Question

(a) Describe a transverse bow thrust unit using a controllable pitch propeller. Mention should be made of how it is supported and how the strength of thrust and reverse thrust are achieved.

(b) State with reasons. a suitable prime mover for the controllable pitch propeller.

(c) State whether the thrust unit delivers a relatively low-pressure head with high-volume output or high-pressure head with low-volume output.

Answer 1

Part (a)

A transverse bow thruster assists in docking, manoeuvring, or positioning a vessel, particularly at low speeds (typically below 4 knots). The most common arrangement is a tunnel thruster, consisting of a pipe tunnel running athwartship with protective guides at the ends and reinforcement bars along the top and bottom for added strength.

In a CPP-based system, the propeller blades’ pitch is controlled using a non-rotating servo motor housed within the gear housing. The servo motor operates based on input from the bridge:

Movement of the bridge lever moves the servo control valve piston, allowing hydraulic oil to flow into the appropriate side of the servo piston via the servo control block and check valve.
The force generated on the servo piston is transmitted via a push-pull piston rod inside the propeller shaft to the crosshead and crank mechanism in the gear housing.

This design enables the blade pitch to adjust, allowing the water flow direction to change as needed for thrust or reverse thrust.

Support for Strength of Thrust and Reverse Thrust:

Solid plate: Strengthens the bottom of the tunnel.
Centre girder: Provides longitudinal support to the tunnel from underneath.
Foot brackets: Reinforce the tunnel and prevent flexing under stress.
Tunnel ends: Welded to the hull plating or fabric piece using butt welding, integrating the tunnel into the vessel structure for increased rigidity and durability.

Part (b)

The ideal prime mover for a CPP-based bow thrust unit is a non-reversing prime mover, such as:

Diesel engine
Single-speed induction motor (e.g., a squirrel cage induction motor).

Reasons for Suitability of an Induction Motor:

The CPP system allows pitch adjustment, so the motor does not need to stop during maneuvering operations. The propeller blades can be placed at neutral pitch when no thrust is required.
Induction motors are reliable and require less maintenance.
Equipped with either a star-delta starter or an electronic soft starter, ensuring smooth operation and minimal wear on components.
The motor provides uninterrupted power, allowing precise and efficient control of thrust direction and strength

✓ Verified Model Answer (Text Solution)

The ideal prime mover for a CPP-based bow thrust unit is a non-reversing prime mover, such as:

Reasons for Suitability of an Induction Motor: