Pumpjets in de binnenvaart

Bigger ships are replacing the small ships, also at the Amsterdam-Rijnkanaal. A direct consequence is the use of larger main propellers and increased installed engine power in order to realize acceptable ship speeds. To improve the maneuverability, the power of auxiliary bow and stern propulsion systems has increased as well. This results in higher flow velocities and thus more damage to bed protections at berthing places. Especially pump jet thrusters as a bow propulsion system may attack the bed protections more severely due to a vertical component of the flow field induced by the pump jet which is directed towards the bed. The aim of this research is, in cooperation with Rijkswaterstaat, to develop an analytical model for the flow field induced by the pump jet thruster. Subsequently, the required dimensions of a bed protection has been determined as well as an investigation on the possible scour effects in case of an unprotected bed of the Amsterdam-Rijnkanaal. In order to meet this goal, an inventory of the bow propulsion systems in inland vessels has been made by conducting an inquiry. Subsequently, the different propulsion systems have been compared qualitatively. Since existing formulas are based on the circular free jet, this system is also treated in the comparison. In addition, a water jet (hydro jets can be distinguished in low-powered and high-powered jets), often used as propulsion system by ferries, has been taken into account due to the strong analogy with the circular free jet. As a result, both the ratio of decrease of axial velocities and diffusion of the jets in radial direction of a circular free jet strongly correspond to a high-powered water jet. However, the free propeller and transverse tunnel thrusters are considered to have a faster decrease of axial velocities and a faster diffusion of the flow field. The applied power and the induced turbulence seem to be the main reasons for the flow field characteristics. An essential part of this research is the derivation of an analytical model for the pump jet thruster induced flow field. The principle is based on the free propeller theory. In the case of a pump jet, part of the contraction takes place inside the unit. Furthermore, the flow field has been subdivided in three parts: initial outflow velocity, axial flow velocity and the flow velocity distribution in the region of the established flow. The coefficients in the model have been empirically determined using a simulation with a CFD model. This model has been compared to flow fields induced by different other propulsion systems. It results in a similar axial flow velocity decrease as a low-powered water jet. However, the pump jet diverges faster and with a greater angle than any other jet. The derived model has been used to investigate the stone stability at the toe of a sheet pile wall. Firstly, the flow velocities at the bed have been evaluated by looking at berthed ships in different conditions (fully loaded, partly loaded and unloaded). Secondly, a stability analysis of the bed material has been executed by using a method presented by Deltares. However, in order to protect the bed against the high flow velocities, large stone diameters are required. An alternative is allowing scour holes to occur, but the length of the sheet piling should be extended in order to maintain stability of the entire quay wall. The predicted maximum scour depths caused by a pump jet are in the same range of measured scour depths at the Amsterdam-Rijnkanaal. This means that scour holes may cause instability problems of the existing sheet pile walls.Hydraulic EngineeringCivil Engineering and Geoscience