4 6 8 D O C . 4 6 6 T H E F L E T T N E R S H I P
The wind blows in the direction indicated by the arrows. It
must make a certain detour around cylinder Z, whereby it
flows past A and B at the same velocity. Therefore, the pres-
sure will be the same at A and B, and no force is exerted by
the wind on the cylinder. Now, however, let the cylinder ro-
tate in the direction of arrow P. The flow of the wind passing
by the cylinder is thereby unevenly distributed on the two
sides of the cylinder. At B the wind’s motion is assisted by
the cylinder’s rotational motion; at A it is impeded. Under
the influence of the rotation, a motion forms at the cylinder that has a greater ve-
locity at B than at A. That is why the pressure at A is greater than at B, and the cyl-
inder is subjected to a force from left to right which is utilized to propel the ship.—
One might think that some inventive mind could have hit upon this idea directly,
i.e., without any external motivation. In reality, though, this matter developed as
follows. It was observed when firing cannonballs that, even during a lull in the
wind, considerable irregularly alternating deviations from the course’s vertical
plane occurred toward either side, deviating from the ¢beginning² initial direction
of the cannonball’s flight. For reasons of symmetry, this strange effect necessarily
had to be connected with the ball’s rotation, because any other reason for a side-
ways asymmetry in the air resistance was clearly inconceivable. The correct expla-
nation for this phenomenon, that was quite baffling for the experts, was then found
by Magnus, the professor of physics from Berlin, around the middle of the last
century.[5]
It is the same explanation as the one just given for the force acting on
the Flettner cylinder in the wind, except that, instead of cylinder Z, there is a ball
rotating around its vertical axis and, instead of the wind, there is the relative motion
of the air against the cannonball in flight. Magnus confirmed his explanation
through trials with a rotating cylinder that did not differ essentially from a Flettner
cylinder.[6]
Somewhat later, the great English physicist Lord Rayleigh again inde-
pendently discovered the same phenomenon, using tennis balls, and likewise of-
fered the correct
explanation.[7]
Recently, the renowned prof[essor] Prandtl con-
ducted precise experimental and theoretical studies on the fluid motion around
Magnus cylinders, in the course of which he conceived and already executed what
amounts to almost the entire design executed by
Flettner.[8]
Flettner saw Prandtl’s
experiments[9]
and was the first to think that this installation could be used as a sub-
stitute for a sail. Who knows whether anyone else would ever have thought of that?
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