ship's hull structure through a thrust bearing (and possibly an axial damper) and the ship's double bottom. 2 Excessive superstructure vibrations worsen the working conditions of the ship's crew and may influence maritime safety detrimentally.
Generally, axial vibrations are only dangerous for propulsion systems with slow-speed, two-stroke engines and propellers directly driven by shaft lines. 3 Nowadays, all slow-speed engines are equipped with an axial damper. If the damper is well regulated, axial vibrations are no danger to the main engine. Nevertheless, these vibrations have to be controlled by checking (by calculations and/or measurements) the vibration amplitude of the free end of the crankshaft. Numerical analyses should determine the operating restrictions for a main engine with a failed axial damper. Usually the maximal rotational speed of the engine is limited.
A determination of the coupling degree of the axial vibration with other vibrations, crankshaft deformation analysis, and torsional vibration calculations are necessary in order to get an appropriate determination of the axial vibrations. An appropriate determination of the dynamic characteristics of the engine's main bearings and thrust bearings, and of the axial dampers of the propulsion system may be crucial for a suitable estimation of the axial vibrations.
However, it is not only the dynamic behavior of the propulsion system which should be analyzed during ship design. Calculations of the ship's hull and superstructure vibrations are nearly as important as dynamic analyses of the power transmission system. 4 These are not separate problems. The thrust bearing and axial damper reactions (coming from the vibrations of the longitudinal propulsion system) are one of the ship's hull and deckhouse excitation forces. Therefore, these dynamic reactions should also be determined.
I created a computer algorithm to analyse the longitudinal vibration problem. Nonlinear algorithms to determine the dynamic stiffness and damping characteristics Abstract Calculations of the axial vibrations of a marine power transmission system are a very difficult problem owing to the complicated couplings and difficulties in determining the boundary conditions. The torsional-bending-axial coupling action of the system should be accounted for when considering its dynamics. A determination of the mutual interference of system vibrations and their boundary conditions is also necessary. A performance analysis of the main engine bearings, the thrust bearings, and the axial dampers should also be carried out. Thus, the effects of additional bending stresses in the crankshaft and possible vibrations of the ship's structure due to the reaction force in the thrust bearings should be considered. I have devised a computer program to analyse the axial vibration problem. The numerical analysis method presented is compared with measurements (performed on real ships) and verified by them.