Question

The stresses in a shaft are:
(i) Shear stress due to torsion
(ii) Normal stress due to bending moments
(iii) Axial stress

Which ONE of the following options lists the stresses that are considered while designing a circular propeller shaft?

• A. (i) and (ii) only
• B. (ii) and (iii) only
• C. (i) and (iii) only
• D. (i), (ii) and (iii)

Hint:

Think about every load a propeller shaft carries: torque from the engine, its own bending, and the propeller's thrust.

Answer 1

The Correct Option is D

A ship's propeller shaft does not carry just one simple load. While it spins, the engine drives it in torsion. The propeller's weight, misalignment and bearing reactions bend it. The propeller's thrust pushes along its axis. All three effects act together and must be added when the shaft is sized.

1. (i) Shear stress due to torsion: the engine transmits torque through the shaft to turn the propeller, so torsional shear stress is always present and is usually the dominant stress in the calculation.
2. (ii) Normal stress due to bending moments: the overhung weight of the propeller, the shaft's own weight between bearings, and any misalignment create bending moments. A bending normal stress develops too, and cannot be dropped.
3. (iii) Axial stress: the propeller pushes the ship forward by sending thrust along the shaft axis, which loads the shaft in direct compression. So an axial normal stress is present as well.

Since torsion, bending and axial thrust all act on a real propeller shaft at once, a proper design combines all three. So option D, (i), (ii) and (iii), is correct.