Question

State how and why the following machinery items are affected when the maximum service speed of a vessel is consistently maintained in heavy weather:

(a) Intermediate shafting.

(b) Propeller shafting

(c) Shafting coupling bolts.

(d) Main thrust pads.

Answer 1

Part (a)

Torsional Stress: Resulting from the rapid changes in propeller speed (propeller racing) caused by variations in water resistance in heavy weather.
Compressive Stress: Due to the end thrust transmitted from the propeller.
Bending Stress: Caused by hull movements such as hogging (upward bending) and sagging (downward bending) in heavy seas.

Part (b)

Torque and Thrust: Caused by the propulsion forces transmitted through the shaft.
Torsional Stress: Due to the rapid fluctuations in engine speed when the propeller races.
Compressive Stress: Generated by the axial thrust from the propeller.
Bending Stress: Arises when the weight of the propeller acts on the shaft as the propeller emerges from the water during rough seas.

Part (c)

Bending Stress: From misalignments caused by hull deformations.
Shear Stress: Resulting from the whirling of the shaft and rapid engine speed changes during propeller racing.
Torsional Stress: Caused by the transmission of fluctuating torque.
Fatigue Failure: Due to the repeated application of fluctuating loads over time, particularly in heavy weather conditions.

Part (d)

Stress from Hull Movements: Misalignment caused by hull hogging and sagging.
Load Fluctuations: Due to variations in propeller thrust during racing and rapid changes in sea conditions.
Axial and Torsional Vibration: Arising from inconsistent propulsion forces and shaft vibrations.
Surface Wear and Damage: Resulting from increased pressure and friction due to fluctuating thrust forces.

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