5 ms·

Comparing hydrodynamics and aerodynamics here is not quite comparing apples to apples: I expect much less surface area is exposed to water on a ship than expos

by graeham 14y ago

Comparing hydrodynamics and aerodynamics here is not quite comparing apples to apples: I expect much less surface area is exposed to water on a ship than exposed to air on a train, and speed are lower. Drag scales with velocity squared. A calculation is required, and would also have to account for the weight of cargo each is carrying.

robomartin 14y ago
Sure it is. Without air or water the cost of shipping would be almost nil. POWER to overcome drag is proportional to the CUBE of the velocity. This is the main reason container ships have been slowing down from about 30 miles per hour to 20 in direct response to the cost of fuel. The power to overcome drag is also directly proportional to the density of the fluid. Air density: 1.2041 kg/m^3 Water density: 1,000.00 kg/m^3 (pure water) Sea water is roughly 830 times denser than air. Therefore, you need 830 times more POWER to move the same object through water at the same speed when compared with air. Here's a practical example: Let's move a Toyota Camry through air and water at 20 miles per hour and compare. We'll ignore friction. http://www.toyota.com/camry/specs.html http://www.toyota.com/camry/specs.html Frontal area: 57.9in x 71.7in = 2.68 m^2 Coefficient of drag: 0.28 Power = 1/2 * density * velocity ^ 3 * area * coefficient of drag Power in air = 1/2 * 1.2041 Kg/m^3 * (8.9 m/s ^ 3) * 2.68 m^2 * 0.28 = 0.43 horsepower Power in water = 1/2 * 1,000 Kg/m^3 * (8.9 m/s ^ 3) * 2.68 m^2 * 0.28 = 353.02 horsepower In other words, you need 830 times more power. How about now moving up to 40 miles per hour? Power in air = 3.4 HP Power in water = 2,824.14 The same 830 to 1 power ratio. However, moving the same object through water at double the speed just became EIGHT TIMES more expensive. If we moved the Toyota Camry through air at 188 miles per hour we would need nearly exactly the same power required to move the same car through water at 20 miles per hour. Bottom line, moving anything on water is very expensive if you have to burn fuel to do it. Unless I am missing something very fundamental there simply is no comparison.
- graeham 14y ago
  My "apples to apples" point is that density is not the only parameter to change when comparing ships to trains. 1. Assuming the same speeds. Ships go slower than trains (say 40kmph compared to 80kmph). 2. The frontal area is not the same as only part of the ship is underwater. Further, for a long train, the frontal area drag is likely less than the skin friction drag. 3. The coefficient of drag is not the same as the CD of a train is different than for a boat. 4. The amount of cargo on each is not the same, and would need to be normalized for a fair comparison. I'd be happy to take a quick look at your materials off-line, please send to jd2planks at hotmail
  - robomartin 14y ago
    I really don't see it as even close. Moving anything through air requires far less power than moving the same thing through water. With regards to the rest of your comments, I've done some of those calculations. Again, there is no comparison. I need to repeat that the idea here is NOT to in ANY WAY use existing truck or cargo rail systems. They just don't perform. Here in the US you are lucky if you can get cargo trains moving on average at 20 to 30 miles per hour. In other words, the move just as slowly as cargo ships, except that they can't pull the same number of containers. The idea is that a new purpose-built system is the solution. Also, please keep in mind that the idea here isn't necessarily to search for the most efficient mode of cargo transportation in complete isolation of the realities of where they derive their energy from. Here's a very interesting paper on the aerodynamics of high-speed trains: http://ompldr.org/vMXlsOA/Aerodynamics_HSR02.pdf http://ompldr.org/vMXlsOA/Aerodynamics_HSR02.pdf Anyhow, I sent you all my files via a shared Dropbox folder. I'd be interested in what you have to say once you have a chance to look at it all. To be sure, there are huge issues with the idea that have absolutely nothing whatsoever to do with science and technology. I could not imagine the problems one might encounter in terms of the various special-interest groups that would get in the way of building such a thing, even if it was so good that it would save the world. I think the phrase "can't get out of our own way to save our lives" rings true in more than one way, at least in the US.
    - leoedin 14y ago
      I think the problem is that you're imagining the same frontal area moving through two fluids. Certainly moving the same toyota camry through air and water will give vastly different results, but if you turned that toyota camry upside down and sealed all the holes, the actual wetted area that needed to be pushed through the water would be significantly less. I'd be happy to look at your calculations. My knowledge of hydrodynamics is somewhat limited, but I do have a fairly good understanding of aerodynamics and a lot of that stuff is transferable. I can't decipher your email riddle though - is [4:5] saying the 4th and 5th characters? Mine is leorampen (at) gmail.
- URSpider94 14y ago
  This is just not right. Unless it's a submarine, a ship doesn't move through water, it floats on top of it. The wetted surface is going to be a tiny fraction of the total frontal area. 90% or more of the hydrodynamic drag of a ship (by area) is in the air, not in the water. For example, here is a nice little 21-foot 1200-lb motor boat that can reach 16 knots with a 25 hp motor, while carrying four people: http://www.southportislandmarine.com/ordereze/1011/page.aspx http://www.southportislandmarine.com/ordereze/1011/page.aspx
  - robomartin 14y ago
    No, the calculations are exactly right given the conditions I presented. The nice little boat you are linking to can do what it does largely because at a certain speed it will start to plane. These are very, very different conditions. Two of the components of drag on a boat are form and parasitic or frictional drag. The first can be thought of as simply the drag created to push water out of the way. Here the shape of the boat is important. The second is the drag created by water "rubbing" against the portion of the hull in contact with it. Think of sandpaper against wood. More wood and more sandpaper means more resistance to motion. As speed increases both friction and form drag increase with the square of the speed. On a planing hull --one that can generate lift and get the boat up and partially out of the water-- form drag decreases significantly once you are "on a plane" (off the water) and friction drag increases. As it turns out, each boat has a sweet spot where it operates best. And, of course, as you get on a plane the power required in order to reach or sustain a given speed decreases significantly when compared to a displacement (non-planing) hull. A Panamax ship carrying 4,400 TEU does not plane. It is, as far as a know, a pure displacement vessel. So, yeah, you need a tremendous amount of power to counteract significant components of both form and parasitic drag. A Panamax container ship can have up to nearly 400 square meters of cross-sectional area under water. That's about 4,000 square feet. So, while it isn't exactly the same thing, only for the purpose of visualization, imagine a 100 foot long x 40 foot rigid piece of plywood fully submerged in water. Not try to push it forward at 20 miles per hour. Right. That's why they have to burn nearly a million and a half dollars of bunker-C fuel to get from Los Angeles to NYC. Physics can be really inconvenient sometimes.

