Study on shaft alignment of propulsion shafting system depending on single reaction force supporting position of aft stern tube bearing

Trends of large-scale ships have seen propulsion shaft and propeller sizes increase. This has enabled shafts to have greater stiffness, yet has caused flexibility to lessen and induce bearing failure at the aft stern tube bearing. In general, shaft alignment is calculated and evaluated in accordance with classification societies’ rule requirements. Especially, positioning reaction support and stiffness of aft stern tube bearing are based on the practical experience of shaft alignment. Therefore, in this study, to evaluate the feasibility of the reaction force supporting position and stiffness of the aft stern tube bearing as recommended by classification societies in shaft alignment, theoretical reaction force supporting positions for various ship propulsion shafting systems were examined and the differences were evaluated. The reaction force supporting position of the aft stern tube bearing provided by classification societies was evaluated and it was found that a propeller shaft diameter less than 600 mm is within the provided range. However, in the evaluated shafting system, a propeller shaft diameter of more than 600 mm tends to cause the ship to move to the forward side due to increased shaft stiffness.