With reference to propellers, explain EACH of the following:
(a) TWO advantages of having high skew;(5)
(b) TWO advantages of aft rake.(5)
What is a Propeller?
In essence, a propeller is a rotating device with blades set at an angle (pitch), designed to generate thrust by interacting with a fluid, typically water or air. Think of it as a spinning wing or fan. As the blades rotate, they push against the fluid, creating a pressure difference that propels the vessel or aircraft forward.
2. (PDF) Propeller thrust explained by Newtonian physics. – ResearchGate
Key Components:
- Hub: The central part of the propeller where the blades are attached. 1. Propeller, screw propeller – Wärtsilä www.wartsila.com
- Blades: The airfoil-shaped blades that generate thrust. 1. Airfoil – Wikipedia en.wikipedia.org
- Shaft: The rotating shaft that transmits power from the engine to the propeller. 1. Propeller Shaft: Diagram, Parts, Function, Types, Applications – Testbook testbook.com
How It Works:
- Lift and Thrust: Similar to an airplane wing, propeller blades generate lift due to their shape and angle of attack. This lift, directed forward, creates thrust that propels the vessel or aircraft. 1. Aircraft Propeller Basics – Southern Wings www.southernwings.co.nz2. Propellers | How Things Fly – Smithsonian Institution howthingsfly.si.edu
- Pressure Difference: As the blades rotate, they create a lower pressure area in front and a higher pressure area behind them. This pressure difference pushes the fluid backward, creating a reaction force that moves the object forward. 1. If the propeller faces the high pressure of the incoming air, how can it generate low pressure in the front end to generate thrust? – | How Things Fly howthingsfly.si.edu2. Aircraft Propeller Basics – Southern Wings www.southernwings.co.nz
- Newton’s Third Law: This principle of action and reaction is fundamental to propeller operation. The propeller pushes the fluid backward, and in response, the fluid pushes the propeller (and the attached vessel or aircraft) forward. 1. Thrust and Newton’s Third Law in Aviation – Cal Aero Blog calaero.edu2. Aircraft Propeller Basics – Southern Wings www.southernwings.co.nz
Types of Propellers:
- Fixed-Pitch Propeller: The blade angle is fixed and cannot be changed during operation. It’s simpler and less expensive but less adaptable to varying conditions. 1. fixed-pitch propeller – ANAC www2.anac.gov.br2. Different Types Of Aircraft Propellers Explained In Detail – Acorn Welding www.acornwelding.com
- Controllable Pitch Propeller (CPP): The blade angle can be adjusted while the propeller is rotating, allowing for greater control over thrust and direction. This is particularly useful for maneuvering and optimizing efficiency. 1. Controllable Pitch Propeller (CPP) Explained – saVRee www.savree.com2. Improve manoeuvrability and efficiency for newbuild feeder vessels with a CPP – Wärtsilä www.wartsila.com
Factors Affecting Propeller Performance:
- Blade Shape and Pitch: The shape and angle of the blades significantly impact thrust, efficiency, and noise levels.
- Number of Blades: More blades generally produce more thrust but can also create more drag and noise.
- Diameter: Larger diameter propellers can move more fluid and generate more thrust, but they also have size and weight limitations.
- Rotational Speed: The rotational speed of the propeller affects thrust and efficiency. Generally, slower-turning propellers are more efficient but might require larger diameters. 1. How does change in propeller speed affect thrust efficiency? – for How Things Fly howthingsfly.si.edu2. Propeller – Wikipedia en.wikipedia.org
Applications:
- Marine Propulsion: Propellers are the primary means of propulsion for most ships and boats. 1. Marine propulsion – Wikipedia en.wikipedia.org
- Aircraft Propulsion: Propellers are used in many smaller aircraft and some larger turboprop aircraft.
- Other Applications: Propellers also find use in fans, wind turbines, and other applications involving fluid movement.
In Summary:
Propellers are essential components for generating thrust and enabling movement in marine and aviation applications. Their ability to convert rotational energy into forward motion through the interaction with a fluid makes them a vital part of various transportation and industrial systems.
What is Propeller Skew?
Blade Shape Variation: In a skewed propeller, the blade sections are not radially aligned. Instead, they are progressively swept back or forward along the circumferential direction as you move from the hub to the tip. 1. Identification of the geometric design parameters of propeller blades from 3D scanning | Journal of Marine Science and Technology – SpringerLink link.springer.com
Skew Angle: The degree of this sweep is measured by the skew angle, which is the angle between a line drawn through the blade tip at mid-chord and a line tangent to the mid-chord line at the projected blade outline. 1. Propeller skew angle – Wärtsilä www.wartsila.com
Advantages of High Skew:
- Reduced Vibration and Noise:
- Uneven Wake: The water flow into a propeller (the wake) is often uneven, especially behind the ship’s hull. This uneven flow can cause pressure fluctuations and vibrations as the blades pass through it. 1. Wake-field – Wärtsilä www.wartsila.com
- Gradual Entry: High skew causes different sections of the blade to enter the wake at slightly different times. This “spreads out” the pressure impulses, leading to reduced vibration and noise levels.
