Boat and Automotive Engines

• Today's inboard boat engines closely resemble automotive engines. They come in a variety of sizes and power ratings. Smaller pleasure boats use four-, six- or eight-cylinder engines made by Ford or Chevrolet. Larger boats use diesel motors built by Cat, Cummins or other manufacturers.
  As in any internal combustion engine, a carburetor or fuel injection system meters and mixes air and fuel. The mixture is then introduced into the cylinder above the piston head. The upward movement of the piston in the cylinder compresses the air/fuel mixture. At just the right moment, spark is applied. The resulting explosion forces the piston back down in what is referred to as the power stroke. Since the bottom of the piston is attached to the crankshaft, the power stroke causes the crankshaft to turn with force. The resulting power delivered by the turning crankshaft is called torque. Other than the cooling system, how torque is delivered to a propulsion system is where boat engines and automotive engines differ.
  
  

Torque

• The automobile engine delivers torque to the wheels via the transmission, drive shaft, rear differential gears and axle. The more powerful the engine, the more torque created. The more torque created, the faster the wheels go around. The concept is similar for a boat except there are no wheels. The crankshaft is connected to the propeller shaft through a set of gears. The more torque created, the faster the propeller goes around.
  On an inboard motor, the propeller shaft angles down though the bottom of the hull and drives the propeller. On an inboard/outboard, or sterndrive motor, the shaft extends through the transom and delivers power to the propeller via the lower unit. This lower unit is steer-able, thus eliminating the need for a rudder.
  You will notice that there is no mention of a transmission in a boat. There are just two gears: forward and reverse. That is because water provides so much resistance it makes a transmission impractical.
  
  

Cooling System

• The other way that boat engines are different from automobiles is the cooling system. Automobiles have limited access to cooling water. Therefore, they must run on a closed cooling system. A chemical/water coolant circulates through a closed, pressurized cooling system and the water is cooled as it passes through the radiator fins. This keeps the engine cool with limited access to water.
  A boat sitting on a lake has unlimited access to cooling water. Therefore, most boats run what is referred to as an open cooling system. The engine draws water from the lake in which the boat sits, circulates the water throughout the cooling jacket, and then deposits the water back into the lake. No coolant or a pressurized system is needed.