Keel Lift- a misnomer

九月 22, 2007 at 12:28 | 張貼於physics, sail | 發表留言
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Catamaran VEGA Sailing School Chapter 11: “Water flowing past the hull, keel, and rudder of a sailboat is subject to the same basic rules as air flowing past the sails. The only difference between the sails and underwater appendages is that the latter are symmetrical while the former are asymmetrical. But the angle of attack (which we call ‘angle of incidence’ for wind hitting the sails and ‘yaw angle’ for water hitting the keel) solves the problem of getting ‘lift’ from the keel. Due to the pressure of the wind in the sails, a sailboat sideslips a little as it goes forward. This is called ‘making leeway.’ Thus the angle between the direction that the boat is heading and an imaginary line indicating its ‘track’ through the water is the ‘leeway angle’ as shown in Figure 1. Since the water has to travel a greater distance on the windward side of the keel, an area of reduced pressure produces ‘lift’ to windward. The more lift from the underwater surfaces, the less leeway the boat makes. In other words, it slips sideways less. Obviously, when sailing to windward we are trying to reach a destination upwind, and any sideslipping that pushes us downwind is undesirable. The slower the velocity of the fluid flowing past the ‘airfoil,’ the less efficiency it has as a lifting surface. So when the boat is going slowly, it sideslips more. This increases the leeway angle and, up to a point, increases the efficiency of the keel. Past that point, though, the water becomes turbulent on the windward side of the keel and a stall results. A good example of this situation is a sailboat sitting on the starting line before a race in a close-hauled pointing angle but with sails luffing, waiting for the starting gun. At the gun, the crew trims in the sails to get the boat moving forward. Instead, the boat goes almost as fast sideways as she goes forward because the velocity of the water flowing past the keel is not sufficient to counteract the sideways push of the sails. Instead, the helmsman should have sailed on a slight reach, where the force of the sails is more in the direction of the boat’s heading, in order to pick up speed and then harden up to close-hauled.


Fig 1: The more lift generated on the keel, the less leeway, or sideslipping.

All the above should not be taken seriously because the force come form the wind above. Below water only drag can be found, not lift.

 


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