In a shot lasting just two seconds in a promotional video for the America's Cup defender Oracle Team USA, a clear bulge can be seen on the centreboard, the hydrofoil. This shape is surprising, as the foils have always been extremely streamlined and streamlined. What is this bulge all about and why can it be seen in a publicly accessible video? Is it a faux pas, has a new development stage in foiling been unintentionally unveiled?
The sword bump can be seen at 0:30 to 0:32
The US team has not yet commented when asked. However, Martin Fisher, designer of the foiling GC32 catamaran, which is also being used in this year's Extreme Sailing Series, has an explanation: "I think it serves to change the pressure field so that in the event of a stall, this does not occur at the same moment along the entire length of the foil. This mitigates a potential crash somewhat," writes the New Caledonia-based foil expert. The bump is therefore a kind of cavitation brake.
Cavitation is a major problem with the current America's Cup cats. It normally only occurs at speeds of around 50 knots and above, which is why this speed was long regarded as the sound barrier for foiling boats. Put simply, at this speed the water on the foil begins to vaporise due to the enormous negative pressure, the current breaks off and the lift is lost.
However, 50 knots is not the target for the AC cats, but cavitation does occur. The aim of the designers is to lift the twin-hulls out of the water even in light winds in order to foil early in the expected conditions off Bermuda. However, this requires a profile with high buoyancy even at low current speeds.
The faster the cat gets, the higher the lift and therefore the pressure on the foil. Although this can be partially regulated by the angle of attack, it is not infinite. As a result, cavitation occurs at speeds of 35 knots and more, which normally starts at the bend of the L-shaped centreboard because this is where the pressure is highest. The air bubble that forms there then spreads rapidly to the end of the foil, which loses its buoyancy and the cat crashes (see diagram).
If it were possible to prevent the cavitation bubble from spreading, a higher top speed could be achieved when foiling. The bump on the US foil appears to serve this purpose. It is a kind of cavitation wall.
This idea is not entirely new; according to Fischer, it has already been experimented with. Although cavitation can be delayed by such bumps, this is at the expense of the resistance that results from the less flow-favourable shape. It is therefore a question of weighing up the advantages and disadvantages.
However, it is rather unlikely that this centreboard will be used in the form shown. Firstly, because it is unlikely that in the strictly monitored and controlled world of America's Cup boat construction, a decisive development would be passed on to the competition via such a video sequence.
Conversely, however, this means that this information was deliberately spread, and that would not be surprising either. Spreading disinformation has always been part of the Cup business. A seemingly accidental revelation like this can mean that the competition's design teams also have to deal with this phenomenon, which ties up time and resources and can lead to uncertainty. In the still relatively young world of foiling and hydrofoiling, which offers designers many new fields of research, one of the most important tasks is to define early on which areas promise the most progress. Anyone who wastes time on the wrong detail, perhaps because of a video, will fall behind.
Lars Bolle
Chief Editor Digital
