Comparing azimuthing thrusters and conventional shaftlines for propulsive efficiency of pushboat- barge convoys


Azimuthing thrusters can serve as an interesting alternative to conventional shaftlines. But how does this affect powering performance and efficiency?


Joost Schot & Erik Rotteveel, j.schot@marin.nl


Pushboat-barge convoys offer great flexibility for inland cargo transport, being able to transport cargo on extremely shallow rivers. However, the meandering nature of rivers requires the convoys to have optimal manoeuvrability performance. Therefore, azimuthing stern drive thrusters (ASD) can provide an interesting propulsion alternative.


Azimuthing thrusters eliminate rudders, shafts and struts, which would normally increase the total ship resistance for a design with conventional shaftlines. However, thrusters require a more complex drivetrain with additional gears. The allowable propeller


diameter of the azimuthing thrusters is reduced as well, because of the space needed to rotate the thrusters. MARIN is investigating the differences between both configurations at multiple water depths, focusing on powering performance, as well as manoeuvrability.


A RaNS-BEM approach is used to calculate the propulsive performance of both configu- rations. The flow around the pushboat-barge convoy is computed with our CFD code ReFRESCO for viscous flows, while the propeller flow is calculated using a Boundary Element Method. The calculations are performed with propulsors underneath the pushboat


Figure 1: Cut-through of flow around fixed-propeller push boat with 6 barges at 3.6 meter draught


and for open-water conditions to better understand the propeller-hull interaction effects.


ASD more efficient in deep water For the shaftlines’ configuration, the nozzle thrust significantly decreases in the in-behind conditions compared to open water conditions. It was found that both the rudder and the propeller tunnel downstream of the propulsors have a negative effect on the thrust provided by the nozzle, leading to a reduction of the efficiency of the propulsion system.


Based on our calculations, in deep water (more than 6 metres), the ASD system is slightly more efficient. In shallow water, the design with conventional shaftlines is more efficient compared to the ASD system. Mechanical efficiencies still need to be included, but the mechanical efficiency is expected to be lower for the ASD system compared to conventional shaftlines.


Apart from powering, manoeuvring perfor- mance is key when selecting an ASD system over conventional shaftlines. We will continue the investigation for both propulsion systems using manoeuvring simulations in typical river conditions.


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