The idea behind this must be that each foil section has it's own unique lift versus drag cutve. For instance a NACA 008 foil has a specific angle of attack where performance is optimum (highest lift to drag ratio). Due to practical design and arragnment considerations boat foils cannot usually be positioned to operate at this optimum angle. The gybing dagger boards are a way to improve the situation and bring the fiols closer to the best angle of attack.
When looking at these angles, the leeway (yaw) angle of the boat (angle of the boat with respect to true path through the water) needs to be added to the the angle introduced by the gybing action of the boards to come up with the angle of attack seen by the foil. In other words - need to take into account that boats don't sail in a straignt line.
Kevin,
The situation looks different.
The angle of attack is a function of the balance of forces. Those vary with wind pressure and point of sailing, mostly. This angle is authomaticaly adjusted by the boat by means of varying its drift speed. The adjustment mechanism is completely independent on the board's angle of incidence.
In other words:
-When the boat "needs" more lift force, the drift speed increases, also increasing the board's angle of attack and, therefore, the lift force.
- When the boat "needs" less lift force, the drift speed is lowered, also reducing the board's angle of attack and, therefore, the lift force.
A gybing board only positions the board at a given angle of incidence. The effect is a reduction of the drift speed necessary to generate the same lift - but the angle of attack between the board and the water flow remains exactly the same as that of a symetric board.
Like in an airplane, the angle of attack is a function of the balance of forces, not of the angle of incidence.
Instead of thinking that the board was rotated in relation to the boat, think of the boat rotated in relation to the board, wind and flow. Visualize it this way:
1) A symetric section board in a normal boat is generating lift at a fixed angle of attack with the water flow.
2) Now we keep the same conditions (wind, water flow and board), so that the drift force remains the same, and rotate only the hull/boat/sails a couple of degrees in relation to the board.
3) Note that the initial situation is that of a normal board and the second situation is that of a gybing board.
4) The main difference is that now the boat does not need to drift due to the angle of incidence. The wind, flow, etc. remain the same.
![[Linked Image]](http://www.catsailor.com/bb_files/145048-symetricalboardxgibedboard.GIF)
This is actually a simplified case. The boat's hull was also generating lift in the first case, differently from the second case, where it is aligned with the flow. Also, the rotated boat gives more room to set the sail at a lower angle of attack with the wind flow. Lastly, in practice it is recommended that the angle of incidence remains smaller than the drift angle, because you don't want a negative angle of drift when less drift force is required.
I hope this helped clarify the concept,
All the best,
