Without a power source and propellers, cruise ships would drift aimlessly at sea. These mega-ships require engines, fuel, and massive propellers just to move through the water and keep all activities on board the ship running. We describe not only the cruise ship propeller size, but also how it integrates with the rest of the engine.
Older cruise ships use diesel engines for propulsion. The power generated from the engine goes through a transmission and into the propeller shafts.
The transmission determines propeller revolution like the transmission in an engine transfers RPM to a manageable speed to power the rear wheels in a car.
Meanwhile, modern cruise ships use diesel-electric engines or gas turbines to propel the ship through water. The ship’s systems are also used to guide and move the ship through the ocean.
The larger the ship, the higher the demand for electrical power. There are cruise ships that rely on two different sources of power. One is dedicated to electrical power, while the other is for propulsion.
In This Article…
- Cruise Ship Propeller Size
- How Do Propellers Work?
- How Do Azipods Work Compared to Traditional Propulsion Systems?
- What Is Propeller Cavitation?
- Effects of Cavitation on a Cruise Ship’s Propellers
- How Is Cavitation Prevented?
- Which Cruise Ships Have the Largest Propellers?
- How Fast do Propellers Spin?
Cruise Ship Propeller Size
Propellers can be up to 20 feet in height. Often referred to as ABB Azipods XO, modern cruise ships tend to have three of them to properly maneuver the ship and propel it forward in water. They are efficient on fuel compared to a traditional system.
As mentioned, they allow for improved maneuverability, allow the ship to maximize speed, reduce bad emissions, and optimize the overall performance of the cruise ship.
An ABB Azipod system has a large impact on the operating efficiency of a cruise ship. It effectively reduces bad emissions and energy consumption by 20%. This propulsion system resides in the ship’s aft outside of the hull. They can rotate by 360° using a rudder, which provides thrust in any direction. This is not possible with a conventional system.
This type of propeller contains three main components:
- Frequency Controller – This is designed to change the supplied power’s frequency, so the rotating motor speed is more easily controlled.
- Supply Transformer – The power, which is produced by a generator, is 6,600 KV. This is stepped down to the required voltage used by a supply transformer. Then, it is transferred to the motor located in the pod.
- Propulsion Motor – Designed to drive or produce thrust, its rotation is generated using an electric motor.
How Do Propellers Work?
Propellers are required to push a cruise ship through water. Commonly referred to as “screws,” they slice through the water. They also provide a reverse and forward motion. Cruise ship propellers do not need to turn quickly since they rely on brute power or torque instead of high speeds or RPM. This is what causes a ship to move slowly, as these vessels rarely reach 30 knots.
Cruise ships that are newer and more cutting edge may use azimuth thrusters. These are pods housing propellers, which rotate 360° and provide optimal maneuverability. They were designed to replace rudders and are touted as more beneficial compared to screw-type propeller systems. Benefits include improved fuel efficiency and shorter stopping distance.
These types of propellers allow cruise ships to turn quickly in water when necessary to avoid collisions, which is why more vessels are being equipped with this technology.
How Do Azipods Work Compared to Traditional Propulsion Systems?
The term “Azipod” combines the words “azimuthing” and “pod.” The phrase “Azipod” is a registered trademark of ABB Oy. They work by having the propeller located on the front of its pod. This allows the cruise ship to be pulled through the water rather than being pushed as in a traditional shaft and propeller system.
With a traditional system, its propeller is rotating in water that has been disturbed by the moving of the propeller shaft, brackets, and framing. With the pod system where the propeller is found in the front of its pod, it is turning in water that has been undisturbed. This allows the pod to be 5%-6% more efficient due to improved water flow.
Traditional systems are noisier when maneuvering due to the cavitations of its propellers. This doesn’t occur with pods since they are tractor pods. Tractor pods comparatively pull a ship through surrounding water. Clean, smooth water moves into the blades, thereby decreasing noise.
Pod systems free up additional space in the hull. Traditional systems meant the motors and propeller shafts had to be placed in the hull. Modern cruise ships not containing pods require stern thrusters to maneuver the cruise ship while undocking and docking.
On the other hand, pod systems use the equivalent of a drive motor and propeller shaft, which are found in the pod as opposed to the hull. There is also no need to use stern thrusters.
Many cruise ships contain two pods. Allure of the Seas and Oasis of the Seas contain three pods. Several may be designed using a combination of azimuthing pods and pods which don’t rotate (fixipods) for forward/backward motion.
What Is Propeller Cavitation?
This is the formation and dissolving of vapor bubbles (vapor filled voids) found in liquid mediums. Since the propeller rotates through water at an angle, a suction and pressure side exist on each of the propeller blades. The faster a blade slices through water, the lower the pressure becomes with the suction side.
At a certain speed, the pressure found on the suction side will become so decreased that water running over it will evaporate. When this occurs, voids or vapor bubbles form. It occurs at a normal air pressure (1013.24 hPa) with the water evaporating at 100°C. At higher pressures, evaporation temperature also increases. The opposite happens at lower pressures.
When local pressure goes down below vapor pressure, the water will evaporate. Then, a cavitation bubble forms, becomes bigger, and is transported by flowing to an area with higher pressure. The bubble then ceases to grow any further. When local pressure exceeds vapor pressure, the vapor will condense, beginning from the bubble’s wall.
