– Technology – Project ‘Cosmos’: Innovation in propeller and rudder design

After delivery, the 80 m long Heesen Cosmos project will be able to reach almost 30 knots and achieve an efficient cruising speed of more than 20 knots. In terms of hydrodynamic efficiency, the Cosmos project promises to be among the most advanced superyachts ever built, with a refined hull shape with low drag and an innovative propulsion system.

With four engines, two shafts, a displacement of 1,100 tons at half load and a required speed of 29 knots, the Cosmos project offers a uniquely sophisticated set of parameters, and minimizing limb drag was not the only challenge facing the design and construction team. “The propellers will be under a very high load at maximum speed,” explains Heesen naval architect Sjoerd van Herk. “And high loads can mean cavitation.”

Cavitation describes when steam bubbles form in the flow of water around and behind a ship’s propeller. It’s especially demanding at high speeds and can not only degrade performance, but also have serious effects on noise and vibration – an important consideration for any superyacht project, especially one rated at nearly 30 knots.

Therefore, the propeller design in the Cosmos project had to be optimized not only to ensure that the speed and efficiency requirements were met, but also to eliminate possible cavitation problems. For this reason, Heesen turned to the Norwegian propulsion specialist Kongsberg for his expertise in propeller design. The company also takes an innovative approach to rowing. The Promas integrated drive system has been under development since 2004. “The advantage is that the rudder and propeller are coordinated with one another,” says Carola Andersson from Kongsberg.

Sjoerd van Herk and Perry van Oossanen during tank tests

The Kongsberg Promas propulsion system has a special hubcap attached to the propeller that streamlines flow onto a piston that is added to the rudder, effectively reducing flow separation immediately after the propeller. The result is an increase in propeller thrust as previously wasted energy is recovered from the flow. Adding the light bulb to the rudder also streamlines the flow behind the rudder, further reducing drag. A twisted rudder provides further improvements in terms of efficiency and maneuverability.

Each Promas propulsion system is tailored to its respective hull. Heesen shared data on drag, tank test results, and information about water flow around the hull from van Oossanen’s CFD simulations to Kongsberg. “The three of us worked closely together,” adds van Herk.

Due to the specific performance requirements of Cosmos, variable pitch propellers were essential. Two new propeller designs were evaluated in the Kongsberg test tank before the blade shape was finally determined. Tests showed that the propellers were virtually free of cavitation at 30 knots and completely free of cavitation at 20 knots.

Promas essentially transforms an element of propeller design that has long been considered a passive participant in ship operations into an active contribution to the hydrodynamic efficiency of a ship – even in a ship that is as optimized for low drag as Cosmos. “The rudder alone creates an additional half a knot at top speed,” concludes van Herk.

Heesen recently announced that the hull and superstructure of the Cosmos project are now connected – the slim profile of the project, written by the renowned British studio Winch Design, can finally be admired for the first time. The yacht will now go through the outfitting process ahead of its market launch in 2021.

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