1.2.1 Flettner rotors
are named after their creator, Anton Flettner. The engineering principle
behind the design existed before the rotor was created (the ‘Magnus’
Effect, see below). However, the technology had never been applied
in practice in the marine industry. In 1922 Anton Flettner filed for
a patent on the ‘Rotor Ship’. With help from other industrialists,
the first experimental rotor ship, the ‘Buckau’, was finished
in October 1924. It was fitted with two large rotors (15m tall, 3m
diameter) and a 50hp electric propulsion system. The ship operated
for around a decade before it broke up at sea in the Caribbean during
a storm in the early 1930s.
1.2.2 Although the vessel
was a success in terms of proving that thrust can be generated and
contribute towards the propulsion of a vessel, at the time there was
no call for reducing fuel cost and emissions. It was proven that the
thrust created by the rotors for the given input of electrical power
was less than that if the power was used to directly drive a conventional
screw; thus the idea sank with the ship.
1.2.3 The basic principle
behind the ‘Flettner Rotor’ was discovered by Heinrich
Magnus, who first described the effect in 1852. However, it should
be noted that the effect was also already observed and the cause (correctly)
inferred by Isaac Newton in 1672 after observing the behaviour of
a tennis ball during a match at Cambridge College.
1.2.4 When a cylindrical
body is spun in a viscous fluid, it creates a boundary layer around
itself. This boundary layer induces a circular motion in the fluid
in its immediate vicinity (creating a vortex flow). If the body is
moved through the fluid with a velocity, V, the velocity of the thin
layer of fluid close to the body is a little greater than that on
the forward-moving side and a little less than that on the backward-moving
side (similar principle to the lift created by an aerofoil). This
happens because the induced fluid flow velocity surrounding the spinning
body is added to the free stream flow velocity over the top of the
cylinder, and subtracted from that below the cylinder.
1.2.5 Applying this to
a cylinder rotating in air, the pressure differentials create the
thrust. Due to the acceleration of air over the top of the cylinder
(and thus increased velocity), compared to below, a perpendicular
component of force is produced. Thus, the combined flow has a higher
velocity, and hence a lower pressure on the top surface, leading to
a pressure imbalance and a net upward force on the cylinder.
1.2.6
Figure 1.1.1 Pictorial representation of Magnus Effect demonstrates the Magnus Effect
fluid flow principles, where U is the free stream velocity and ω
is the rotational speed of the cylinder. For ship applications, U
is a function of vessel speed and wind speed.
Figure 1.1.1 Pictorial representation of Magnus Effect