3 ms·

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

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.

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.