Village Science - Teacher Edition


Outboard Motor Cooling System

Teacher Edition Contents

Skill, Tools, & Craftsmanship

Cutting & Drying Fish
Nails, Pegs, & Lashings
Falling Trees &
     Small-Scale Logging
Chainsaw Clutch & Chain
Ice Pick


Wood Stoves
Wall Tents
Insulation & Vapor Barriers
Gas Lamps & Gas Stoves


Piloting A Boat
Boat Design
Magnetos & Spark Plugs
Outboard Motor Lower Unit

Outboard Motor Cooling System
Snowmachine Tracks
Snowmachine Clutch
Winter Trails


  1. Take a water pump apart. Identify the three main parts. What causes the impeller to turn as the shaft turns?

    The drive shaft has an indentation that holds a key. The key also fits into the impeller in the pump. As the shaft turns, the water pump also 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?

    It is hard. It must be durable to withstand the abrasion from silt.

  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?

    Aluminum housings are cheaper, but less durable. Chrome housings are very durable but more expensive.

  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.

    There is usually a screen to keep rocks and grass out. The cooling system, particularly the thermostat would plug up if it were not there.

  6. Check the copper tubing that carries the water from the pump to the upper unit. Are there seals where it connects to each?

    Yes, there is pressure in the cooling system from the pump, and the seals hold the pressure.

  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?

    The response is visible.

  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

  1. What would happen to an outboard if water didn’t circulate around the cylinders? Is this change permanent?

    Parts would warp because of overheating. Yes

  2. What are the three parts of a water pump?

    Impeller, housing, bottom plate

  3. With a drawing of the top view, show how a water pump works.

    It should show the impeller off center.

  4. What is the purpose of the screen on the lower unit?

    Keep rocks and grass out

  5. What is the purpose of a thermostat? How can it malfunction?

    Control the waterflow so the engine can work at the best temperature. If a rock gets stuck, it can refuse to open or close.

  6. What does the indicator do? If it is plugged, does that mean the water pump isn’t working?

    The indicator shows if there is water coming out of the cooling system of the engine. If the indicator is not showing water, the water pump may or may not be working.

  7. Why should an outboard motor be tilted up when not in use? During what season is this not so?

    Silt settles in the waterpump if the engine isn’t tilted up. This isn’t true in late fall when the water is no longer silty.

  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?

    If his improvised method doesn’t work well, the engine might still run, but parts will be warped and damaged from the heat.


  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?

    The stainless one.

Questions or comments?
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