Q8
(a) Describe the fundamental principle of a propeller.
(6)
(b) A propeller 6m in diameter has a pitch ratio of 0.9, BAR 0.48, and, when turning at 110 rev/min, has a real slip of 25% and wake fraction 0.30. If the propeller delivers a thrust of 300 kN and the propeller efficiency is 0.65, calculate:
(10)
(a) Blade area
(b) Ship speed
(c) Thrust power
(d) Shaft power
(e) Torque
Reference Answer
### Part (a): Fundamental Principle of a Propeller A marine propeller generates thrust to propel a vessel by converting the rotational power from the main engine into hydrodynamic force. The fundamental principle is based on a combination of Bernoulli's principle and Newton's Third Law of Motion, applied to the propeller's blades which act as rotating hydrofoils. 1. **Hydrofoil Action (Bernoulli's Principle)**: * Each propeller blade has a cross-section similar to an aerofoil, known as a hydrofoil. The face of the blade (pressure side) is relatively flat, while the back (suction side) is cambered or curved.
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