Heesen Yacht’s latest project is Cosmos, an 80.7 metre aluminium superyacht, and features some remarkable innovations.
With a designed top speed close to 30 knots and an efficient cruising speed in excess of 20 knots, Cosmos will use a development of the Fast Displacement Hull Form (FDHF) coupled with innovative construction and advanced propulsion solutions to set new standards for large aluminium yachts.
Although the keel-laying ceremony is due to take place later this year in May, this is not the start of the project – indeed, an extraordinary amount of design and testing has already been undertaken and completed.
The proof-of-concept and testing process has comprised several stages, starting with the base naval architecture of the vessel and its hull form, progressing through extensive computational fluid dynamics (CFD) work and physical tank testing of a scale model, and culminating with the design of the propellers and rudder system. For these aspects, Heesen has drawn not only on the expertise of its in-house design and engineering department but also that of partners Van Oossanen Naval Architects, and propulsion specialists at Rolls-Royce (RR).
FDHF Concept with Backbone
The first stage was to develop the FDHF concept and ensure not only that Cosmos benefitted from the efficiency and speed gains of the design, but also that the yacht could retain inherent stiffness – a challenge because Cosmos features all-aluminium construction in order to achieve the client’s desired maximum speed of close to 30 knots.
The solution was for Heesen to develop the Backbone – a method of construction that could deliver the required stiffness in the yacht without adding weight that would have impacted on the yacht’s performance. Such has been the success of the design that the Backbone has now been patented.
Physical Tank Tests
A 3.7 m scale segmented model of the yacht was built to use in physical tank tests at the Wolfson Unit in Southampton. After initial data on hydrodynamics, resistance and speed had been gathered, the model has been radio-controlled and self-propelled in order to measure the various bending forces acting within the hull and to prove the Backbone concept. That provided valuable data for fine-tuning the structural design and to further optimise the construction method.
Watch a video of the tank tests.
Knowing the Real Forces
For such a large aluminium yacht, it is particularly important to know the real forces created by the sea on the hull, not just to meet Lloyd’s Register requirements, but to exceed those and construct the boat to the highest loads that could be encountered.
Deeper Keel Depth and Higher Efficiency
Using the FDHF design and the Backbone for such a large aluminium yacht requires a slightly deeper keel depth, but this actually brings further benefits. Due to the slightly increased draft, Project Cosmos has a very low block coefficient, which means that the underwater body is very slim with nice verticals in the aft ship area, for an optimal flow of water towards the propellers. This results not only in improved performance, but also in higher efficiency, ensuring fast cruising with comparatively low fuel consumption.
Refining the propeller design is the ongoing job of research and design engineers at RR. Alongside the propeller design itself, the Promas system – where propeller and rudder are integrated into a single unit to optimise hydrodynamic efficiency – has been implemented in the design.
RR engineers started by gathering CFD information of the water flow towards the propeller. Then the design process began with RR proprietary software: different iterations of propeller and rudder design were tried before arriving at the optimal solution, which was then tested in physical experiments.
Watch a video of the propeller test.
By creating a 1:11 scale model of the Promas rudder and propeller in the test tank facilities of the RR Hydrodynamic Research Centre (HRC) in Kristinehamn, Sweden, the propeller design was fine-tuned not only to ensure the speed and efficiency requirements were met, but also that any potential problems with cavitation could be eliminated.
Cavitation is essentially where air bubbles form in the flow of water around and behind the propeller. It is particularly challenging at high speed. In addition to affecting performance, it can also have severe impacts on noise and vibration – a critical consideration for any superyacht project, especially for one close to 30 knots. Cavitation issues were identified and eliminated by adjusting the rudder design. Finally, the tests proved that the propeller performed above expectations.
Variable Pitch Propellers
Project Cosmos will feature variable pitch propellers that will enable the yacht to handle both impressive cruising speeds in excess of 20 knots, as well as reaching a maximum speed close to 30 knots – a considerable achievement for an 80-metre superyacht.
Currently, the hull design has been optimised, the construction engineering has been finalised and the propulsion system is going through the last stages of testing.