by graeham 14y ago

robomartin 14y ago

Sure it is. Without air or water the cost of shipping would be almost nil. POWER to overcome drag is proportional to the CUBE of the velocity. This is the main reason container ships have been slowing down from about 30 miles per hour to 20 in direct response to the cost of fuel. The power to overcome drag is also directly proportional to the density of the fluid. Air density: 1.2041 kg/m^3 Water density: 1,000.00 kg/m^3 (pure water) Sea water is roughly 830 times denser than air. Therefore, you need 830 times more POWER to move the same object through water at the same speed when compared with air. Here's a practical example: Let's move a Toyota Camry through air and water at 20 miles per hour and compare. We'll ignore friction. http://www.toyota.com/camry/specs.html http://www.toyota.com/camry/specs.html Frontal area: 57.9in x 71.7in = 2.68 m^2 Coefficient of drag: 0.28 Power = 1/2 * density * velocity ^ 3 * area * coefficient of drag Power in air = 1/2 * 1.2041 Kg/m^3 * (8.9 m/s ^ 3) * 2.68 m^2 * 0.28 = 0.43 horsepower Power in water = 1/2 * 1,000 Kg/m^3 * (8.9 m/s ^ 3) * 2.68 m^2 * 0.28 = 353.02 horsepower In other words, you need 830 times more power. How about now moving up to 40 miles per hour? Power in air = 3.4 HP Power in water = 2,824.14 The same 830 to 1 power ratio. However, moving the same object through water at double the speed just became EIGHT TIMES more expensive. If we moved the Toyota Camry through air at 188 miles per hour we would need nearly exactly the same power required to move the same car through water at 20 miles per hour. Bottom line, moving anything on water is very expensive if you have to burn fuel to do it. Unless I am missing something very fundamental there simply is no comparison.

