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Some boat engine breakdowns are unavoidable but those caused by lack of maintenance or regular checks can be avoided. Failure to maintain an engine’s cooling system is a well known example of this, so it is well worth spending time checking over the cooling system both when the boat is ashore and afloat.

Indirect cooling

Many boat engines are cooled by a combination of fresh water and sea water, also known as raw water, using a system known as indirect cooling. Raw water refers to the water that the boat is floating in, hence on an inland waterway this may be fresh water and at sea it will be salt water.

An indirect cooling system works as follows. Raw water enters the boat via a seacock and passes through the raw water filter to remove weeds and debris. It then passes through hoses and a pump to a heat exchanger and back out of the boat via the exhaust system.

The fresh water is contained in an enclosed system similar to a car’s cooling system, using a combination of fresh water and antifreeze supplied by a header tank which can be topped up when required. The main difference between the car and the marine engine is that the marine engine uses a heat exchanger instead of a radiator. The fresh water circulates around the engine and passes through the heat exchanger, which usually forms part of the header tank on smaller engines but may be separate on larger engines. Once in the heat exchanger the fresh water is cooled by the raw water which is pumped through small diameter cooling tubes – note that the raw water and fresh water do not mix together. The sea water is then pumped into the exhaust system where it mixes with the exhaust gases and ends up being pumped out of the exhaust into the sea.

Direct cooling

The more basic system of engine cooling is direct cooling, where raw water is pumped through the engine block and back out to sea, with no secondary freshwater system used at all. This might sound more straightforward but the problem of corrosion can become a big issue.

It can get confusing when several different names are given to parts of a boat and the cooling system is no exception. To summarise, sea water cooling is also raw water cooling. As a bonus it is also called a direct cooling system if no fresh water is involved. Fresh water cooling is also known as indirect cooling if raw water is involved. Confused? Hopefully the illustrations will help explain this better.

Inspection of the cooling system

     

Here are some things to check for:

  • Seacock hose – check the condition of the hose that connects the seacock to the raw water filter. Any signs of swelling in the hose is a sign that it is deteriorating. Check there are double hose clips in place and that they are free from corrosion.
  • Raw water filter – although this should be checked throughout the season, give this an extra special clean and double check the condition of the wire strainers, the hose connectors either side of the filter and for any signs of leaks in the system.
  • Raw water pump – check for any signs of leakage around the pump. Open it up and inspect the condition of the impeller. Some people remove the impeller completely for the winter to help preserve its shape and condition. Replace the impeller on an annual basis is normally recommended (see below Replacing the raw water pump impeller).
  • Sea water hoses – check all sea water hoses for condition and signs of corrosion at the connections. If in doubt replace these every two years as a sensible precaution.
  • Gaskets – check the cylinder head, thermostat housing and manifolds for any signs that salt water has been weeping through the gasket surfaces.
  • Fresh water circulating pump – the pump itself does not need servicing, but check the condition of the drive belts and beware of shaft bearing and seal failures.
  • Heat exchanger – the heat exchanger tube stack needs to be removed and cleaned annually. The reason for this is that fine seaweed and other debris can get past the raw water filter and then get trapped in the tube stack. Pieces of a disintegrated rubber impeller can also get trapped in the tube stack, so this is the place to look and remove them if the impeller has failed. Refer to your engine manual for advice on how to do this.

Replacing the water impeller

Every boat owner knows to check that water is flowing from the exhaust pipe after starting the engine as this indicates the raw water cooling system is working properly. If water isn’t flowing it may simply mean that the raw water seacock has not been turned on, but it could also mean that the raw water pump has failed.

The most common reason for the failure is that the rubber impeller has disintegrated. To be on the safe side, most manufacturers advise the impeller is replaced annually. Certainly it needs to be inspected when the boat is ashore, even if it has been replaced during the season, and many boat owners remove it during the winter as this helps to prolong its life.

You will need a replacement impeller and gasket. When ordering remember to have your engine number at hand to ensure you get the correct parts. This is how to replace it:

  1. Remove the circular faceplate by undoing the screws holding it in place.
  2. Note how the impeller fits into the pump – taking a photo will help.
  3. Withdraw the rubber impeller from its drive shaft using a pair of pliers, gently coaxing the impeller bit by bit from the pump. Take care not to scratch the inside surfaces of the pump.
  4. Smear the new impeller with washing up liquid and push it on to the shaft, making sure that the rubber vanes are facing the same way as the old ones did – checking the photo you took will help if you’ve forgotten.
  5. Fit the new gasket, if necessary use some washing up liquid or a little vaseline to hold it in place.
  6. Then place the cover plate back on and screw it tight.

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