There are two types of sailing hydrofoils; fully submerged and surface piercing.
Fully submerged designs, for example the Moths, have no inherent ride height stability and require a surface sensing wand to control the hydrofoil incidence. This wand is physically delicate and prone to excitation by waves making this system unsuitable for offshore conditions.
The surface piercing system achieves height stability by varying the immersed hydrofoil area to suit the conditions, dispensing with the requirement for a complex control system. The hydrofoils have fixed incidence and therefore mechanically robust and in addition the exposed hydrofoil area above the water surface provides large reserves of lift when encountering waves. This makes surface piercing hydrofoils the natural choice for offshore sailing and is the type used on C-FLY.
Most surface piercing hydrofoil craft use a tailplane to stabilise the craft in pitch in a similar manner to aircraft. However unlike aircraft sailing hydrofoils experience large pitch down moments due to the high rig centre of effort. This requires the tailplane to exert a downforce to balance the craft which is a dangerous situation since the downforce can be suddenly lost in the event of the tailplane broaching the surface in a wave trough or gust, resulting in a pitchpole.
C-FLY reverses this configuration by using a canard bowfoil which exerts an upthrust to achieve pitch control. In the event of the bowfoil broaching the water surface the rig pitching moment pushes the bowfoil back in, restoring its lift with no loss of control.
Previous attempts to build canard stabilised hydrofoils have generally failed due to inappropriate bowfoil design. By necessity the bowfoil is set at high incidence to achieve pitch stability and consequently conventional wetted sections suffer from intermittent ventilation with loss of lift. The key to C-FLY’s success is the use of superventilated hydrofoil technology pioneered by the Canadian Bras D’Or project in the 1960s.
This approach uses a very unusual foil section designed to ventilate stably by drawing air down from the free surface. The flowfield is tolerant to the large orbital water motions encountered in waves resulting in small changes in lift which do not upset the pitch control of the craft. This gives C-FLY rough water capability with a real functioning wave piercing hydrofoil system.