Ships and submarines move by making thrust, which means they push water one way so the vessel moves the other way. The two common systems in this infographic are screw propellers and waterjets. Both obey the same physics, but they handle water in different ways.
Comparing them helps explain why slow heavy ships, submarines, and fast patrol boats often use different propulsion designs.
A screw propeller uses rotating blades to accelerate water backward in an open flow behind the hull. A waterjet pulls water through an intake, speeds it up with an internal pump, and ejects it through a nozzle. Propellers are efficient for many large vessels and can work well at low to moderate speeds, while waterjets are useful for high speed craft, shallow water operation, and quick steering.
The best choice depends on speed, draft, maneuvering needs, noise, and how the vessel will be used.
Key Facts
- Thrust comes from momentum change: F = Δp/Δt.
- For a waterjet, thrust can be estimated by F = ṁ(v_out - v_in), where ṁ is mass flow rate.
- A screw propeller pushes water backward with rotating blades, producing forward reaction force on the vessel.
- A waterjet uses an intake, pump, and nozzle to accelerate water into a high speed jet.
- Propellers are usually efficient at low to moderate speeds and for heavy loads, while waterjets often perform best at high speeds.
- Cavitation happens when pressure drops enough for vapor bubbles to form, reducing efficiency and increasing noise or damage.
Vocabulary
- Thrust
- Thrust is the forward force produced when a propulsion system pushes water backward.
- Screw propeller
- A screw propeller is a rotating set of blades that accelerates water to move a vessel.
- Waterjet
- A waterjet is a propulsion system that draws water into a pump and ejects it through a nozzle to make thrust.
- Draft
- Draft is the depth of a vessel below the waterline, which affects where it can safely operate.
- Cavitation
- Cavitation is the formation of vapor bubbles in low pressure water near blades or pumps.
Common Mistakes to Avoid
- Thinking a propeller works by pushing on air, which is wrong because marine propellers make thrust by accelerating water backward.
- Assuming waterjets are always more efficient, which is wrong because they are often best at high speeds but may be less efficient than propellers at lower speeds or for heavy displacement vessels.
- Ignoring draft when comparing systems, which is wrong because exposed propellers and rudders can sit deeper in the water while waterjets can allow shallower operation.
- Treating speed and maneuvering as the same design goal, which is wrong because a vessel may turn well using nozzle steering even if its maximum speed or fuel efficiency is limited by other factors.
Practice Questions
- 1 A waterjet takes in 80 kg of water each second and increases the water speed by 12 m/s. Use F = ṁ(v_out - v_in) to find the thrust.
- 2 A propeller produces 6000 N of thrust on a 2000 kg test boat. If water resistance is ignored, what is the boat's acceleration using F = ma?
- 3 A patrol boat must operate at high speed in shallow coastal water and make tight turns near docks. Explain whether a screw propeller or a waterjet is more suitable, and give two physics or design reasons.