Images included come from the excellent book « Au large » written by Alain Gliksman with a foreword by Eric Tabarly (Arthaud Neptune - Editions maritimes et d'Outre-Mer).
Quotes from this book :
Figure 1 : A boat in motion is subjected to the balance of 4 forces equal and opposite 2 by 2, and the couples they form between them. Two have positive action : the result of the wing (pushing force) R and buoyancy (Archimede push) F. The two others have negative actions : the resulting dynamic forces exerted on the hull (drift force) D and the weight of the boat P.
R & D on one hand, F and P on the other are equal and opposite.
The pair formed by the forces R and D is variable depending on their point of application. The values of F and P are constants and the application point of P fixed (gravity center of the boat), the center of F (center of buoyancy), which will move to balance the couple R - D.
Figure 2 : The forces R, F, D and P projected in 3 planes : transverse, longitudinal and horizontal, one observes the following phenomenas :
- In the transverse plane, R & D are a couple tilting balanced by the righting moment, constituted by the forces F and P.
- In the longitudinal plane, the couple formed by the longitudinal tilting R & D is balanced by the righting moment of the longitudinal forces F and P.
- In plane, when the boat sails straight ahead, there is no torque, the forces R and D are opposite on the same axis, while F and P, perpendicular to the plane, have no influence.