Village Science

An Outboard Cooling System


A 15
B 3
D 1, 3


Forms of energy

water intakeAs gasoline is burned in an outboard motor, the temperature quickly rises in and around the piston and cylinder. The chemical energy of the fuel is changed to the energy of motion of the piston, as well as heat and sound energy. If the heat were not carried away, it would soon warp and melt engine parts. The engine may run again, and seem all right for a while, but the damage is never reversed. Engines don’t heal like people, plants, and animals.

Airplanes, chainsaws, and most snowmachines rely on air flow to carry the heat away.

Outboard motors can efficiently use water for cooling. Water is has greater mass and is a better conductor of heat than air.

Automobiles, trucks, and high-tech snowmachines are also water-cooled.

The water comes into the outboard’s engine through the lower unit, which is below water level as the boat is moving. Without a pump of some kind, water couldn’t flow up to the engine. The water pump is the heart of the cooling system.

Main Parts

pumpThere are three main parts to the pump:

  • Bottom plate
  • Impeller
  • Housing


If the impeller were exactly in the center of the housing, it would turn with the drive shaft, spinning water within the housing, but it would not pump water.

However, it is designed with an offset impeller. When the impeller turns, it has a lot of room on one side of the pump, and not much room on the other. As the drive shaft and impeller spin, water enters on the side that has a lot of room, and is pressured out on the side that doesn’t have much room.

offset impellerWhile this seems very simple, it is the basis upon which many other kinds of pumps work, particularly fuel pumps.


A screen protects the opening to the water pump on the lower unit. It keeps grass, sticks, and small stones from plugging the cooling system, and destroying the engine.


When an engine is cold, it doesn’t run well at all. Ignition requires three things: fuel, oxygen, and heat. If any of these is lacking, combustion will not occur efficiently. The thermostat prevents the cooling water from getting to the cylinder walls until there is enough heat for efficient combustion. Once the cylinder temperature is up, the thermostat allows the cooling water to flow from the water pump through the water jacket and out the exhaust. If a small stone gets jammed in the thermostat, it can be stuck closed which causes the engine to overheat, or it can be stuck open, making warmup very difficult.

If a motor overheats from cooling system failure, quickly remove the spark plugs and pour oil into the cylinder to keep the piston rings from ceasing up in the cylinder. Gently pull the starter rope to keep parts moving.



On the side of most motors there is a small hole that emits a stream of water. This is only an indicator that the water pump is working. Most of the water that comes from the cylinders is exhausted through the lower unit. During cold weather, the indicator might freeze even though the pump is working well.


silt pumpMany of Alaska’s rivers are silty. Friction with the silty water wears the impeller and housing much quicker than clear water. Some water pump housings are lined with very durable chromium steel. Others are made of soft aluminum. Aluminum wears quickly and the jagged pieces of the worn housing cut the impeller to pieces.

When the motor is stopped and left in an upright position, silt in the water settles to the bottom of the water pump. When the engine is started again the impeller is severely worn by the silt. To prevent this, good pilots tilt their motor up after stopping. This drains the water pump.

A good pilot doesn’t do this when the temperatures are freezing. The motor is left down with the pump below the water line to keep it from freezing and cracking. Fortunately the silty rivers become clear once freezing temperatures cause the water level to drop. The hazard of silt in the pump no longer exists. It is important, however, to pull the starter rope very slowly before starting the engine in cold weather. To pull on the starter rope quickly when the water pump is frozen could strip the connection between the impeller and the driveshaft, ruining the impeller and stranding the pilot.

When a water pump freezes, pour hot water on the lower unit housing until it is thawed. Usually one teapot of boiling water is enough to free it. Some people pour a little gas in a coffee can, throw in a match, and hold the burning gasoline under the lower unit with pliers. Gasoline is very dangerous and this method is not recommended.

Inexperienced pilots often hit bottom, running their motors in the mud trying to get to deeper water. Even if there isn’t gravel to damage the prop, the water pump draws tremendous amounts of silt, wearing the impeller and housing.

Common Problems

scoopThe most common problems with water pumps are:

  • Worn impellers
  • Worn housings
  • Bottom plates that are worn rough and thin by silt

An oldtimer made a scoop from a metal can and bolted it in place of the intake screen on the lower unit. He removed the worn impeller in the pump. Forward motion of the boat forced water into the scoop up through the upper unit. As long as he ran full throttle in forward he had enough water to cool the engine. It was a good temporary solution.

water jacketSome people bail water on their engines to cool them until they get home. This cools the outer jacket of the engine, but doesn’t cool the cylinder walls well at all. The operator might get home, but not without internal damage to the engine.

An extra impeller and housing are not too expensive to have as spare parts in a tool box as they are so important to an engine’s operation.



  1. Take a water pump apart. Identify the three main parts. What causes the impeller to turn as the shaft turns?
  2. Touch the bottom plate of the water pump with a file. Is it harder or softer than the file? Why do you think this is so?
  3. There are two types of metal that water pumps are made of. Find examples of each. Why do you think there is a difference?
  4. Put an impeller in a pump housing. With a stick in place of the drive shaft, turn the impeller. Watch the impeller blades extend and compress. Imagine the water coming in the side where the blades are extending and leaving the side where it is compressing.
  5. Find the intake for the water on the lower unit. Is there a screen of any kind? Why do you think the screen is there? Ask people in the village what it is for and what would happen if it were missing.
  6. Check the copper tubing that carries the water from the pump to the upper unit. Are there seals where it connects to each?
  7. Can you find the thermostat? They are in different places on different motors. If possible, remove it. Alternately, pour hot and cold water on it. Can you see it move in response to the temperature difference?
  8. If you live near a silty river, collect some water and let it settle overnight. Is there sediment on the bottom of the container? Pour most of the water off and rub your hand on the bottom of the container. Can you see why silt will wear the impeller inside the pump housing?
Student Response

Student Response

  1. What would happen to an outboard if water didn’t circulate around the cylinders? Is this change permanent?
  2. What are the three parts of a water pump?
  3. With a drawing of the top view, show how a water pump works.
  4. What is the purpose of the screen on the lower unit?
  5. What is the purpose of a thermostat? How can it malfunction?
  6. What does the indicator do? If it is plugged, does that mean the water pump isn’t working?
  7. Why should an outboard motor be tilted up when not in use? During what season is this not so?
  8. A water pump doesn’t work. The operator improvises a way to get home. What are the consequences if the improvised water cooling system doesn’t work?


  1. A cheap aluminum impeller housing costs $12. An expensive stainless steel one costs $37. However, a housing made from stainless steel lasts 4 times longer. Which is cheaper in the long run?

Questions or comments?
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