Figure 2 Figure 1: Dependency of power need on vessel draft


100 120


20 40 60 80


0 5 Figure 3 i 4 Shi 10 15 20 speed [kn] Figure 4: Ship power utilizationtili ti 25 30


count at an early stage and the bulbous bow has to be developed for a wider range of operating speeds, trim and draught con- ditions. Results of such studies can be found in Figure 1.


Propulsive efficiencies Although it is not a big achievement to design a propeller with an efficiency rate of close to 90%, it does become an unprecedented challenge when the main engine and aftbody shape constraints are added into the equation. The delicate balance between efficiency and cavitation-induced vibrations and erosion is typically met today by designing propellers with contemporary tools, such as lifting line and lifting surface codes. Cavitation nuisance assessments (usually vibrations and erosion but in the future likely to include noise con- straints) are typically being dealt with in a heuristic manner, using experience to assess whether the observed cavity extent and dynamics are likely to lead to problems. But this approach can lead to large uncertainties. Recent exploratory studies have demonstrated for instance, that propeller efficiencies can be increased when allowing for larger cavity extents (see Figure 2).


R&D efforts are ultimately aimed at exploit- ing the full potential of the optimum integration of propeller and aftbody design, thus allowing for increased efficiencies at


similar comfort and safety levels. For exam- ple, integration can lead to better use of pre-rotation in the wake (asymmetric aft- body) or to the application of a recess in the hull for an increased propeller diameter. If the current R&D developments are success- fully implemented in the industrial environ- ment, the attainable propulsive efficiencies are expected to increase by some 5% to 10% in the next decade. Today, the advanced propeller analysis and design tools outlined are partly being used in the design process of propellers and hulls. MARIN is focusing efforts on extend- ing the use of these tools to further explore propeller and hull design space, leading to a better exploitation of the tools and adding to our long-standing experience in propeller and hull design.


Energy-saving devices Another way of increasing efficiency is the use of energy- saving devices as retrofit measures. Most of these devices are used to enhance the flow over the aftbody, thereby enhancing the flow into the propeller and consequently, reducing vibration and increasing efficiency. If a new propeller design is also used, over- all efficiency can increase even more. An example is the vortex generator. This small device is attached to the hull up- stream of the propeller at such a location that the vortex generated by the device can


positively influence the flow into the propeller plane. It can reduce cavitation and vibration problems and together with an updated propeller design, improve efficiency.


Another way of energy saving is by reducing energy losses due to flow obstruction, ero- sion and vibration on appendages. A lot is known about flow alignment of appendages such as bilge keels, stabiliser fins and struts but nowadays, the rudder too, is an impor- tant appendage to look at. Due to the rotation of the flow behind the propeller, the rudder encounters different angles of attack leading to high-pressure peaks on the rudder surface, cavitation and eventually rudder erosion. By twisting the rudder over its length in such a way that the local angle of attack is reduced, the pressure peaks will decrease and cavitation and erosion will be avoided. Together with optimised rudder profiles, the overall propul- sive efficiency will increase. An integrated design approach is the answer to the ever-increasing energy reduction demand. By combining established design approaches and new sophisticated technol- ogies, the energy management of ships can be brought to an even higher level, making them well prepared for the coming decades.


1. IMO.MEPSC 59/INF.Y & MEPC 59/INF.Z report 11


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