Propulsion shaft alignment method and analysis for surface crafts

Abstract

This paper presents an approach to determine the changing displacement of the propulsion shaft due to the vessel's hull deflection. The model considered is a 60 m long shafting system of a ship consisting of 10 bearings, one gearbox and one main engine. This paper presents a comprehensive alignment analysis process for a long shafting system. A deflection analysis program, Shaftflex, is to shorten the analyzing time and to maintain consistency in the analysis process. Alignment of the first four bearings, which are exposed to the sea, has to be determined first and while on the launch way. As the ship hull is flexible, hogging occurs when it is afloat. It is observed that there is an inherent upward tilting of the shaft as such pre-tilting the projected shaftline downwards is necessary in order to counter the problem. Furthermore, as the shaft is more flexible than the ship's hull, it will be able to deflect as much as the hull and as a result causing variation in the loading at bearings. Therefore, the hull deflection characteristic of a vessel is a major concern in the analysis and alignment of a long shaft system. It is found that the alignment of the first four external bearings greatly affects vertical position of the gearbox and the main engine, which are aligned afloat and taking reference from the bearings. Numerical algorithm is also developed for such an analysis. The work presented in this paper is based on analytical geometry, arithmetic and trigonometry expressions. The derived mathematical expressions are incorporated into commonly used and comprehensive software for a quick analysis and parametric study.