Reader Question: Rotor Ships

The Buckau, the original Flettner rotor ship

lazenby asks:

Can you explain how the magnus effect makes rotor ships move?

When a spinning body is placed in a flow, the body experiences a force perpendicular to the direction of the flow. This is called the Magnus effect and is, for example, why baseballs, soccer balls, and tennis balls veer from the path we expect them to take. To understand why a spinning body experiences this force, take a look at the streamlines around a rotating cylinder.

In this picture, the flow goes from left to right and the cylinder is spinning in the clockwise direction. The red dots represent the stagnation points of the flow. Air over the top of the cylinder gets accelerated by the spinning, shown here by the narrowing of space between streamlines. On the underside of the cylinder, the surface is moving in the opposite direction of the air, which decelerates the flow. We know from Bernoulli that this means there is low pressure on the top of the cylinder and high pressure on the bottom. As a result, the cylinder experiences a upward force – lift! You can explore the effect of rotation on the streamlines yourself using this neat demo from Wolfram.

Rotor ships, invented in the 1920s, used this effect for ship’s propulsion. They used a regular motor to begin moving, and, once they had some wind, used motors to spin giant cylinders on the deck. As the rotors spun, the ships were pushed in a direction perpendicular to the wind. They could apparently tack 20-30 degrees into the wind while conventional ships could only manage 45 degrees. Unfortunately, so much energy was required to spin the rotors that the design was pretty inefficient and never caught on.

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