Monster Kites

skysail.jpg

So there is a buzz about kites pulling boats (ships) and saving energy… I LOVE technologies that are old fashioned and then make a big comeback (good example = steam engines). Questions – how do these things work when the wind in not in the right direction? They say they are computer controlled – what does the computer control? Why not sails which can give you a better angle? mrm

UPDATE:

The first Skysail has left Germany on a cargo ship headed to Venezuela.
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7 Responses

  1. This is a very interesting idea. The computer control steers the sail in the same way a paraglider can pull his chute to turn left and right… by pulling the strings to adjust the shape of the sail. They claim that the system can work up to 50 degrees into the wind theoretically… 70 degrees into the wind in practice.

    There seem to be a couple of reasons why a kite is more effective than a sail. The first being increased wind speed due to elevation. Secondly, because of the mast, a large sail has more torque to cause “heeling”, which is the tipping of the boat caused by the wind force. The kite is anchored at the deck of the ship, so the R of F x R is smaller. Heeling apparently reduces the efficiency of the boat because of increased friction and potentially drag through the water.

    http://skysails.info/fileadmin/user_upload/Pressedownload/Dokumente/EN_Technology_Information.pdf

    jk

  2. To go along with the increased wind speed due to elevation, how large is the boundary layer for air flow over a free surface like an ocean? I am assuming that the Re is pretty high and if the relation for boundary layer thickness for flow over a flat plate holds, I would imagine it would be pretty small. So perhaps with the kite we have less frictional losses as compared to a sail, which may stay within the boundary layer.

    Also, by having the kite, can the kite height be lowered into the BL and thus somehow control speed if need be?

    On the flip side, I’m also wondering if a kite can drive the boat fast enough. I am assuming that big boats carry important things that need to be delivered in a timely fashion. But I guess this problem would exist with sail boats too.

  3. If we consider wind speed of about 15 m/s as it is mentioned in the reference Josh has provided, with the boat length of approximately 15 m and kinematic viscosity of 15*10-6 m^2/s at 1 bar and 20 C, Re number of the air flow over the boat will be 15*10+6. Calculating Boundary Layer Thickness, considering boundary layer assumptions (High Re number and small B.L thickness compared to the characteristic length) we will come up with the B.L thickness of less than 2 cm., which seems reasonable for the boat scales. Therefore, I do not think we can consider the sail within the boundary layer thickness.
    I guess we can study the force prepared by these huge kites in terms of less friction in higher levels and more wind speed but I doubt they can be replaced by other types of powers. Perhaps as suplementary forces to REDUCE our energy usage to ride the big boats.

  4. In reference to Prakash and Sara,… and my above news posting.. it does seem like it is a supplemental source of energy for driving the ships… its more about keeping momentum which then requires less additional force from the engines of the ship to keep it moving at required speeds… it also seems like they have recently dropped the speed of these ships from 23 knots to 20 knots in order to save fuel….

  5. Here’s an interesting concept that I just thought of. Does anyone have any idea of what the wind patterns surrounding a storm might look like? This might be a bit dangerous or unrealistic in practice, but would it be possible for a ship to ride the wind patterns of a storm at sea so to speak. I’m going to try to find wind profiles surrounding a storm from meteorological sites, but if there is an increased wind speed near a decently moving storm, do you think it would be possible for a ship to ride that wind and gain faster speeds than they would in a sunny day? Even if that is the case, would any extra speed be lost due to increased turbulence in the vicinity of the storm.
    I am aware that the path of the storms might not be entirely predictable and ships need to run on a schedule, but phenomena such as this could give a speed boost to ships that happen to be in the vicinity of a storm and heading in the same direction, at least for the moment.

  6. I believe that storms have a tremendous amount of vorticity spiraling about the eye – meaning the ship would just go in giant circles. I believe that a storm propels itself in a direction due to pressure differences. All of the above only comes from watching the weather channel – so I could be completely wrong.

    -pr

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