Eventually, the surface breaks down, beginning with the weakest location, thereby causing the bubble to implode. While it is imploding, water fills the space where the void is, causing a pressure surge. This creates pressure waves along with high-pressure peaks.
Should the vapor bubbles be close to or directly on a solid wall surface like the blade of a propeller, the implosion generates a liquid microjet exerting high stress on the surface. This sudden energy release creates pressure loads found on the surface, generating crate-shaped erosion of material in a propeller blade.
The bubble layer pack on a surface of a blade will also make them thicker, thereby increasing resistance moving through water. This impacts the propeller’s performance. Cavitation also generates vibrations and popping noises. This, in turn, affects guests and ships.
What Causes Cavitation?
The primary cause of cavitation is an unevenness in the blade’s leading edge, too much sharpness of the edge, increased curvature of the propeller blade, or poor finishing of the surface of the blade.
Effects of Cavitation on a Cruise Ship’s Propellers
The effects on the propellers are:
- Eroding of the blades
- Decreased performance
As cavities collapse, they generate noise and high-frequency vibrations. This can be unpleasant to crew and passengers if they are located at the aft of a cruise ship.
Vibration is generated from the unsteady characteristics of cavitation. It is the periodic deflections of the propeller in a horizontal, vertical, or torsional manner.
Vibrations are further reduced by using Schneekluth or Mewis ducts. These are fitted on the hull of the ship, or before the propeller. This provides a uniform wake flow into the propeller thus reducing vibrations.
This can be a serious effect. Cavitation bubbles once formed are unable to persist if they move into an area where net pressure goes back to a value which exceeds the water’s vapor pressure. It can happen toward the blade’s trailing edge or as it moves from top to bottom of its circle, thereby gaining hydrostatic pressure.
The way they collapse creates a new phenomenon. Bubbles will then contract to minute sizes prior to disappearing. This causes the entire energy to collapse, causing a change of state to be concentrated into a tiny location.
At the beginning, the damage looks like it has been hit with a tiny hammer, leaving small, circular indentations. If the damage is prolonged, the result is serious pitting in the metal. This looks like a definite crater on the blade’s surface. Continual pitting can erode the entire thickness of the propeller blade, creating a hole.
When it comes to performance, the impact can be considerable. Cavitation begins at the tips of the blades and gradually consumes the remainder of the blade as propeller loading increases. As cavitation extends to .75 of the radius, there is significant loss in the thrust plus a reduction in torque. There will be a significant increase in revolutions with a given power.
Thrust breakdown will begin quicker than a change in the torque. This can result in a loss of efficiency.
How Is Cavitation Prevented?
The hull of a cruise ship influences the flow going around the propellers since they fully work in a cruise ship’s wake. To avoid this, the follow measures are undertaken:
- Reducing revolutions per minute. Since the thrust varies, reducing revolutions decreases cavitation. However, the downside is a loss of speed.
- Generate a maximum immersion.
- Reduce the blade’s thickness. This is done by utilizing stronger materials, which are resistant to cavitation.
- Avoid suction peaks close to the leading edge. This is done by utilizing a suitable amount of camber and a good shape for the entrance.
- Design the stern so that it achieves a uniform wakefield.
- Avoid high sections on the blade’s backside. This is achieved by using section shapes providing a uniform pressure distribution.
- Pitch can be varied over the blade’s length. This diminishes loading in important areas.
- Decrease angles of incidence and blade angles by creating a slightly bigger diameter.
- Make the total blade area larger. This is done by increasing the propeller’s diameter with a reduction of revolutions or your blade area ratio with a constant diameter.
Which Cruise Ships Have the Largest Propellers?
Royal Caribbean’s Oasis Class
These cruise ships tend to be the largest in the world. Their propellers are designed to have a diameter of six meters. The ship’s propulsion contains three 26,800-horsepower Azipods, which is an electric azimuth thruster.
How Fast do Propellers Spin?
Since a cruise ship’s engines turn their crankshaft at extremely high rates of speed – hundreds or thousands of revolutions per minute – the engine connects to the ship’s propellers using gears. The propellers themselves are created to turn slower, which tends to be around 250 revolutions per minute or less.
Carnival Cruise Line Fantasy Class
This cruise line’s service speed is 22.3 knots. It was the first to use a unique electric azimuth Azipod system for propulsion. This means that the ship is pulled as opposed to pushed by its propeller. The Carnival Elation contains two 14 MW Azipod units.
When it comes to a cruise ship propeller size, they can be as large as 20 feet high. This is no surprise when you consider the size of the engines and everything that is necessary to move a ship. Thanks to the introduction of Azipods, cruise ships are better able to navigate around any obstacles they may encounter in the water.
Read Also: How Big is a Cruise Ship?
Not only are they better equipped to move around potential hazards, but this improvement in technology also allows them to better move into ports, work more efficiently, and save on fuel costs. With constant pressures to become more environmentally friendly, this may be a good thing for cruise lines.
While they aren’t great for the environment, they aren’t emitting as much pollution as fuel is being conserved. It also makes this natural resource stretch for a longer duration. Ideally, LNG-powered cruise ships can be used to prevent harmful emissions. This is the best method to protect the environment.
However, with the drawbacks of using this fuel type, changing the propulsion technology may be the next-best thing.