For all displacement sailboats there are three criteria that determine the effectiveness of an underwater appendage package. These are maneuverability, lift and righting moment. Over the decades and even centuries of evolution, the keel/rudder system or configuration has become the most common solution where the rudder is utilized for maneuverability and the keel is left to address the lift and righting moment. Unfortunately, in this configuration the effectiveness of the keel to do these two tasks is an inversely proportional relationship depending on heel angle. Enter the introduction of CBTF, a technology addressing the problem with a completely fresh and empirical solution.
CBTF starts by isolating the righting moment to an articulating/canting strut with a bulb of ballast secured at the tip. By canting the strut up to 55 degrees to weather, the necessary righting moment for a given hull and sail plan is comfortably achieved with 25-50% less ballast than required for a conventional keel. Net result: lighter overall displacement for a given hull and sail plan, hence higher power-to-weight ratio.
The second part of CBTF is the use of the twin foil configuration, which addresses maneuverability and lift. Enhanced maneuverability over the conventional system is quickly attributed to the handling of the steering by two turning foils and lighter displacement as opposed to a conventional single rudder boat. For optimized lift efficiency (Cl/Cd) the foils are placed in the high-pressure zones created by the hull. Not only does this increase the underwater appendage package lift efficiency, but also reduces the boat’s overall wavemaking drag.
Another benefit of the twin foil system is the collective control.
This adjustment allows the boat to sail with virtually zero leeway upwind and superior directional stability downwind due to its ability to shift the CLR fore and aft, balancing the hydro and aero forces.
One of the most common misconceptions of CBTF technology is the belief that there is an increase in wetted surface area over the conventional rudder/keel geometry because of the addition of the forward foil. However, the reality is the wetted surface of a CBTF design is less than or equal to a conventional design for several reasons:
In conclusion, the CBTF appendage package outperforms the rudder/keel configuration:
