Kiteboard Theory - Thread 9: Force, Energy, Power, etc

Here are some definitions and examples and stuff. I know that none of this info replaces good technique, but I believe that a decent grasp of the physical principles at play in any endeavour can help to advance the boundaries of performance. If any of it sinks in, I guess it's worthwhile.


Velocity is a vector which represents speed and direction. The term is often used erroneously to represent speed, which is a scalar value (having no specific direction), but in most discussions that isn't a big deal.


Force is typically measured in pounds or Newtons. Like velocity, force has both magnitude and direction, but the direction is often ignored in very simple discussions. Lift and drag generated by a kite represent force, as does the tension in the lines and what is often erroneously termed "pressure" on the bar. Force mustn't be confused with pressure, energy or power. They are all completely different concepts.


Pressure is the force per unit of area. In a kite bladder, that's usually anywhere from about 5-10 psi (pounds per square inch), which is about 1/3 - 2/3 of atmospheric pressure at sea level (~15 psi). If your kite bladder is inflated to 7 psi, that means "gauge pressure" which is the difference between the "absolute pressure" inside the bladder (~22 psi) and outside it (~15 psi).

The tension, or force, acting on the fabric of a tubular bladder casings is proportional to the diameter of the tube. The fabric and stitching of an 8" diameter tube has to resist twice the tension, or force, of that of 4" diameter tube, given the same pressure.

The aerodynamic pressure acting on the surface of a kite will vary across its surface, but on average, if there are 200 pounds of tension in the lines of a 12 m kite, then the average pressure acting on the surface of the kite would be about 200 lbs/18,600 sq inches = 0.01 psi. At about 1/1,000 of bladder pressure, that's pretty light pressure. This is "dynamic" pressure, caused by the flow of a fluid, as opposed to "static" pressure, such as inside the bladder.


Energy can be found in different forms, including kinetic energy (proportional to mass times the square of velocity), gravitational (potential) energy (proportional to mass times height), heat energy, wave energy, electrical energy, chemical energy and atomic energy. It is normally measured in Joules, BTU's, watt-hours or kWh. Energy can be converted between forms, but it is always conserved, and it is a scalar value (having no direction). Kinetic energy and gravitational potential energy are most relevant to kiteboarding, although heat and wave energy also play a role.

When energy is converted from one form to another, it can be said that "work" is performed. When a force is exerted, causing a particular object to move, then the energy converted, and work performed, can be calculated by multiplying the force by the distance that the object is moved. The energy converted (from kinetic to potential) when a kiteboarder jumps, can be calculated by multiplying the force acting on him (his weight) by the height that he is elevated by that force.

BTW, weight is defined as the mass of an object multiplied by the force of gravity. The mass of an object is constant (notwithstanding the effects of general relativity) but it's weight varies depending where it is. It weighs more on Mount Everest than it does on the moon, and slightly more again at sea level.


Power is the rate at which a device can convert energy from one form to another, and is usually measured in watts or horsepower. It can be a confusing term, because the relevant types of energy have to be defined. A 100 watt lightbulb converts electrical energy to light energy and heat energy at a rate of 100 watts, but if the lightbulb is only 20% efficient, that means it only converts electrical energy to light energy at a rate of 20 watts.

A bouncing rubber ball, and a jumping kiteboarder, both convert energy alternately between kinetic and potential forms, but because they aren't sustainable processes, power ratings aren't relevant in these examples.

In some circumstances, power can be quantified by multiplying a force by the speed at which that force is exerted. Here are some examples of power calculations based on specific examples:

1. Heavy(ish) kiteboarder on freestyle board going 25 mph with 30 lbs resistance (independent of wind speed and kite size): power = 2.0 hp

2. Light kiteboarder on high L/D ratio board (eg Spleene Session) going 18 mph with 20 lbs resistance: power = 0.96 hp

3. Same kite as in example (1.), attached to a Honda Civic with tires overinflated to reduce resistance, on a broad reach on an airport runway, with wind blowing like !@#$%, chicken loop mechanically fastened to car (pilot couldn't possibly hold it), going 50 mph with 40 lbs resistance (rolling resistance only, apparent wind over the beam): power = 5.3 hp

A kite alone does not generate power. Power is generated when a traction kite and surface vehicle interact as a system, whether that vehicle is a kiteboard and rider, a buggy and rider, a car, an ice boat, or whatever.

The maximum achievable power is based largely on what the vehicle can handle. A given vehicle will have a maximum practical upper limit to power generation. If the vehicle is a particular kiteboard and rider, they may have an approximate upper end of 1.5 hp, regardless of whether they achieve it with a big kite in light wind or a small kite in high wind.

The heavier and stronger the rider, and the more efficient the board, the higher the maximum power will be. A 200 sq m monster kite wouldn't help a kiteboarder generate more power than with a normal kite, but a freighter or a heavyweight ice boat could generate huge power with the monster kite.