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Grades 3–5 · Machines

How does a spinning propeller move a boat?

Behind many boats, a propeller spins underwater. Its blades are angled and shaped to push on the water as they turn. They do more than stir it in a circle: a working propeller sends water backward. The water pushes forward on the blades at the same time. That forward force passes through the propeller's shaft and supports to the boat.

M93 visual explanationIn a steady ideal flow example, 10 kg of water per second changes backward speed from 1 to 3 m/s. The water gains backward momentum at 20 N; the equal and opposite force on the propeller is 20 N forward. The motor supplies energy. Mass flow and velocity change are supplied, not predicted from the drawn blades. Boat speed, drag, swirl, pressure details and cavitation are excluded.A propeller pushes water back; water pushes it forwardTurning bladesBoat pushed forwardWater pushed backExample: 10 kg each second gains 2 m/sEqual force pair: 20 N each way.

In the diagramA propeller pushes water back; water pushes it forwardTurning bladesBoat pushed forwardWater pushed backExample: 10 kg each second gains 2 m/sEqual force pair: 20 N each way.

Follow the green forward-force arrow on the boat and the blue backward-flow arrow in the water. The two forces act on different objects.

These two pushes are a pair. The propeller pushes the water one way, and the water pushes the propeller the opposite way. They do not cancel on the boat, because one force acts on the water and the other acts on the propeller. The motor supplies the energy needed to keep turning the blades and changing the water's motion.

Moving more water backward, or giving it a larger backward change in speed, can produce a larger forward push. The illustration uses supplied quantities to show that relationship. It does not predict a boat's speed: the boat also pushes through water and air, which resist its motion. A boat can therefore move forward by giving the surrounding water a push in the other direction.

About this explanation

Starting knowledge: Recognize opposite directions and that motors supply energy.

Sources

  • Massachusetts Institute of Technology, Mission 2005 — Propulsion
  • MIT OpenCourseWare — Marine Power and Propulsion: Lecture Notes
  • NASA Glenn Research Center — Propeller Thrust
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