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-0 – Technology – Project ‘Kosmos’: Innovation in propeller and rudder design

The 80-meter motor yacht from Heesen will have an integrated Promas propulsion system …

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

With four motors, two shafts, a displacement at half load of 1,100 tons and a required speed of 29 knots, the Cosmos project is a uniquely demanding set of parameters, and minimizing the towing drag wasn’t the only challenge facing the design and construction team. “The propellers are very heavily loaded at top speed,” explains Heesen’s naval architect Sjoerd van Herk. “And a high load can mean cavitation.”

Cavitation describes the formation of steam bubbles in the water flow around and behind a ship’s propeller. It is particularly demanding at high speed and can not only degrade performance, but also have a significant impact on noise and vibration – a critical consideration for any superyacht project, especially one designed to travel close to 30 knots.

As a result, the Cosmos project’s propeller design had to be fine-tuned not only to ensure that the speed and efficiency requirements were met, but also to rule out potential cavitation problems. Heesen therefore turned to the Norwegian drive specialist Kongsberg for their expertise in propeller design. The company also takes an innovative approach to rowing. The Promas integrated drive system has been developed 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 directs the flow onto a ball attached 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 optimizes the flow behind the rudder, which further reduces drag. A twisted rudder provides further improvements in efficiency and maneuverability.

Each Promas propulsion system is tailored to its specific 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. “There was a strong working relationship between the three of us,” adds van Herk.

Due to the specific performance requirements of Cosmos, controllable 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 optimized for low drag like Cosmos. “The rudder alone makes up an additional half a knot at top speed,” summarizes van Herk.

Heesen recently announced that the body and superstructure of the Cosmos project have now been put together – the slim profile of the project, penned by the renowned British studio Winch Design, can finally be admired for the first time. The yacht will now be equipped in advance of its market launch in 2021.

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Project ‘Kosmos’: Innovation in propeller and rudder design

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