Question 56 1AE01 - First Assistant Engineer

The Correct Answer is C **Explanation for Option C (Cycloidal propeller):** The description provided—a flat disc set flush with the under surface of the vessel's hull, with a number of vertical, rudder-like blades projecting from it—perfectly describes a cycloidal propeller system, such as the Voith-Schneider Propeller (VSP). In this design, the vertical blades (often called cycloidal blades or foils) are rotated around a central axis while their pitch and angle are continuously and cyclically varied (hence "cycloidal motion") by an internal mechanism. This configuration allows for instant and precise control over thrust magnitude and direction (360 degrees) without needing a separate rudder, making it ideal for highly maneuverable vessels like ferries, tugboats, and offshore supply vessels. **Explanation for Incorrect Options:** * **A) Tandem propeller:** This refers to two separate propellers mounted on the same shaft, one behind the other, rotating in the same direction. It aims to improve efficiency by utilizing the slipstream of the forward propeller but does not involve a flat disc with vertical blades. * **B) Helicoidal propeller:** This is an alternative, archaic, or mathematical term for a standard screw propeller (the vast majority of marine propellers), characterized by blades twisting along a helical path (the helix). It consists of blades radiating from a hub, not vertical blades projecting from a flat disc. * **D) Contra-rotating propeller:** This system uses two propellers mounted on concentric shafts rotating in opposite directions (counter-rotation). This configuration is used to recover rotational energy lost in the slipstream of the forward propeller and increase efficiency, but it still uses standard screw-type blades, not vertical rudder-like blades on a disc.