As with any lifting surface of finite span, the keel generates a tip vortex analogous to ones produced by airplane wings. This recirculation is associated to an additional component of the drag (the induced drag), which can be reduced by means of suitably designed winglets, whose placement over the bulb is itself a matter of careful design. Streamlines visualization helps to evaluate the effectiveness of such devices and to suggest new strategies to reach an optimal configuration.

(Image courtesy of CMCS (Chair of Modeling and Scientific Computing-EPFL-Lausanne))