"I cherish the wild grass growing by the stream, above which orioles sing in the deep woods. The spring tide, carrying rain, rushes in late, and at the deserted ferry, the boat lies adrift." This well-known poem, "West Stream of Chuzhou," was written by the Tang Dynasty poet Wei Yingwu when he served as the prefect of Chuzhou (present-day Chuzhou City, Anhui Province). "The spring tide, carrying rain, rushes in at dusk; at the deserted ferry, the boat lies adrift, unmanned," describes a small, unmanned boat moored at a rural ferry crossing, adrift in the swift current of the stream formed by the evening tide and spring rain. This vividly and realistically depicts the small boat adrift on the river, which, in order to maintain a stable balance, always lies adrift. Why do small boats always end up sideways in a river? In fluid mechanics, a small boat can be simplified as an elliptical cylinder. When the boat is in equilibrium with the current (either in the direction of flow or across the river), its stability is quite different. In the case of a forward-flowing current, if for some reason the angle between the hull and the current changes by even a tiny angle, the torque on the small boat will cause the angle to become larger and larger, so this equilibrium position is unstable. When there is a transverse current, if for some reason the angle between the hull and the current is deflected by any small counterclockwise or small clockwise angle, the torque on the small boat will cause the deflection angle to decrease more and more, so the equilibrium position is stable. The experiment confirmed the results of the above analysis, indicating that the elliptical cylinder is indeed in a stable equilibrium position when its major axis is perpendicular to the incoming flow. This principle regarding the stability of a stationary boat also applies to a boat in motion. When a boat moves forward with the current, its balance is unstable, and the helmsman needs to constantly adjust the steering to maintain the boat's course. This is why a novice rower often struggles to keep a boat sailing straight. When first learning to row, the boat often spins in the water.
An unmanned ferry moored in the countryside, in the swift current, always seems to drift sideways, swaying near its position when the current is flowing across. This is the reason for the phenomenon of "an unmanned ferry drifting sideways." Of course, the stability of a small boat during navigation also needs to be considered, along with its inertia. These aspects constitute the in-depth study of the stability of modern guided ships and aircraft during navigation. It is one of the theoretical foundations of modern navigation, aviation, and aerospace technologies. "The spring tide, carrying rain, rushes in late; at the deserted ferry, the boat lies adrift." More than 1,200 years ago, the Tang Dynasty poet Wei Yingwu made a meticulous observation of the stability of a boat, vividly depicting it in just 14 characters. This not only provides us with aesthetic enjoyment but also allows us to appreciate the marvelous laws of nature.