Axial vibration of the crankshaft and propeller shafting of motorships 1

Abstract

The crankshaft of every reciprocating marine engine and the propeller shafting driven by it are subject to axial as well as torsional vibration. The larger the number of cylinders and cranks in the engine, the more pronounced this axial vibration becomes. It is caused both by the radial forces acting on each crank and by the periodic thrust of the propeller. In the shafting of a cargo vessel of 12,600 gross tons propelled by a twelve-cylinder two-stroke diesel engine of 18,000 h.p., a troublesome fourth-order resonance of the axial vibration was reduced by about two-thirds by opposing the excitation from the propeller to that from the diesel engine. Rotation of the propeller relative to the engine through an angle which was fixed to an accuracy of a few degrees was quite sufficient to produce a substantial improvement. Analysis of the vibration measurements in three different relative positions of propeller and diesel engine finally proved that the propeller had been turned into its optimum position. No further improvement was possible.