The main goals of achieving a good propeller shaft alignment are to:

• Ensure uniform load distribution: This makes sure that the loads on the bearings are distributed evenly and stay within the limits specified in the design.
• Achieve smooth power transmission: By minimizing vibrations, noise, and power losses, the system operates more efficiently.
• Prevent excessive wear: This protects the bearings, seals, and couplings from wearing out prematurely.
• Avoid system damage: Proper alignment prevents shaft bending, crankshaft deflection, and stresses from misalignment that could lead to cracks or eventual failure.

To get a good alignment, several conditions must be met:

• Correct bearing heights: The bearing offsets must be precisely adjusted so the shaft forms a smooth, continuous curve.
• Proper bearing contact: The shaft must have adequate contact with the bearing surface to prevent "edge loading," where the weight is concentrated on the edges of the bearing.
• Allowance for hull deflections: The alignment must account for the ship's movements like hogging (bow and stern droop) and sagging (center droops) as well as changes due to thermal expansion.
• Correct coupling alignment: Accurate "sag and gap" measurements at the coupling faces are essential to ensure the shafting sections connect correctly without stress.

Fair curve alignment means the propeller and intermediate shafts form a smooth, continuous curve when placed in their bearings. There are no abrupt bends or steps at the bearing points. Instead, each span of the shaft is slightly deflected so it rests naturally on the bearings, distributing loads evenly. This method is crucial because it prevents localized stress concentrations and avoids putting excessive loads on any single bearing.

Sag and gap are measurements used to calculate and verify the alignment of shafting sections, particularly at the coupling flanges.

• Sag: This is the vertical offset measured between the top and bottom of the coupling flanges. It indicates the vertical angular misalignment between the shafts.
• Gap: This is the horizontal offset measured between the coupling flanges on the port and starboard sides. It indicates the horizontal angular misalignment.
• Together, these values are used to adjust the shaft alignment so that the shafts mate precisely and transmit power without inducing bending stresses in the system.