- Comfort & Longevity: Less vibration translates to improved passenger and crew comfort and reduced wear and tear on the vessel’s structure and machinery.
- Improved Efficiency:
- Reduced Cavitation: Cavitation, the formation and collapse of vapor bubbles, can occur when blade sections experience low pressure. High skew can help reduce cavitation by altering the pressure distribution along the blade, leading to improved efficiency and reduced erosion damage.
- Smoother Pressure Distribution: By spreading out the pressure pulses, high skew can lead to a more even pressure distribution on the blades, potentially improving overall propeller efficiency.
- Reduced Pressure Pulses:
- Less Fluctuating Forces: High skew lessens the fluctuations in pressure and forces acting on the blades and the propeller shaft, which can lead to smoother operation and less stress on the propulsion system.
- Reduced Tip Vortex Cavitation:
- Tip Vortices: The tips of propeller blades create strong vortices that can lead to cavitation and noise.
- Delayed Tip Vortex Formation: High skew can delay the formation of tip vortices, potentially reducing tip vortex cavitation and associated noise.
- Potential for Increased Thrust:
- Optimized Blade Loading: In certain cases, high skew can allow for better distribution of load along the blade, potentially leading to increased thrust for the same power input.
Considerations:
- Design Complexity: Designing and manufacturing highly skewed propellers is more complex, potentially increasing cost.
- Hydrodynamic Challenges: The effects of skew on propeller performance can be complex and interact with other design parameters. Careful hydrodynamic analysis and optimization are needed.
In Conclusion:
High skew offers several advantages in terms of reduced vibration, noise, cavitation, and improved efficiency, making it a desirable feature in propellers, especially on large vessels and those operating in challenging conditions. However, it’s essential to consider the specific application and balance the benefits of high skew against the increased design complexity and potential challenges.
What is Aft Rake?
Blade Inclination: Aft rake refers to the inclination or tilt of the propeller blades towards the rear (aft) of the vessel. 1. Propeller Rake Explained: It’s all About the Angle – BoatTEST boattest.com
Rake Angle: The angle formed between a line perpendicular to the propeller shaft and the line connecting the blade tip’s leading edge to the trailing edge at the hub is called the rake angle. In aft rake, this angle is positive. 1. Propeller Rake Explained: It’s all About the Angle – BoatTEST boattest.com
Advantages of Aft Rake:
- Improved Efficiency and Speed:
- Reduced Drag: Aft rake helps to lift the stern of the vessel, reducing the wetted surface area (the portion of the hull in contact with water) and, consequently, the hydrodynamic drag. This can lead to increased speed and improved fuel efficiency. 1. Propeller Rake Explained: It’s all About the Angle – BoatTEST boattest.com
- Enhanced Propeller Performance:
- Reduced Ventilation and Cavitation: Aft rake can help to minimize propeller ventilation (air being drawn into the propeller) and cavitation (formation of vapor bubbles), especially at higher speeds and when the vessel is trimmed up (bow raised). This improves thrust and reduces noise and vibration. 1. Propeller Rake Explained: It’s all About the Angle – BoatTEST boattest.com
- Better “Bite”: The angled blades provide a better “bite” on the water, especially in rough seas or during maneuvers that can cause propeller ventilation. 1. Propeller Rake Explained: It’s all About the Angle – BoatTEST boattest.com
- Smoother Operation:
- Reduced Vibration: By altering the timing of blade entry into the water flow, aft rake can help to reduce pressure fluctuations and vibrations, leading to a smoother and more comfortable ride.
- Maneuverability:
- Improved Bow Lift: The stern-lifting effect of aft rake can enhance the vessel’s responsiveness to steering commands, especially at higher speeds.
- Clearance:
- Increased Tip Clearance: Aft rake provides more clearance between the propeller blade tips and the hull, reducing the risk of vibration and potential damage.
Degree of Aft Rake:
- The amount of aft rake used in a propeller depends on various factors, including the vessel’s type, hull design, speed, and operating conditions.
- Typically, aft rake angles range from 5 to 30 degrees.
- Higher rake angles generally offer greater benefits in terms of efficiency and performance but might also increase drag and complexity of manufacturing.
Conclusion:
Aft rake is a design feature in propellers that tilts the blades towards the rear of the vessel. This inclination provides several advantages, including improved efficiency, reduced vibration and noise, enhanced maneuverability, and increased clearance between the propeller and the hull. It’s particularly beneficial for high-speed vessels and those operating in conditions prone to propeller ventilation or cavitation.
1. Propeller Rake Explained: It’s all About the Angle – BoatTEST