Kiteboard Threory - Thread 12: Speed and Kites


This is a follow up to Thread 11, which touched on the fundamantal advantage of current kiteboard designs at the high speeds that cause most laminar hydrofoils (fins) to cavitate and/or ventilate. Before I move on, I'll summarize the basic advantages and disadvantages of kiteboards, relating to performance, as compared to sailboards and other high performanace sailcraft:

1. GEOMETRIC CONVERGENCE OF FORCES

Kite driven craft have an immense advantage over most other sailcraft in that sail forces, lateral resistance forces, gravitational forces and bouyant forces almost intersect. This is the holy grail of performance sailing, beacuse it virtually eliminates heeling, which has been a fundamental impediment to speed in sailcraft since the beginning of time. Heeling forces limit the amount of sail that a craft can carry in a particular set of conditions, and therefore the maximum practical propulsive force. Multihull sailboats, moveable keels and ballast, and more recently, reactive negative-lift hydrofoils (eg Talking Yellow Pages) are all designed to counter heeling forces while preserving hull and/or foil efficiency. Some historic sailboats were designed with complicated canting rigs in an attempt to intersect dynamic forces, but modern kite driven craft achieve the same thing with elegant simplicity. It is becasue of this that modern kiteboards can compete, in some circumstances, with other high performance craft including sailboards and catamarans, even though most kiteboards and kites have comparatively primitive performance attributes. Put simply, a kiteboarder can carry immensely more sail area per pound than a sailboarder or other craft, which helps to overcome the inefficency of the kiteboard and kite. Could you imagine a kiteboarder trying to race a sailboarder in 18 kts true with a 5 or 6 m kite? KITEBOARD: ADVANTAGE

2. BOARD DESIGN

In Thread 11, I explained the advantage that kiteboards have at very high speeds, but at typical speeds, most kiteboards are optimized for control, maneuverability and freestyle, at the cost of dynamic performance. If you want to kiteboard fast in a straight line, you'd be much better off learning to kite while riding a good sailboard, flat, with the very-efficient laminar fin resisting lateral forces. It might be hard to control, and tricks would suck, but you'd leave your twin-tip and/or ventilating short-fin friends in the dust, unless you cavitate or ventilate, or happened to be riding in very shallow water. KITEBOARD: DISADVANTAGE AT NORMAL SPEEDS, ADVANTAGE AT VERY HIGH SPEEDS OR SHALLOW WATER

3. SAIL/KITE EFFICIENCY

Current traction kite designs are designed for maneuverability, safety, launching, relaunching and convenience. Furthermore, leading-edge-inflatable (LEI) kites are still in their relative design infancy. Consequently, relative performance falls far short of more evolved sail designs found on other high performance sailcraft, including sailboards and modern racing yachts. For reference, consider that some modern monohull and and multihull yachts can cross the Atlantic Ocean at an average speed of over 30 kts, and peak speeds of over 40 kts, even in relatively heavy seas. Even though these craft don't benefit from the geometry outlined above in (1.), their vastly superior hull and rig efficiencies, represented by their respective L/D ratios, more than compensate. KITEBOARD: DISADVANTAGE


So, having the fundamental advantage described in (1.), can you imagine how unbeatable kiteboarders could be if they overcame the design disadvantages of (2.) and (3.)? Having touched on boards in the last thread, I'll get into kites now. Simply put, modern kites are heading in the right direction for recreational and freestyle riding, but for pure speed, they're way off the mark. Most current LEI kites have relatively low L/D ratios between 5:1 and 7:1, which I've explained in detail in previous theory threads. I confidently predict that we will see speed records obliterated by efficient wing kite designs, probably paired with more controllable (and likely stepped) boards.

Here's why. Modern performance gliders have L/D ratios of over 50:1, and can resist forces of several thousand pounds without breaking. Adapting these designs to kiteboarding would be a much smaller undertaking than some of the ther speed contraptions that are built to challenge speed records. Wing kites could be scaled down, lighter versions of glider wings, with suitable control surfaces for maneuverability and active inversion resistance.

To illustrate the benefit of a 50:1 L/D ratio kite, versus a conventional 7:1 L/D high performance kite, assume that recent 50 kt kiteboard runs have been achieved at an angle of 45 degrees off a beam reach in 40 kt true wind, which is actually just about right. The apparent wind speed would be 35.67 kts, 52.48 degrees from the kiteboarder's direction of travel. A 7:1 L/D ratio kite, flown low, would pull at an angle of about 45.65 degrees from straight ahead.

A 50:1 L/D ratio wing kite, on the other hand, could achieve the same force, in the same direction relative to the rider's course, in about 32 knots of wind. In this particular case, the rider would be riding at only about 35 degrees off a beam reach. Considering that a 40 kt wind is over 56% more powerful than a 32 kt wind, this represents an immense power reserve. The frequency of reliable 40 kt true wind is a fraction of that of reliable 32 kt wind, and 32 kt wind doesn't cause as much trouble with control and waves.

Other areas that could use improvement for speed riding include better streamlining of the rider's suit and the kite lines.

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