The RAKE revolution
(Revolutionary Adaption of Kinetic Energy)
Magnus Effect stabilizers have been getting much attention during the last years. The “specialty” of this phenomena in the shipping industries lies in the fact that it can create Lift at very low speeds. Both RotorSwing as Quantum have been the originators of retractable Magnus stabilizers used for underway and ZeroSpeed applications.
But as many other inventions, the system also has a few drawbacks. For the typical displacement vessels, the roll damping system with rotating cylinders works extremely well up to speeds of around 8-12 knots. But at higher speeds the resistance starts to pay its toll, which is excessive fuel consumption. However, our answer was the patented RAKE function which can be Manually operated or directed through a GPS signal.
The drawing above shows how the water “sees” the rotating cylinders. From left to right it shows the basic position, perpendicular to the hull. The circumference is a circle and the diameter is the basis of the Magnus effect. The second picture shows the RAKE being 30 degrees with the direction of flow. The red colored circumference is changed from a circle to an elliptical shape. The circumference has increased with 7,5%. This means that with the same speed of rotation the water flow gets a 7,5% higher speed and thus the Lift increases with the same amount.
We have now several ships installed that have a RAKE setting of 45 degrees and the results are astonishing. Far less drag and still a perfect roll damping force. The 18,5% larger circumference counts for a larger Liftforce that merely compensates the loss in moment arm. Several other tests are underway where we will increase the RAKE angle up to 60 degrees.
Another advantage of the RAKE is the shorter arm that protrudes outside the vessel. As the same time the Drag diminishes considerably.
As can be seen from the picture, taken from a publication about Cd (coefficient of drag) the resistance of an elliptical shape is 50% of a cylindrical profile.
We are continuously performing rigorous testing in cavitation tunnels and towing tanks. However, nothing beats the the real world experience of rough sea water! Our RotorSwing demonstrator, a 50 foot Altena Pilothouse displacement yacht, has been of great help.
We have tested and measured that by folding the rotors backwards (RAKE) like the wings in fast airplanes, the resistance (Drag) diminished considerably while not affecting the Lift too much. We found the best of both worlds; less drag with supurb roll damping!
The RAKE function is now a standard for vessels that want to take advantage of this patented idea. We are confident that this Patented RAKE function is one of our greatest inventions in roll damping. It makes the RotorSwing stabilizers a truly unique piece of equipment that provides maximum roll damping with minimum drag!