Shipping is the dominant form of transportation, carrying approximately 80% of world trade. This is because, despite air, road and rail, most of the long distance and a great deal of the short distance transportation is made by ships. This includes all kinds of goods, from highly industrial products, meat, and fruit to grain, ore, and oil.
Safe and efficient ships and sea transportation are essential for the modern world. These topics are dealt with in the Section of Maritime Engineering at the Department of Mechanical Engineering. One of the topics is propulsion of ships, in particular propellers. The propeller must transfer the power generated by the ship’s engine into power to propel the ship with as little loss as possible. Worldwide, the total power installed in ships is approximately 6,800,000 kW and the annual fuel consumption is more than 7,000,000 tonnes. Even a relatively small increase in efficiency will mean a large saving in oil, accompanied by a similar reduction in the emission of exhaust gases.
The KAPPEL propeller is a new, innovative propulsor with higher efficiency than a conventional state-of-the-art propeller. Whereas traditional ship propellers have blades modelled on the basis of helical surfaces, the KAPPEL propeller has modified blade tips smoothly curved to the suction side of the blade. There is a parallel development within aircraft design where many modern aircraft, from high-performance jet liners to sophisticated gliders, have similar modifications of the wing tips in the form of winglets. These are separate lifting surfaces attached more or less perpendicular to the wings on the wing tips. Numerical methods, as well as experiments, show that the effect of winglets is to increase the lift-drag ratio of the wing.
Aircraft have relatively well-defined design conditions such as climb, cruise and descent. The flow to the propeller is more complicated since the propeller works in the flow abaft the ship hull. This is in particular the case for single screw ships. At each revolution a section of a propeller blade will experience a highly varying inflow. This means that the pressure on the hull varies in time, giving rise to noise and vibrations in the ship. The pressure variation is exacerbated by cavitation, a phenomenon that occurs when the suction of the propeller locally evaporates the water.
One of the challenges of the KAPPEL propeller design was the optimization of the propeller with respect to efficiency. When modifying the geometry of the blade tip, relative to a conventional propeller, it was of paramount importance that the beneficial effects of the modified blade loading were annulled by the detrimental influence of friction on the relatively larger blade area in the tip region. This optimization was made on the basis of numerical fluid dynamics by which the flow field around the propeller was computed and hence the performance of the propeller. The calculations were complemented with model experiments. Further model tests were necessary to examine the interaction between ship hull and propeller, in particular the extent and time history of cavitation and the pressure field on the ship hull. On the basis of a vast number of calculations and comprehensive model testing, a design was developed for a 35.000 dwt product carrier.
KAPPEL propeller behind the 35000 tdw product carrier M/T Nordamerika of Dampskibsselskabet Norden, A/S
A full-scale KAPPEL propeller for this ship was manufactured. It was tested at sea immediately after tests with the conventional propeller originally designed for the ship. Both sea trials took place in April, off the coast of Portugal, in good weather and under comparable conditions. The results confirmed the model test predictions that the improvement in efficiency of 4 per cent aimed at was achieved. Furthermore, the pressure pulse level was slightly lower with the KAPPEL propeller than with the state-of-the-art comparator propeller.
Numerical boundary element model of KAPPEL propelle
The project was partly sponsored by EU and included: J.J. Kappel Marine Concept, Danish Maritime Institute, Department of Mechanical Engineering DTU, Stone Manganese Marine, Hamburgische Schiffbau-Versuchsanstalt Gmbh and Dampskibsselskabet Norden A/S. DTU was responsible for the theory and software of the design and optimization of the KAPPEL propeller.