graeham 14y ago
My "apples to apples" point is that density is not the only parameter to change when comparing ships to trains. 1. Assuming the same speeds. Ships go slower than trains (say 40kmph compared to 80kmph). 2. The frontal area is not the same as only part of the ship is underwater. Further, for a long train, the frontal area drag is likely less than the skin friction drag. 3. The coefficient of drag is not the same as the CD of a train is different than for a boat. 4. The amount of cargo on each is not the same, and would need to be normalized for a fair comparison. I'd be happy to take a quick look at your materials off-line, please send to jd2planks at hotmail
- robomartin 14y ago
  I really don't see it as even close. Moving anything through air requires far less power than moving the same thing through water. With regards to the rest of your comments, I've done some of those calculations. Again, there is no comparison. I need to repeat that the idea here is NOT to in ANY WAY use existing truck or cargo rail systems. They just don't perform. Here in the US you are lucky if you can get cargo trains moving on average at 20 to 30 miles per hour. In other words, the move just as slowly as cargo ships, except that they can't pull the same number of containers. The idea is that a new purpose-built system is the solution. Also, please keep in mind that the idea here isn't necessarily to search for the most efficient mode of cargo transportation in complete isolation of the realities of where they derive their energy from. Here's a very interesting paper on the aerodynamics of high-speed trains: http://ompldr.org/vMXlsOA/Aerodynamics_HSR02.pdf http://ompldr.org/vMXlsOA/Aerodynamics_HSR02.pdf Anyhow, I sent you all my files via a shared Dropbox folder. I'd be interested in what you have to say once you have a chance to look at it all. To be sure, there are huge issues with the idea that have absolutely nothing whatsoever to do with science and technology. I could not imagine the problems one might encounter in terms of the various special-interest groups that would get in the way of building such a thing, even if it was so good that it would save the world. I think the phrase "can't get out of our own way to save our lives" rings true in more than one way, at least in the US.
  - leoedin 14y ago
    I think the problem is that you're imagining the same frontal area moving through two fluids. Certainly moving the same toyota camry through air and water will give vastly different results, but if you turned that toyota camry upside down and sealed all the holes, the actual wetted area that needed to be pushed through the water would be significantly less. I'd be happy to look at your calculations. My knowledge of hydrodynamics is somewhat limited, but I do have a fairly good understanding of aerodynamics and a lot of that stuff is transferable. I can't decipher your email riddle though - is [4:5] saying the 4th and 5th characters? Mine is leorampen (at) gmail.
URSpider94 14y ago
This is just not right. Unless it's a submarine, a ship doesn't move through water, it floats on top of it. The wetted surface is going to be a tiny fraction of the total frontal area. 90% or more of the hydrodynamic drag of a ship (by area) is in the air, not in the water. For example, here is a nice little 21-foot 1200-lb motor boat that can reach 16 knots with a 25 hp motor, while carrying four people: http://www.southportislandmarine.com/ordereze/1011/page.aspx http://www.southportislandmarine.com/ordereze/1011/page.aspx
- robomartin 14y ago
  No, the calculations are exactly right given the conditions I presented. The nice little boat you are linking to can do what it does largely because at a certain speed it will start to plane. These are very, very different conditions. Two of the components of drag on a boat are form and parasitic or frictional drag. The first can be thought of as simply the drag created to push water out of the way. Here the shape of the boat is important. The second is the drag created by water "rubbing" against the portion of the hull in contact with it. Think of sandpaper against wood. More wood and more sandpaper means more resistance to motion. As speed increases both friction and form drag increase with the square of the speed. On a planing hull --one that can generate lift and get the boat up and partially out of the water-- form drag decreases significantly once you are "on a plane" (off the water) and friction drag increases. As it turns out, each boat has a sweet spot where it operates best. And, of course, as you get on a plane the power required in order to reach or sustain a given speed decreases significantly when compared to a displacement (non-planing) hull. A Panamax ship carrying 4,400 TEU does not plane. It is, as far as a know, a pure displacement vessel. So, yeah, you need a tremendous amount of power to counteract significant components of both form and parasitic drag. A Panamax container ship can have up to nearly 400 square meters of cross-sectional area under water. That's about 4,000 square feet. So, while it isn't exactly the same thing, only for the purpose of visualization, imagine a 100 foot long x 40 foot rigid piece of plywood fully submerged in water. Not try to push it forward at 20 miles per hour. Right. That's why they have to burn nearly a million and a half dollars of bunker-C fuel to get from Los Angeles to NYC. Physics can be really inconvenient sometimes.