How a single rudder can turn a destroyer

Why did Fletcher-class destroyers use a single rudder if destroyers need agility?

A rudder's effectiveness depends on the water flow reaching it, so one well-fed rudder can outperform two receiving poorer flow.

Water flowing across a rudder creates the force that turns a ship. A propeller arrangement that sends a strong stream onto one central rudder can give it considerable authority. Two rudders offer no automatic advantage if each receives a weaker flow. Fletcher-class agility therefore depended on the relationship between propellers, water flow, and the steering surface.