Optimum Phasing of Engine and Propeller in Marine Propulsion Systems with Direct-Coupled Two-Stroke Engines

In marine propulsion systems with direct-coupled two-stroke large diesel engines, the angular position of the propeller with respect to the crankshaft remains unchanged during the whole operation of the engine.

The interference between major harmonic orders of the engine torque is avoided by choosing a number of blades, different from any divisor of the cylinder number. However, interferences between other harmonic orders of the engine torque and the major harmonic order of the propeller torque -which is equal to the number of blades- may increase the amplitude of torsional vibrations.

The paper shows that there is an optimum Phasing between crankshaft and propeller for which a decrease, up to 20%, of the dynamic shear stress of the shafting -in comparison with the worst possible phasing- may be achieved. This results points out the importance of being able to calculate, in the design stage, the best phasing of engine and propeller