The theory of hydrodynamics tries to explain why hydrofoils like the fin are generating lift. Lift plays a key role on all courses of the windsurfer (not on the run!).

The performance of the fin is interconnected with the hydrodynamic drag of the fin. For maximum speed we would like to have an ideal fin without any resistance. But, as we will see, lift without resistance is not possible. So, what we should be aiming at is the combination of maximum lift with minimum drag. Therefore, it seems to be quite rewarding to find out a few details about lift and drag. For this purpose we need to understand some hydrodynamic terms.