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:
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..."
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.
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.
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.