Kiteboard Theory - Thread 11: Speed and Ventilating Foils


BACKGROUND


As of early 2008, some kiteboarders have been exceeding 50 knots, and even 55 knots for short durations. Ideal conditions for very high speed runs involve smooth offshore wind (for smooth water) and a course angle of about 130-140 degrees off the true wind (40-50 degrees below a beam reach).

In October 2005, I posted this on Kiteforum:

"In principle, kites have a geometric advantage over windsurfers, but windsurfers are generally more refined in their aerodynamic, planing and lateral resistance aspects. But I think that kites will permanently leave the others behind in the not-too-distant future with more refinements to equipment and technique...

As far as kiteboards are concerned, they do not rely on fins for lateral resistance as windsurfers do. At lower speeds a windsurfer's fin is much more efficient than an edged board, as evidenced by the lack of energy-wasting spray. However, at very high speeds these fins encounter cavitation and/or ventilation, which causes sudden loss of control and efficiency. But here kiteboards have an advantage because their lateral resistance relies on a planing surface, which is basically a permanentantly ventilating foil, and it's performance and control is linear and predictable.

I don't think it'll be long before kites pull into the lead..."


VENTILATING FOILS

Sailboards and other sailing craft have struggled for years to surpass the 50 knot mark. The fundamental problem with most of these craft is that they rely upon "laminar" fins or other foils to resist lateral forces, and somewhere around 50 knots, pressure on the windward (low pressure) side of these foils is reduced to such an extent that the flow of water tends to "detach" from the foil. What actually happens in such a situation is a drop in pressure to such an extent that the water "boils" at its ambient temperature, causing pockets of nothing more than vapour to form on the windward side of the foil. It's called "cavitation". At the point of cavitation, the foil's efficiency drops, and the craft is likely to go out of control, possibly with catastrophic consequences. Furthermore, air can get sucked down to create a cavity beside the foil, known as "ventilation". Ventilation is similar to cavitation, but it can happen as soon as the pressure on the windward side of the foil drops to atmospheric pressure.

Cavitation is not relevant to most kiteboards because they do not rely on submerged foils to resist substantial lateral forces. Kiteboard fins are typically used for tracking only, and aren't normally subject to large forces. Lateral forces, as well as gravitational forces, are generally resisted by edging a kiteboard at an appropriate angle. Because atmospheric pressure normally acts on the top surface of a kiteboard, it can be considered a ventilating foil in almost all circumstances. Its lift (both vertical and lateral components) is a function of its speed through the water, its submerged surface, and its angle of attack relative to the free flow of water.

At relatively slow speeds, laminar foils, such as those used by sailboards, are much more efficient than "ventilating" kiteboards. In fact, a typical kiteboard is a rather crude and inefficient device at lower speeds, much the same way that aircraft designed for supersonic flight are very inefficient when flown at subsonic speeds. But at speeds that would cause sailboards, and other craft optimized for laminar dynamics, to transition to cavitation or ventilation modes, kiteboards suffer no such peril. Theoretically, laminar craft can continue to operate when subjected to cavitation or ventilation, but their foils are typically too small to be efficient or controllable under such influences.

So, there is no inherent barrier to the maximum speeds that properly designed kiteboards might achieve. 50, 60, 70 knots, or higher, should be possible. Current speed boards are dsigned with minimal rocker, and minimal resistance to both air and water. My guess is that future, faster boards will employ new innovations, such as "steps" that minimize wetted surface and serve to optimize "effective" aspect ratio while maintaining controllability.

©2007