In the drive to reduce the fuel consumption of inland vessels, one of the main limitations, the risk of propeller ventilation was investigated. The aim was to reduce the resistance of the vessel, while at the same time an acceptable margin against propeller ventilation had to be assured. A typical inland vessel has up to two high loaded relative large propellers, with tip regions in the lighter loading condition above the water line. To eliminate the risk of propeller ventilation, these ships often have inbuilt shape features such as tunnels and skirts, only having the intention to avoid air being drawn by the rotating propeller.
These devices however have a noticeable own resistance, imposing a mortgage to the ship’s total resistance. Current design practice for these devices is that as long as you cannot quantify a risk, take care of a sufficient safety margin. With the research done within the EU-funded STREAMLINE project new insights were gained into the prediction of air suction. As a result, the margin against ventilation could be reduced which resulted into a large reduction of the resistance of the vessel.
Reducing the high loading of propellers means to gain efficiency with at the same time reducing the risk of propeller ventilation as the suction by the propeller(s) can be decreased as well. This leads to the idea of a “distributed thrust” concept (DTC). Within STREAMLINE DST developed a sample case for such a novel design with six thrusters, with the aim to reduce ventilation against zero and to achieve maximal performance improvements. The measurements were carried out in MARIN’s Depressurized Wave Basin (DWB). These tests were carried out with a 1:10 scale ship model, in sailing condition, and depressurized conditions. In this way, the correct representation of cavitation and possible ventilation bubbles and vortices is ensured, resulting in a correct physical behavior. At the same time synchronized high speed video recordings were made to acquire insight in the occurring phenomena.
Within the project CFD calculations were carried out, aimed at characterizing the performance, loads, cavitation nuisance and ventilation risk in full-scale operating conditions. Validation of these calculations was done using the towing tank results.