No stone unturned - Part II

Designer Steve Killing looks at controlling and then measuring the performance of the world’s first dedicated C-Class foiler - Fred Eaton’s Off Yer Rocker

Controls
The now common practice of CNC cutting of sheet material makes the creation of parts like the wand driving mechanism chosen for Rocker quite practical (although in some cases the shape was so simple that the builder simply glued the drawing straight to a carbon plate and cut it out by hand). Rocker’s wand control is modelled on the system developed for the Moth. The wand’s job is to control the ride height; when the boat is too low the wand is rotated aft by water pressure, which pushes on the control cable and deflects the flap on the daggerboard wing downwards to raise the boat.

However, if the wand is set too far from the daggerboard then there is an increased risk of false readings. For example, if the ride height is already optimum (with the daggerboard exposed by about 12in) but the bow is pitched slightly up, then a wand mounted forward on the hull will indicate the hull is too high and reduce the flap angle - not the desired response.

To avoid this problem, on Rocker the wand pivot is set 6ft forward of the wing that it controls. In fact, with so much hull forward of the daggerboards we had complete freedom on where the wand should be, but the initial location that we chose proved suitable and was never changed. After talking over several control techniques, we decided to give the rudder ­elevator control to the helmsman. The dynamics between the rudder elevators and the daggerboard wings were unknown, and while we didn’t want to increase the helmsman’s tasks, it seemed like the best solution. It turned out that our helmsman Fred Eaton could easily control the elevators and thereby the hull trim angle. We used double tiller extensions leading into a tiller-mounted gearbox to provide the manual pitch control through the elevator flaps.

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Below:

The trapezing racks added to Rocker bring the overall beam back out to the maximum allowed. The hulls are closed together to allow the foils to also fit within the C-Class beam limit of 14 feet. Before the fragile and expensive solid wing was installed on Rocker she was towed (right) to confirm dynamic balance and stability