# Changes in Propeller Shaft Behavior by Fluctuating Propeller Forces during Ship Turning
## 1 Introduction  
The first studies related to securing the stability of ships' shafting systems began in the United States Navy in the late 1950s and progressed in various ways throughout the 1970s [1]. In the 1960s, when shaft alignment was performed according to the conventional straight alignment theory, it was found that the shaft system could be damaged by the uneven distribution of the bearing load [2]. With the introduction of the fair curve alignment theory, which can guarantee the proper spacing and height adjustment between the shaft support bearings, the support bearings could be made to effectively share the loads of the propulsion shafts, leading to a significant reduction in the bearing damage caused by poorly distributed loads on the bearings [3; 4; 5]. In addition, a series of shaft optimization studies conducted in the late 1970s, along with the development of shaft alignment computation programs, played an important role in establishing the shaft alignment theory that is still used to this day [6; 7; 8; 9]. Meanwhile, the application of high-tensile steel plates in shipbuilding in the 1990s offered a great deal of flexibility to the hulls of large ships. This trend resulted in a higherdegree of deformation of ship hulls, whereas the strength of the propulsion shaft tended to increase to cover larger engines with high power. In other words, the engine compartment located in a double-bottom hull has become thinner and is easier to deform as a consequence of hull optimization; the opposite trend occurred with propulsion shafts [10]. Similarly to the target ship of this study, the oil tanker is a linear type of ship that exhibits the greatest hull deformation due to draught changes due to the characteristics of transporting oil cargo. As a result of such hull deformation, the offset of the propulsion shaft support bearing will change and the reaction force of the support bearing will also significantly change in a chain, resulting in damage to the shaft bearing. There are two parts that would suffer bearing damage in the propulsion system due to offset changes. Firstly, in the case of main engine bearing damage, damage to the affmost bearing in the vicinity of the turning wheel occurs as a wiping phenomenon due to overloading, whereas crank throw end bearing (the affmost crank throw bearing) damage occurs as a hammering phenomenon due to unloading. These types of damage have as their main cause the ship's deformation due to a change in the ship's draught. These types of damage can be reduced significantly in collaboration with main engine manufacturers who could recognize and address the load sharing problem in the crankshaft bearing [11; 12]. Meanwhile, the second issue, damage involved in the second after-stern tube bearing, remains serious. The major causes of such damage to the after-stern tube bearing are primarily the misalignment and excessive local load at the end of the after-stern tube bearing due to hull deformation [13; 14]. Therefore, in consideration of the major damage issues mentioned above, shafting system design has been carried out focusing on alleviating the local load of the rear end [15] and maintaining the minimum oil film thickness of the after-stern tube bearing [16; 17; 18] in accordance with the Rules for the Classification of Steel Ships and the guidelines of the shipyard. However, despite such design considerations in the rules and guidelines, if eccentric propeller thrust force fluctuations frequently occur in a dynamic or transient state, the after-stern tube bearing is highly likely to be damaged, regardless of how much the bearing local load fluctuation is predicted and prepared for by means of sophisticating machining with single-sloped or double-sloped processing. Another notable recent shaft system design perspective is to apply an ultra-long stroke engine to increase the ship's operating efficiency.