On the working principles of Energy Saving Devices

The aim of this paper is to explain the principal mechanisms for the reduction in power demand by so called “Energy Saving Devices” (ESDs). This paper follows a similar approach as taken by Wald (1965) and Dyne (1995), where the authors use considerations of the momentum and energy equations.
Different energy saving concepts are evaluated on their contribution to the abatement of energy losses. The discussion on hydrodynamic mechanisms is then focused on Pre- or Upstream Ducts where four different mechanisms are hypothesized. The effect of these mechanisms is then investigated for a systematically varied series of Pre-Ducts on an axisymmetric body in deeply submerged conditions. It is concluded that the benefits of this Pre-Duct configuration do not come from the same principles as used by ducts around propellers, nor that they come from an improved propeller-hull interaction. It is recommended to further study the remaining hypothesis that a Pre-Duct conditions the flow into the propeller in a favorable way so that the propeller operates at a higher efficiency.