The keel fin is the only major steel part of the Bondi 36. For the safety of the boat and crew it is important that this part is reliable over many years. A lead bulb of more than 2 t weight is mounted to the tip of the keel fin to keep the boat upright. Stress ranges are therefore high, especially during tacks, gusts and wave action which can potentially lead to fatigue issues. 

To ensure safety and high fatigue performance a very innovative design was developed by KJ Engineering. The keel consists of a CNC machined top plate to which the load carrying strut is welded. The strut is made from two plates bent into a C - shape. This ensures that all welds on the strut are parallel to the main stress direction and located in the neutral axis to avoid fatigue issues. 

The most critical weld is the connection of the strut to the top plate because of high stress levels. For optimum performance the top plate is shaped in such a way that the weld connection to the strut is a butt weld located away from the stress concentration due to the radius between the top plate and the strut. Butt welds have good fatigue strength compared to other weld types, ensuring maximum fatigue life. Non-destructive testing was carried out after each weld run to ensure this critical weld is crack free.

To complete the aerofoil shape, another C - shaped sheet is welded to the front and two sheets to the back. The inside of the keel was zinc coated after manufacturing to prevent corrosion.