3 ms·

The rule of thumb for well streamlined objects in a liquid is that the power required goes up with the square of the speed. So twice the speed costs 4 times the

The rule of thumb for well streamlined objects in a liquid is that the power required goes up with the square of the speed. So twice the speed costs 4 times the power. This works for ships, aircraft and ground vehicles. So it is possible that the future will be significantly slower because we are currently like to run well above the most efficient speed for things.

javiramos 7y ago
Power required to overcome a drag force is proportional to the cubed velocity (drag force is proportional to the squared power). So twice the speed equals 8 times the relative power. Nevertheless, it makes your point even stronger.
- esotericn 7y ago
  Your factor of two is removed by the fact that the power is applied for half the time. A car travelling 70mph gets to its' destination twice as quickly as a car travelling 35mph.
bzbarsky 7y ago
That quadratic behavior is only true for high Reynolds numbers, because the drag coefficient in https://en.wikipedia.org/wiki/Drag_equation https://en.wikipedia.org/wiki/Drag_equation is itself dependent on the Reynolds number (and hence on the velocity). The quadratic behavior actually sets in at _lower_ Reynolds numbers for less-streamlined objects. Now as it happens the Reynolds number for a large cargo ship is almost certainly quite large, because it involves the product of the ship's velocity and some linear measure of its size, and container ships are huge. https://en.wikipedia.org/wiki/Reynolds_number#Similarity_of_flows https://en.wikipedia.org/wiki/Reynolds_number#Similarity_of_... lists ~5e9 for a large passenger ship, and the quadratic behavior sets in somewhere in the 1e3-1e7 range depending on how streamlined you are. So we're well into the quadratic regime in this specific case. But assuming that you're there in all cases is not a great idea. For example, it's not clear to me that a walking human is in the quadratic regime (though I am pretty sure a running human typically is). Small fish are typically _not_ in the quadratic regime, I expect.

javiramos 7y ago

Power required to overcome a drag force is proportional to the cubed velocity (drag force is proportional to the squared power). So twice the speed equals 8 times the relative power. Nevertheless, it makes your point even stronger.

esotericn 7y ago
Your factor of two is removed by the fact that the power is applied for half the time. A car travelling 70mph gets to its' destination twice as quickly as a car travelling 35mph.

bzbarsky 7y ago

That quadratic behavior is only true for high Reynolds numbers, because the drag coefficient in https://en.wikipedia.org/wiki/Drag_equation https://en.wikipedia.org/wiki/Drag_equation is itself dependent on the Reynolds number (and hence on the velocity). The quadratic behavior actually sets in at _lower_ Reynolds numbers for less-streamlined objects. Now as it happens the Reynolds number for a large cargo ship is almost certainly quite large, because it involves the product of the ship's velocity and some linear measure of its size, and container ships are huge. https://en.wikipedia.org/wiki/Reynolds_number#Similarity_of_flows https://en.wikipedia.org/wiki/Reynolds_number#Similarity_of_... lists ~5e9 for a large passenger ship, and the quadratic behavior sets in somewhere in the 1e3-1e7 range depending on how streamlined you are. So we're well into the quadratic regime in this specific case. But assuming that you're there in all cases is not a great idea. For example, it's not clear to me that a walking human is in the quadratic regime (though I am pretty sure a running human typically is). Small fish are typically _not_ in the quadratic regime, I expect.