Trial-and-error… developing a new foil system for flying multihulls is not for the faint-hearted. Although developers of the Vampire foiling prototype (left) are making great strides it is far from a relaxing process. But if foiled cat sailing is to advance beyond the de rigueur J-foils then it’s important that groups outside the ‘traditional’ foiled classes continue to support and thoroughly trial new ideas
lift decreases. But the suction on the wind- ward side of the vertical strut is still acting to increase lift. This means we have two components that are fighting one another.’ Ellway says the result is a foil system that has the following characteristics: 1. When the foil is deeply immersed (eg hull just clear of water) the change in lee- way with height is small, and so the change in lift with height is small and flight is unstable. 2. The change in lift with height becomes sufficient for stable flight only when the boat is very high and making large leeway angles. In these circumstances the foil drag increases and the foil becomes inefficient. Smith believes that the wand-to-flap control mechanism used on the Moth would address many of the control prob- lems experienced by the new generation of foiling multihulls. ‘The Moth system with the wand is a powerful control solution. You can develop a very large change of lift as a function of change of height. And, almost more importantly than that, you develop a change of lift as a function of a boat’s pitch attitude.
‘And this produces a stable system where you’ve got a lot of lift if you’re too low, and as you climb that lift is backed off as a function of the rate of climb as well as actual height. And that situation is what is required for stable height control. Now, when you look at a J-foil, when it’s fully submerged it’s only got lift just above the weight of the boat. As it climbs out the lift decreases, but it’s got a poor aspect in that a lot of that lift comes from side force on the J-foil. And as it rises the lift initially tends to increase rather than decrease.’ Ellway says that in comparison with a Moth foil system the J-foil offers much poorer height-to-lift feedback. ‘The lift to drag ratio of the Moth is significantly better than with the J-foil,’ he says. ‘So for a J-foil it’s only by getting a foil design that balances the change of lift – because of the increase in leeway – that the system works. It’s very poorly damped and only margin- ally stable.
– Part II
definition the foiling used by the AC72s in San Francisco was compromised. Why, ask Ellway and Smith, would you want to take compromised foiling technology as the basis for developing a new design? Ellway offers this assessment of AC foiling technology: ‘These foils regulate lift by what is termed “leeway coupling”. Basically, once the windward hull is lifted the foils have to resist the more or less con- stant sideforce that is developed by the rig. As the boat rises higher out of the water there is less of the vertical section of strut
in the water so the boat makes more lee- way. Now the trick the AC designers used was to inversely link the vertical lift pro- duced by the foils to the amount of leeway. If you take a pure L foil, the lift on the hor- izontal section is augmented by the suction on the windward side of the vertical sec- tion that is produced as a consequence of leeway. So with a pure L foil, the vertical lift increases as the leeway increases. This is the opposite of what is required and it compromises pitch and height stability. ‘So the designers started to reduce the angle between the vertical and horizontal sections of the foil, so that the lifting part points up towards the water surface at a dihedral angle. We now have the J-foil. If we just look at the dihedral part of the foil, then as leeway is increased, the flow angle it sees (angle of attack) reduces, and so the
‘Oracle’s AC72 and the latest AC45F boats also allow the helm to adjust the angle of attack of the front foil in flight. This is done by an incremental push- button system. I think this extra control significantly contributed to Oracle’s supe- rior upwind foiling performance over Emi- rates TNZ at the 2013 AC. This manual pilot control is, however, coarse and slow and thus unlikely to allow good control for smaller boats in comparatively larger sea- ways. It also increases the risk of danger- ous pitchpoles in the event of pilot error.’ Smith says it’s useful to think of any foiling boat as a low-flying aircraft. ‘If you think about something as small as a Moth, it’s obviously important the foil stays under the water surface, but you want the foil as close to the water surface as possible so that you haven’t got much of the struts in the water, which are adding drag. Your boat foil should hug the surface of the
SEAHORSE 37
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