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))