Propeller Terminology Explained:
(a) Skew:
- Skew refers to the angle at which the propeller blade sections are twisted relative to the plane of rotation. Imagine looking at the propeller from the front. In a propeller with no skew, the blade sections would appear flat. With skew, the blade sections are angled off-axis, typically leaning either forward or aft.
(b) Rake:
- Rake refers to the angle of the propeller blades relative to a line perpendicular to the propeller shaft axis. There are three main types of rake:
- Zero Rake: The blades are perpendicular to the shaft axis.
- Aft Rake: The tips of the blades are angled slightly backward relative to the shaft axis.
- Forward Rake: The tips of the blades are angled slightly forward relative to the shaft axis.
(c) Pitch:
- Pitch is a concept similar to the lead of a screw. Imagine the propeller advancing through a solid medium like a giant corkscrew. The pitch is the theoretical distance the propeller would advance in one revolution if there were no water slippage (explained in term (d) below). A higher pitch angle results in a larger theoretical advance per revolution, but requires more torque from the engine.
(d) Slip:
- In reality, propellers operate in water, which yields and does not provide a solid medium for perfect forward thrust. Slip refers to the difference between the theoretical advance (pitch) and the actual distance the vessel travels forward in one revolution of the propeller. Slip is expressed as a percentage of the pitch. For example, a propeller with 10% slip would travel 90% of its theoretical pitch distance per revolution. Slip is inevitable, but a well-designed propeller aims to minimize it for optimal efficiency.