Propeller Advantages: Skew and Rake
(a) Advantages of High Skew:
High skew refers to a propeller blade design where the cross-section of each blade is angled significantly off-axis relative to the rotational plane of the propeller. Here are two advantages of using propellers with high skew:
- Reduced Cavitation: Cavitation is the formation and collapse of vapor bubbles around a propeller blade, which can cause noise, vibration, and damage to the blade. High skew disrupts the smooth flow of water across the blade face, making it more difficult for cavitation bubbles to form and persist. This is particularly beneficial for propellers operating at high speeds or with high blade loads.
- Improved Efficiency at Oblique Angles: Unlike a straight-bladed propeller, which is most efficient when pushing water directly aft, a high skew propeller can maintain better efficiency even when the vessel is not traveling in a straight line. This is because the angled blades can still grip the water effectively even at oblique angles, reducing efficiency losses during maneuvers or when steering.
However, it’s important to note that high skew designs also have some drawbacks, such as increased drag at low speeds and slightly reduced overall propulsive efficiency compared to lower skew propellers.
(b) Advantages of Aft Rake:
Aft rake refers to the design where the tips of the propeller blades are angled slightly backward relative to the plane of rotation. Here are two advantages of using propellers with aft rake:
- Reduced Shaft Vibration: Propeller rotation can induce vibrations that travel through the shaft and into the vessel. Aft rake helps to mitigate these vibrations by altering the way the blades interact with the water. The angled blades tend to enter and exit the water more smoothly, reducing the forces that contribute to shaft vibrations.
- Improved Clearance: In vessels with limited propeller clearance between the hull and the bottom of the propeller, aft rake can provide some additional clearance. The angled blades are positioned slightly higher relative to the shaft axis, reducing the risk of the blade tips striking the hull, especially during rolling or pitching motions.
Aft rake also has some minor drawbacks. For example, it can result in a slight decrease in propulsive efficiency compared to a straight rake design.
In conclusion, both high skew and aft rake offer specific advantages for propeller design. High skew is beneficial for reducing cavitation and improving efficiency at oblique angles, while aft rake helps minimize shaft vibration and provide additional clearance. The choice of propeller design with specific skew and rake angles will depend on the vessel’s operational needs and priorities.