The presence of a vortex circulating the sails affects the flow pattern of the air for up to about two boat lengths in every direction. Boat A has positioned itself so that Boat B is at a disadvantage. The vortex around A's sails have changed the direction of the air hitting B's sails. As a result B must steer at angle further from the desired northward direction to attain the greatest speed. Efficiency of Lift Lift depends on a transfer of momentum to the air.
Airplanes essentially employ the first method.
The vortex set up around an airplane wing affects a large mass of air, but the downward defection of the airflow is fairly nominal. Helicopters, on the other hand, take the second approach. But the change in velocity is extreme. This is linear in mass, but quadratic in velocity. Hence, as the velocity imparted to the air goes up, the energy goes WAY up.