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I don't understand why lift gives an advantage, it could be working the same

enter image description here even if it was just flat.

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    $\begingroup$ When you use it for steering, the wing-like shape improves the effect reducing the pressure coming from the side opposite to the direction you are taking. It is basically the same principle of wings (the shape reduces the pressure from above the wing). You manage to steer with less angular displacement of the rudder $\endgroup$ – AoZora Apr 24 at 6:25
  • $\begingroup$ Just to clarify: you do realize that "lift" for the rudder is in the sideways direction for the hull, don't you? $\endgroup$ – D. Halsey Apr 24 at 19:55
  • $\begingroup$ Flat wings on airplanes would also work, but not very well, because flow would separate from the surface at the leading edge, causing a stall at low angles of incidence with the flow. When that happens, lift decreases while drag increases. To get the best lift/drag ratio the flow has to follow the surface to the trailing edge. $\endgroup$ – Mike Dunlavey Apr 25 at 0:20
  • $\begingroup$ @ Mike Dunlavey What you say is usually true, except at very low Reynolds numbers (<50,000 or so). The flat wing is actually better on very small models (& insects of course). $\endgroup$ – D. Halsey Apr 25 at 1:16
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A wing is an object which is designed to generate lift while keeping drag low. That's exactly what you want from a rudder: lift from the rudder turns the ship, while drag from it means you need to burn more fuel.

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