The effect of waves on engine-propeller dynamics and propulsion performance of ships

Abstract

This paper investigates the effect of waves on the propulsion system of a ship. In order to study the propulsion in different wave conditions, a procedure for wake estimation in waves has been implemented. A clear drop in the propulsion performance was observed in waves when engine propeller dynamics, wake variation and thrust and torque losses were taken into account. This can explain the drop in vessel performance often experienced in presence of waves in addition to the effect of added resistance. Therefore, performance prediction of ships in rough weather can be improved if the effects of waves on the propulsion system are considered. Specific problems causing drop in performance have also been identified. System response in case of extreme events like propeller emergence has been simulated for analyzing the performance and safety of the propulsion system. The framework of enginepropeller coupling demonstrated in this paper can also be used to analyze different components of propulsion system (e.g. propeller shaft, control system) in higher detail with realistic inputs. This paper is a step towards optimizing the propulsion of ships for realistic operating conditions rather than calm water condition for energy efficient and economic ships.

Effect of waves on cavitation and pressure pulses of a tanker with twin podded propulsion

“…Although the current vessel and the propulsion system is much different from the one used by Taskar et al [17], this conclusion can still be valid,since wake varies much slower than the propeller RPM. Hence the changes due to wake variation can be assumed to be quasi-steady from the propulsion system point of view.…”

“…The amount of fluctuation depends on the inertia of engine, propeller and shaft, control system and the wake variation [17]. In the absence of engine-propeller model for the propulsion system of this particular vessel, the amount of RPM fluctuation has to be approximated.…”

“…Torque variations due to wake variation were taken from [18] to calculate RPM fluctuations using the method described above. Results were compared with RPM fluctuations calculated in [17] using engine-propeller coupled model. The above method predicts 4% fluctuations in RPM whereas fluctuations using engine-propeller coupled model are close to 3%.…”

“…170, 173 and 170 RPM, which correspond to an instant of highest average wake velocity (at t/T=0, 0.51 and 0) in λ/L= 0.6, 1.1 and 1.6 respectively. Simulations reported in [17] show that the RPM fluctuates in phase with the average wake velocity meaning that RPM is largest when the average propeller inflow is largest.…”

Formalization of design for physical model of the azimuth thruster with two degrees of freedom by computational fluid dynamics methods

“…It is often referred to as the cross-combination of thrusts [8]; -the presence of cavitation for heavy loads on propellers (intake of air) leads to the decrease in pressure on the propeller blades and can occur at low immersion of the propeller due to the motion of ship across the waves [9]; -extreme conditions with large amplitudes of the motion of the ship perpendicular to the surface of water leads to the sudden drop in thrust and torque with the hysteresis effect [10]; -simultaneous reduction of thrust and reverse thrust can occur due to the interaction between the flow from SP and the hull, caused by the effects of pressure, when the small thrust of SP passes along the hull. This is referred to as the Coanda effect [11][12][13];…”