeller is not a difficult job. Changing an impeller, on many engines, usually takes less than 20 minutes even if you are fairly slow. The most complicated part of it is access to the pump.

Impellers are a ‘wear’ item so unfortunately they do not always abide by a "time" or "hours of use" schedule. A simple mishap like forgetting to open the engines raw water intake or a plastic bag over the sea water inlet can kill an impeller very quickly. They will run dry and overheat, If this happens for long, they usually loose  a blade or a chunk that can lodge in the hose or the heat exchanger. If this happens you have a much larger job on your hands of finding the piece to prevent further damage or overheating issues.

It is advisable to change your impeller every year regardless of condition. This may seem unnecessary, but changing your impeller is cheap insurance! When an impeller breaks, the parts can get into your engine and cause considerable damage as an overheated engine or a thrown blade can cost many, many times more in both time and money than the few minutes and the cost of this simple maintenance.

NB…. One important point, before starting on this job you must always remember to CLOSE the sea water intake, and once you have finished, don’t forget to OPEN it again.



When you are replacing your impeller, cover it in washing up liquid, this will make it easier to slide in and once you are ready to restart your engine, get a crew member to check your outlet for bubbles, this will confirm that your water pump is running fine!



There are many problems you can fine with salt water pumps, but all are usually repairable.

·         Check the cover plate - The Cover plate of a water pump can be considered another wear item. If it is scored and worn you should replace it or usually it is possible to turn it over (depends on manufacturer) as in time the impeller will wear a groove in it.

·         Cover plate screws - These often corrode with the salt water and can shear off so be careful not to over tighten.

·         Shaft – the internal shaft in you w/p can also wear, so if you need seals at any time, get them to check this also, unfortunately, it often means a new water pump as the shafts can cost a large percentage of a new pump.

·         Seals – the seals in you pump will give up at some point, if your pump is leaking, this is often the problem. (you may need to take these to a marine engineer as the pump needs to be dismantled and may need a press to get the seals out)

·         Gasket – you may need to change the gasket when you change the impeller (if it is a paper one) or you may have an “O” ring type, which will last longer.


Check your impeller periodically for signs of wear, lookout for the problems shown on the diagram, if you find any of these, your impeller needs changing.



The photo below shows a worn impeller with missing pieces! Always make sure that you can account for all the pieces of an impeller when it breaks!

The other photo shows the wear on the cover plate, once it gets like this, it is a good idea to turn it over if possible.


SPARES you should carry!

It is a good idea to keep a full set of spares for your water pump, Such as:

·         Impellers,

·         cover plate,

·         gaskets,

·         a full set of cover screws

·         seals




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Marine Maintenance Ltd, 5 The Square, Marina Bay, Gibraltar. Tel - +350 200 78954  Fax - +350 200 74754