The introduction of the Energy Efficiency Design Index in recent years has dictated the use of larger diameter slower rotating propellers. As a direct consequence a new generation of two-stroke diesel engines was released which have resulted to increased vibration issues for various reasons.  Even medium sized modern engines operate at speeds below 100rpm and it is not uncommon to see projects with an operating speed of the order of 65 rpm. The usable operating range therefore becomes limited and in the presence of a barred speed range it becomes even smaller. Finally, the use of the slow steaming principle as a means of lowering operating costs, has introduced more unknowns by changing the operating point of existing vessels. A detailed vibration analysis therefore becomes extremely important for the reliable operation of the propulsion system. In propulsion systems three major vibration modes are encountered which are described below:

1. Torsional Mode. The torsional mode is mainly excited by the main engine and in a secondary fashion from the propeller. The greater torsional excitation forces stemming from the ultra long stroke engines and the greater propeller inertia have resulted to increased vibtation amplitudes even in the presence of a torsional vibration damper. Additionally, the reduced power reserves during acceleration through the barred speed ranges result to increased passage durations and a high fatigue loading. Stress rising points such as filllets and coupling bolts have to be carefully designed in order to achieve an adequately high component life. The calculations determine the shear stresses on the shafts which should be within IACS limits and indicate the location and extent of barred speed ranges. 
2. Axial Mode. The axial mode is mainly excited by the engine crankshaft. The greatest amplitudes occur at the free end of the engine where an axial vibration damper is usually present. Axial deflections of crank webs during power strokes are closely coupled with torsional and therefore at least a two degree of freedom analysis is necessary in order to calculate forced damped results. Modern slow-speed engines present increased axial vibration amplitudes which stem from the ultra long stroke crankshafts. Problems usually arise in the case where a failure occurs in the vibration damper. In such cases, a barred speed range is imposed where vibration amplitudes exceed maximum values determined by engine manufacturers.  
3. Lateral Mode. The lateral mode (or whirling) is excited by lateral propeller dynamic forces and moments. Gyroscopic effects become significant at the propeller region and introduce forward and backward whirling modes whose natural frequencies differentiate as the rotating speed increases. During the design stage, care should be taken to avoid having resonances close to the Maximum Continuous Rating (MCR) of the vessel. Conventional vessels with two stern tube bearings and four bladed propellers rarely faced such issues under normal operating conditions. Modern vessels however which often employ only one stern tube bearing and large diameter five or six bladed propellers very often have a resonance very close to the MCR. In such cases, a forced damped anallysis is requested by the Classification Societies in order to prove that vibration amplitudes do not exceed certain limits.

Nautilas utilises fully 3D 6-degree of freedom FEA models of the complete propulstion system in order to analyse the vibration behaviour. Such a solution is beyond the current standards of the industry where single or two-degree of freedom models are usually the norm. The mass elastic model resembles closer the real system resulting to enhanced accuracy in natural frequency and forced damped amplitude calculations. Finally, our analysis reports are readily approvable by any major classification society.