- Without the power of a propeller, large cruise ships would drift aimlessly over the sea.
- The options for power are diesel, electric, or gas turbine engines for propulsion.
- The larger the ship, the greater the need for electric power.
- We discuss how a cruise ship propeller works in moving these mammoth cruise ships through the water.
A recent news article published in the Cruise Hive states that How Fast Does a Cruise Ship Propeller Spin?
How Does a Cruise Ship Propeller Spin?
Older cruise ships may use a reciprocating diesel engine for propulsion. Transmissions influence the propeller’s revolutions in the same manner that transmissions transfer engine RPM to manageable speeds to power a car’s rear wheels.
However, modern cruise ships use either diesel, electric, or gas turbine engines for propulsions. The bigger the cruise ship, the larger the demand for electric power. Some cruise ships may even use two different sources for power, with one for propulsion and the other exclusively for generating electrical power.
Aero Derivative Gas Turbines
These engines create heat that goes from mechanical energy to electrical power. To do this, compressed air becomes ignited in the combustion chamber, while hot exhaust becomes forced over turbines that spin to drive a shaft mechanically. Then, the power is used to spin an electrical generator.
Diesel Electric
These engines operate in the same manner yet utilize a direct drive system rather than a turbine. Their output shafts connect to electrical generators, producing electrical power. The faster pistons located on an engine go up and down, making the propeller shaft turn quicker. The ships then, in turn, move faster.
On mid-sized and smaller vessels, the concept of motion isn’t dramatically altered. They use similar mechanics, but the difference rests in the creation of power.
Steam in these vessels is no longer used to make the pistons move. Instead, fuel ignition generates pressure, pushing the pistons down and up while turning a crankshaft that’s connected to the propeller shaft.
Both types of engines require a great deal of fuel. An example of this is the Queen Elizabeth 2, which uses 433 tons of fuel each day – 18 tons per hour – while traveling at 28.6 knots. It carries adequate amounts of fuel to sail without stopping for 12 days. However, the iconic vessel has since retired from sailing and remains in Dubai.
Cruise ships will fill up with fuel while visiting various ports. They use fueling barges, which are like a floating gas station for cruise ships. A lower-grade diesel fuel is often used that doesn’t burn as cleanly as ones designed for a diesel-powered car. When gas prices rise, these vessels feel the financial pinch.
One advantage of direct drive diesel systems is being able to use a shaft generator (a device that uses a circular motion of a propeller shaft to create electricity used for hotel services like cooking and lighting).
These are only used when the vessel is moving at a constant speed. If the shaft isn’t moving, then the generator isn’t working, which means no energy is being created.
Liquified Natural Gas (LNG) Powered
Carnival Corporation announced in June 2015 that its contract with Fincantieri was established to create four LNG-powered cruise ships for its largest passenger capacity. The four newly constructed ships created the industry’s first LNG-powered cruise ship in a hybrid engine.
Gas in the ship is consumed and generates all the power necessary while at sea. Its engines are not exclusively composed of this product, but rather follow a dual fuel model that provides natural gas and liquid marine fuels. The design itself was genius in reducing storage space used for fuel.
LNG is 90% methane and 10% ethane. Cooled down to -160°C, it moves from a gas to a liquid. The volume decreases 600 times, making distribution efficient. This gas reduces bad emissions from sulfur oxides and soot. It also reduces fuel costs.
Azipod Propulsion Systems
This contains three components. The propulsion motor drives or produces thrust. The propeller is moved by an electric motor. Supply transformers reference the power created by generators. This is 6,600 KV. A frequency controller changes the frequency of the supplied power. This allows a rotating motor speed to be better controlled.
These types of propulsions combine propulsion and steering systems. They use a two-stroke engine with a shaft, which passes through the stern tube and the shaft tunnel. It then connects to a propeller outside the ship’s hull located in the stern, aft. Steering is done by a rudder.
Since the propulsion is compact and located in the engine room, the saved space is often utilized for cargo.
Propellers in this system turn in a variety of directions. This helps with crash maneuvering, which is significantly better than conventional methods. Propulsion systems are located below a cruise ship’s height for greater efficiency.
Should a ship have a large breadth, two-plus systems that operate independently can provide subtle maneuverability. It also has lower fuel consumption, lower lube oil, lower vibration, and low emissions.
This type of system is commonly used on Royal Caribbean’s Oasis-class ships.
ABB propulsion is used by Crystal cruise ships, which are powered by two units of an Azipod D. Each Star Cruise Global Class contains three of them. This type of system tends to be seen with the designer MV Werften, which is a German shipbuilder. Most of his large-sized vessels, ice-class cargo, and icebreaking ships contain this propulsion type.
What Does a Propeller Do?
Propellers push ships through water. Often called screws, they slice through water and give reverse and forward motion.
Unlike an airplane propeller, which requires a tremendous amount of speed to advance forward, a ship’s propeller isn’t required to turn as quickly.
A ship’s propeller relies on brute power or torque instead of RPM (high speeds) to move. This is what causes them to rarely move over 30 knots.
Cutting-edge vessels are now using azimuth thrusters. These are pods that house propellers that can rotate 360°, allowing them to maneuver optimally in the sea. They have replaced rudders and are believed to have benefits over a conventional screw-like propeller system.
The benefits to this are greater fuel efficiency, increased ship safety in bad weather, and decreased distance required for stopping. They can be used for both types of engines.
How Fast Does a Cruise Ship Propeller Spin?
Since its engines turn the crankshaft at exceptionally quick speeds (hundreds or even thousands of revolutions each minute), a ship’s engine connects to propellers through a series of gears. Propellers turn more slowly (250-800 revolutions/minute or less).
Gearing allows an engine to move at a more efficient and faster speed. It allows propellers to move at slower, efficient speeds.
How Large Is a Ship’s Propeller?
The largest cruise ships have propellers up to 20 feet tall. Referred to as azipods, each cruise ship contains three of these propellers to maneuver them through the waters and into ports. These massive propellers spin at 250 revolutions per minute.
Largest Ship Propellers Globally
The following ships have the biggest propellers in the world.
- Royal Caribbean Oasis Class – These vessels are the largest in the world. The Allure of the Seas and Ocean of the Seas contain propellers with a diameter of six meters. Propulsion is generated by 26,800 horsepower Azipods.
- Queen Elizabeth 2 – This floating hotel was created for Cunard Line. Since 2018, it has been a floating hotel operating in Dubai. Its two large variable pitch propellers have a diameter of 22 feet. Each propeller weighs 43 tons and is fitted using grim wheels. Grim wheels are blades that spin freely, and each is fitted behind a main propeller. They are used for recovering thrust and reducing consumption of fuel.
- Carnival Cruise Line Fantasy Class Cruise Ship, Elation – Service speed on this vessel is 22.3 knots. It is the first to use an electric azimuth Azipod system for propulsion. The difference with this system is the cruise ship is pulled rather than pushed using the propeller. This ship has two 14 MW Azipod units for propulsion.
Summary
To move a large, weighty cruise ship, the propellers need to be very large. These cruise ship propellers are powered by a variety of fuel sources, ranging from diesel to energy efficient methods or dual fuels.
Dual fuels move toward a cleaner emission and are increasingly environmentally friendly. While there are numerous advantages to this, not all cruise ships have come on board with the notion.
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Source: Cruise Hive
