Investigation of Keel Bulbs for Sailing Yachts.

Ljungqvist, Kasper, Björn Axfors, Hanna Tunander, Michal Orych, Florian Vesting, and Lars Larsson.

4th High Performance Yacht Design Conference, Auckland, 12-15 March, 2012


An experimental investigation of four sailboat keels is presented. Three of the keels have bulbs of different shapes: an L-keel, a T-keel and an integrated bulb keel. These shapes are found on most sailing yachts designed during the past twenty years. The fourth keel is a standard fin keel. Lift and drag of the four keels have been measured in a wind tunnel and the results are compared. To obtain more practical results the measured data have been input into a VPP, and the performance of a 40 foot yacht equipped with the different keels is evaluated. Although the performance differences are not great, they are clearly of interest in yacht racing. Since one purpose of the experimental campaign was to develop a data base for CFD validation, emphasis is placed on uncertainty estimation. A numerical optimisation of the most common bulb, the integrated one, is also reported. A CFD code was linked to an optimiser and a CAD program, and the drag to lift ratio of the keel was minimized at an angle of attack of 4°. The optimisation was preceded by a CFD code validation for the keel. It is shown in the paper that the proposed technique improves the performance such that the worst keel becomes competitive with the best non-optimised one.