At that point, Marlon had dropped the vessel’s speed to more like five knots. This was not the absolute slowest they could go, but if they went any slower they lost all sense that they were making forward progress, and the boat seemed to wallow and wander (an impression that could be confirmed by zooming in on the track and observing the way it staggered across the screen). The rudder, it seemed, was not capable of doing its job unless water was flowing across it with at least some minimum speed.