MadSci Network: Engineering

Re: What is a supersail and how does it work

Date: Sat Jul 1 04:01:29 2000
Posted By: Paolo Omodei-Zorini, Secondary School Teacher, Aerospace dept., OZP Aerospace R&D
Area of science: Engineering
ID: 957362594.Eg

Paolo Omodei-Zorini, OZP Aerospace R&D
Dear Todd, I don't know exactly who is building that kind of ship, so I can only write about its general principles. When a "well shaped" surface (the airfoil) interacts with an airflow, the resulting effect is an aerodynamic force. If you are sailing with a boat with a “tail” wind component, wind interacts with the fabric of the sail, creating a force called “Drag”. This force, that lays windward, is the principal component of the aerodynamic force created by the ail. If you decide to sail with an airfoil, as you have written, it will generate “Lift”. This force, that is the other component of the aerodynamic force (in a 2 dimensional view), is perpendicular to drag (90° from the direction of the airflow, called relative wind in aerodynamics), is greater than drag in a typical plane’s wing profile and can be used as “pulling” force in a “sea” application instead of drag. So both airfoils and sails use the aerodynamic force, that can be divided into Drag and lift. I have taken this image from the Nasa website I have written the address below.

Lift increases proportionally with the angle between the relative wind and the chord of the airfoil (the chord is the line from the “nose” to the “tail” of the airfoil, called “leading edge” and “trailing edge”): this angle is called “angle of attack”. The increasing of lift is limited to a value (generally 15 – 18 degrees for wing’s profiles) of “angle of attack”: if the limit is passed, lift will rapidly decrease because the airflow around the airfoil is unstable and will not cover in the right manner the shape of the airfoil: this situation is called “stall”. (In the real world, the angle between the relative wind and a straight sail will generate some lift too, but it’s not our topic now...) So if you decide to use lift generated from an airfoil shaped sail, your boat will be limited in some parameters :
- Due to the weight and to the drag of a boat, it work well only with strong winds (30 mph or more, I suppose)
- Lift, as I have written before, is perpendicular to the relative wind, so you can only sail with crosswind.
- Limitations about angle of attack will create problems with the range of wind directions you can use to sail
Of course, I hope this prototype will be successful ;o)
I hope to have been useful, and I will happy to answer other questions if this is not the one you were looking for.
Paolo Omodei-Zorini
More information on the topics of aerodynamics can be found at this url:

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