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Changing an impeller – a brief guide

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Most marine engines are cooled by utilising the seawater around the boat through a heat exchange system to cool the coolant that is within the engine itself. Bizarrely most smaller engines only monitor the heat on the engine side of this system with no exhaust temperature monitoring, so often the first sign of an overheat will be the temperature gauge shooting up (very easy to miss) followed by the shriek of the alarm. If your boat is on Outdrives most of this exhaust side of things are actually in the water so there are no early clues. Shaft drive boats have a few early hints that something is wrong, such as a noisier exhaust sound as the water cooling also acts as a silencer, or a hot rubbery smell as the exhaust system pipes get hot. Simple things that you can do to keep the seawater system happy and efficient are to clear out the seawater strainers regularly and especially before a longer trip, have the Impellers changed at least once a season, as lack of use makes the rubber blades go brittle which can result in damage.

If you are unfortunate enough to have an overheat what can you do? Well first off is to remove what caused it – blocked sea strainer filters are usually the culprit however that’s only half the problem, the seawater pump that is actually bringing in this cooling water has an impeller pump which has rubber vanes for efficiency, sadly they don’t like to run without water in them, so that lack of coolant will probably have shredded the Impeller. To change it is relatively simple when tied up but hard work at sea as the engine will be very hot and the boat’s motion unsettling. All that is needed is to remove the faceplate, pry out the old Impellor and replace it. Sounds simple! And in reality it is, but some simple steps could make it much easier:

  • Make sure the boat’s location is the best you can manage, consider anchoring.
  • Close the Seacock, (not all outdrive boats have a seacock).
  • Undo the Faceplate of the impeller pump.
  • Prise out the old Impeller; an Impeller Puller is a very handy tool to have in your tool kit.
  • Inspect it, if bits are broken off they may be in the system and cause further problems, follow the pipe to work out where they have gone.
  • Lubricate the new Impeller, most come with some, washing up liquid works in an emergency.
  • Insert the new Impeller, try to get it the correct way round and don’t forget the gasket.
  • Open the Seacock and start the engine, if your exhaust is visible check the output of cooling water.

Some sensible things to do so you are prepared are:

  • Locate the raw (sea) water filters and practise clearing them.
  • Carry the correct tools complete with an Impeller puller.
  • Carry two spare Impellers.
  • Locate the pump and work out how to access it whilst nicely alongside.
  • This winter change your own Impeller, once done the mystery will be gone and if it ever happens to you it won’t be an issue.

To learn more about Diesel Engine Maintenance courses see here and if you are not already, then being a member of Sea Start (the AA on the water!) is very reassuring.


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