As the foil is more submerged the drag increases. When it touches the water surface the drag increases significantly. How will a plot of drag vs height above/partly submerged/under the surface look? As airplanes has less drag when flying high, I assume the drag in the plot will be minimum high up in the air, then drag increases as the height comes down towards sea level. Will it create more drag if it is half submerged than if it is fully submerged? I feel like I've learned more than I was asking.Īssumed a symetric vertical foil moving horizontally with 0 angle of attack. In contrast, however, supersonic airfoils are necessarily as thin as reasonably possible, but the body plan of a ship stays pretty fat since it needs lateral stability and actual cargo space. In other words, the body plan of a ship hull will have features such as a sharp tip that are more analogous to supersonic airfoils than to subsonic airfoils. A sharp edge is going to do a better job of "cutting" through the water and form an attacked wave (similar to an attacked oblique shock) that has less drag. The bow waves are similar to shock waves on a supersonic aircraft, and a blunt tip is going to cause a large bow wave (analogous to a supersonic bow shock) that greatly increases drag. The physics of surface waves and surface hydrodynamics are actually intimately related to supersonic aerodynamics. In contrast, surface ships move along the surface of the water and generate surface waves in the process. As Ketch mentioned, the bow of a surface ship that remains under the water often looks like this, too. There are therefore many parallels between the external aerodynamics of a wing and the external hydrodynamics of a submarine. Submarines have blunt tips and look more "airfoil-ish" because they operate underwater where the distinction between moving through air and water is minimal. Ketch also mentions this at the end, which is nice. That's unfortunate because it is an excellent analogy. It may help to look at a pressure profile, though I'm not sure they are easy to find for a sharp nose/leading edge (they are for normal profiles).Īpparently posted about supersonic airfoils and got their comment deleted. But on the back end, the air has to come back together smoothly a blunt back end would have the air converging faster than it should, creating a low-pressure area at the back. Air needs to be smoothly directed around the fuselage, but having a sharp point (on a subsonic plane anyway) doesn't really help that. A wing needs to maximize lift while minimizing drag, and the shape isn't otherwise that important for the functionality of the plane, whereas the fuselage of a plane needs to have enough volume to hold its cargo while minimizing drag. It's written that the round shape makes the fluid follow the foils surface better than with a sharp front, but I don't understand why it is like that in applications where you want the flow on both sides of the foil to be equal (lift = 0).Ĭross sectional area and volume enclosed are part of this. This is comparable to the shape of the nose or the leading edge of the wings of supersonic jets which are sharp. The sharpness of the bow is in relation to the angle of the bow wave and the faster the ship the sharper the bow at the waterline. The wave makes an angle with the bow dictated by the speed of the boat and the speed of the wave. Ships and boats tend to move faster than the speed of the surface waves, so the bow wave is a shock wave. Doing so produces surface waves, and how you produce those waves can take more or less energy. Folding it aside at the surface is even more important.Įven at the surface you can’t just think of pushing the water aside. This makes it much harder to move through water as the water has to have somewhere to go. This is very different than being surrounded by fluid.Īnother important difference is that water is essentially incompressible while air is very compressible. There is a place for water to be pushed aside where there isn’t already more water in the way. The surface presents an opportunity that doesn’t exist when you are immersed in the fluid. The sharp part of the bow is about slicing the surface and folding the water to the side. The bow of most ships are actually bulbous beneath the water line and sharp at the water line.
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