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.
Understanding Ships and Submarines: Propeller vs Waterjet
A propeller blade acts like a rotating wing. Its curved shape creates a pressure difference between its two sides. That difference pulls the blade through the water while the blade redirects water toward the rear.
Blade pitch matters. Pitch is the distance a propeller would move forward in one turn if it travelled through a solid material. In real water, the vessel travels less than this ideal distance.
This difference is called slip. Some slip is normal because water must be accelerated. Too much slip can mean the propeller is overloaded, poorly matched to the engine, or operating in disturbed water behind the hull.
The hull changes the water before it reaches the propeller or waterjet intake. Water close to a moving hull has a slower boundary layer because of friction. It may also swirl after passing around the hull shape.
Designers try to give a propeller smooth, even incoming flow. Uneven flow makes each blade experience changing forces during a rotation. This can cause vibration, noise, and loss of efficiency.
Waterjet intakes need clean flow too. If air enters an intake near the surface, the pump can lose grip on the water.
This is called ventilation. It is different from cavitation, though both can reduce thrust.
A waterjet pump raises the pressure of water inside the vessel. The nozzle then converts much of that pressure into a fast stream. Nozzle size is a tradeoff.
A smaller nozzle sends out water faster, but it restricts flow. A larger nozzle moves more water, but the exit speed may be lower. For a given thrust, moving a larger mass of water with a smaller speed increase can waste less energy than moving a small mass at extreme speed.
This helps explain why large displacement ships often use big, slowly turning propellers. Fast craft can accept the higher jet speed needed for their operating conditions.
Cavitation deserves careful attention because it affects performance, safety, and stealth. When local pressure falls below the vapor pressure of water, tiny vapor bubbles form. They collapse when they reach higher pressure regions.
Their collapse can make a crackling sound and can strike metal surfaces hard enough to cause pitting over time. A submarine must limit cavitation because sound travels far underwater and can reveal its position. Engineers reduce the risk by using larger propellers, lower rotation speeds, carefully shaped blades, and sufficient depth.
Students can connect this topic to a ceiling fan or bicycle gear. Changing blade size, speed, or load changes how effectively a rotating system transfers energy to the surrounding fluid.
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.