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Why do rudders and propeller brackets create eddy-making resistance?

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naval architecture A cargo ship passing the Golden Gate Bridge, near San Francisco.

In naval architecture, rudders and propeller brackets create eddy-making resistance because they are appendages that disrupt the smooth flow of water around the hull. When water encounters these structures, the flow may separate from the surface, creating a region of turbulence and eddies behind them.123

This phenomenon is a form of pressure resistance; as the water separates, the pressure in the resulting eddying zone drops below the ambient pressure, effectively creating a suction effect that pulls backward on the ship. This process consumes energy, which is dissipated as heat and contributes to the total resistance the ship must overcome to maintain forward motion.213

To minimize this resistance, naval architects typically employ the following engineering strategies:
* Streamlining: Giving appendages an airfoil shape to encourage the water to flow smoothly around them without separating.1
* Alignment: Orienting the appendages so that the approaching water meets them at a low angle of attack, which reduces the likelihood of flow separation.1