# Drag power speed relationship

### Drag Power -- from Eric Weisstein's World of Physics

Makes various speeds independent of altitude. e.g. stall speed, minimum drag speed etc. Example: variation of true airspeed stall with altitude. The hard part of this relationship lies in the detailed way speed affects drag. . Replace the generic force variable with a generic power equation for drag. Explore the relationship between your cycling power (wattage) and speed. . Aerodynamic drag: As you cycle through the air, your bike and body need to push .

While it is possible to drive a displacement hull faster than a speed-length ratio of 1.

Most large vessels operate at speed-length ratios well below that level, at speed-length ratios of under 1. Ways of reducing wave-making resistance[ edit ] Since wave-making resistance is based on the energy required to push the water out of the way of the hull, there are a number of ways that this can be minimized. Reduced displacement[ edit ] Reducing the displacement of the craft, by eliminating excess weight, is the most straightforward way to reduce the wave making drag.

Another way is to shape the hull so as to generate lift as it moves through the water. Semi-displacement hulls and planing hulls do this, and they are able to break through the hull speed barrier and transition into a realm where drag increases at a much lower rate. The disadvantage of this is that planing is only practical on smaller vessels, with high power-to-weight ratios, such as motorboats.

### An interactive model-based calculator of cycling power vs. speed

It is not a practical solution for a large vessel such as a supertanker. Fine entry[ edit ] A hull with a blunt bow has to push the water away very quickly to pass through, and this high acceleration requires large amounts of energy. By using a fine bow, with a sharper angle that pushes the water out of the way more gradually, the amount of energy required to displace the water will be less.

A modern variation is the wave-piercing design. It should be noted that the total amount of water to be displaced by a moving hull, and thus causing wave making drag, is the cross sectional area of the hull times distance the hull travels, and will not remain the same when prismatic coefficient is increased for the same lwl and same displacement and same speed.

The bulb alters the waves generated by the hull, by changing the pressure distribution ahead of the bow. Because of the nature of its destructive interference with the bow wave, there is a limited range of vessel speeds over which it is effective. According to our sensible model derived from Bernoulli's sensible equation, drag should sensibly be proportional to the square of speed. Drag is a complex phenomena. It cannot always be described with equations that are simple.

## Wave-making resistance

My first guess would always be that drag is proportional to the square of speed since I understand and like the derviation I've presented, but I would not be surprised if over some range of values drag and speed were found to be directly proportional, proportional to some power other than 2, or related by some polynomial.

Welcome to the world of empirical modeling — where relationships are determined by actual physical experiments rather than an ideology of pure theory.

More on this subject later in this section. A nice equation to work with — or is it? Well, yes and no.

Yes, but it works only as long as the range of conditions examined is "small". That is, no large variations in speed, viscosity, or crazy angles of attack.

The way around this is to reduce the coefficient of drag to a variable rather than a constant. I can live with this. Say that C depends on some yet to be specified set of factors.

- Drag (physics)
- Parasitic drag
- Aerodynamic Drag

It is totally acceptable to say that it varies with this that or the other quantity according to any set of rules determined by experiment. No, since speed is squared. Have you ever tried to solve a nonlinear differential equation? Well, welcome to hell. Wait, let me rephrase that — Welcome to Hell! Just wait till you see what's in store for you when you try to solve the differential equations. The mathematics will consume you.

What the hell was that all about? I might not know how to solve every kind of differential equation off the top of my head, but so what.

I can always look for the solution in a book of standard mathematical tables or an on-line equivalent.