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A Flettner rotor can be used to provide propulsion for ship from the wind.

Would it make sense to fit a Flettner rotor to a full displacement expedition trawler in the 55-70 foot class like a Nordhavn or Bering? These vessels are deep draft ships that move slow, about 8 knots typically, and are designed for stable ocean travel over long distances. The idea is that the rotor would augment the diesel propulsion to make the ship faster and more fuel efficient.

Below is a photo of a 65-foot Bering with a rectangle showing the imaginary location of a Flettner rotor. A 4 inch drive shaft would go through the decks to the engine room and be coupled to the engine or main drive shaft via a gear box.

enter image description here

The drawbacks I can see are that it could potentially decrease the stability of the ship and would be added weight. Also there would be a 6 inch column going through the center of the lounge and a loss of space on the top deck.

The question would be how much benefit would the rotor bring?

For example, if the rotor could increase cruising speed by 20% and decrease fuel usage by 30% on a transoceanic crossing, that would be significant benefit.

Another question would be how would the rotor affect the roll stability of the ship? Would it tend to dampen roll or increase it?

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    $\begingroup$ Not an expert but I suspect it is not something practical as a retrofit, and especially not to that boat. Like a sail, if its centre of pressure is aft of the lateral centre of resistance, handling will be poor. You probably need to demolish that superstructure and move it forward. Unlike a sail, you can't furl it in a storm. And consider the safety aspects of a fast spinning cylinder on an open deck. My Seagull outboard flywheel is bad enough! $\endgroup$ – Brian Drummond Nov 7 '16 at 10:12
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It is very unlikely to create enough motive power for that vessel. Two fundamental issues are stability (heavy structure high above the waterline) and the absence of a keel (required to reduce lateral drift or leeway). Adding a rotor to that type of vessel would be IMHO inefficient and dangerous.

The flettner and similar devices are not particularly efficient at capturing wind power when compared with a typical sail. High performance sailing vessels (e.g. Admirals Cup racers) have high aspect ratio, semi-rigid sails. A flettner would need to be very large indeed to be effective.

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