Why Are Big Ships Faster Than Small Ships?

这段视频解释了为什么大型船只通常比小型船只更快，尽管“更重的物体应该更慢”这种想法似乎有悖常理。 解释的核心在于船体、船只在水中航行时产生的波浪，以及“船体速度”的概念之间的关系。 当船只移动时，它的船体会将水推开，从而产生波浪。 这些波浪从船首（前端）开始，因为船体将水推开，并在船尾（后端）产生，因为水涌入填补留下的空间。 这两个波浪系统，一个从船首产生峰值，另一个从船尾产生波谷，相互作用。 在低速时，产生的波浪很短。 随着船只加速，波长（波峰之间的距离）增加。 波长与船速成正比：速度越快，波长越长。 引入的关键概念是“船体速度”。 随着船只加速，它会达到一个点，即它产生的波浪的波长等于船的长度。 在这个特定的速度下，船首波的波峰和船尾波的波谷对齐，导致相消干涉和最小的尾流。 然而，船体经历最大的湿表面积，因此也经历最大的阻力。 这就是船体速度，其计算方法约为水线长度（以英尺为单位）的平方根的 1.34 倍。 这意味着更长的船只固有地具有更高的船体速度。 例如，玛丽皇后二号具有显著的水线长度，因此其船体速度远高于小型休闲船。 视频接着讨论了超过船体速度的情况。 当船只试图以高于其船体速度的速度行驶时，它会达到一个被称为“驼峰速度”（Hump Speed）的点。 在这个阶段，波长达到船体长度的一倍半。 船尾下沉，产生一种“爬坡”的感觉，因为产生了相当大的波浪，使得这个速度效率最低。 克服这个“驼峰”后，存在获得更快速度的潜力，称为“滑行”（Planing）。 然而，由于需要极大的能量，对于排水型船体（例如货船上的船体）来说，通常无法实现滑行。 滑行主要可以通过专门为此目的设计的船体设计来实现，例如在快艇上找到的设计。 该视频描述了一个图表，描绘了在不同速度下，由波浪模式引起的船体阻力。 这个图表表明，这种关系不是线性的。 相反，存在“驼峰”和“颠簸”，导致某些速度下出现“最佳点”。 这些最佳点是阻力低于预期的速度范围。 随着船只长度的增加，这些“驼峰”和“颠簸”发生的 speeds 也会增加。 这进一步解释了为什么大型船只有可能行驶得更快：它们在更高的速度下体验到它们的最佳点。 该视频详细阐述了用于减少波浪阻力的方法。 一种方法是使用球鼻艏，它产生第二个船首波，旨在与主要的船首波发生相消干涉，从而降低阻力。 另一种方法是调整船体形状，以尽量减少波浪的产生。 具有锋利船首的细长船体设计用于更轻柔地将水推开，这可以降低阻力并有助于超过驼峰速度。 这些改进解释了为什么一些较小的船只，例如 300 英尺的渡轮，尽管具有较低的理论船体速度，也能达到显著的高速度。 总而言之，大型船只并非天生就更快，但由于其更大的水线长度，它们拥有更高的船体速度。 虽然小型船只可以通过专门的船体设计和强大的发动机来实现高速，但在商业领域，燃油效率和运营成本至关重要。 因此，大型船只通常被设计和运营以利用其更高的船体速度，这源于它们的尺寸。 本质上，大型船只“更容易”保持更快的速度，因为它们会因水线长度而遇到高得令人望而却步的阻力。

The video explains why larger ships are generally faster than smaller ships, despite the counterintuitive idea that a heavier object should be slower. The explanation centers on the relationship between a ship's hull, the waves it generates as it moves through water, and the concept of "hull speed." When a ship moves, its hull pushes water aside, creating waves. These waves originate at the bow (front) as the hull pushes water out of the way, and at the stern (rear) as water rushes in to fill the space left behind. These two wave systems, one originating with a peak at the bow and the other with a trough at the stern, interact with each other. At slow speeds, the waves generated are short. As the ship speeds up, the wavelength (the distance between wave crests) increases. The wavelength is directly proportional to the ship's speed: faster speed, longer wavelength. The crucial concept introduced is "hull speed." As a ship accelerates, it reaches a point where the wavelength of the waves it generates equals the ship's length. At this specific speed, the bow wave peak and the stern wave trough align, causing destructive interference and minimal wash. However, the hull experiences maximum wetted surface area and, consequently, maximum resistance. This is the hull speed, which is calculated as approximately 1.34 times the square root of the waterline length (in feet). This means that a longer ship inherently has a higher hull speed. For example, the Queen Mary 2, with a significant waterline length, has a hull speed much higher than a small recreational boat. The video then discusses exceeding hull speed. As a ship tries to go faster than its hull speed, it reaches a point known as "Hump Speed." At this stage, the wavelength reaches one and a half times the ship's length. The stern sinks, creating a feeling of "running uphill" because a considerable wave is generated, making this speed the least efficient. After overcoming this "hump," there exists the potential for much faster speeds, called Planing. However, Planing is typically unattainable for displacement hulls, like those found on cargo ships, due to the extreme amount of energy required. Planing is primarily achievable with hull designs tailored for it, such as those found on speedboats. The video describes a graph depicting hull resistance, caused by wave patterns, at different speeds. This graph shows that the relationship is not linear. Instead, "humps" and "bumps" exist, resulting in "sweet spots" at certain speeds. These sweet spots are speed ranges at which the resistance is lower than expected. As a ship's length increases, the speeds at which these "humps" and "bumps" occur also increase. This further explains why larger ships have the potential to travel faster: they experience their sweet spots at higher speeds. The video elaborates ways that are being used to reduce wave resistance. One approach is using a bulbous bow, which generates a second bow wave designed to destructively interfere with the primary bow wave, reducing resistance. Another method is to adapt the hull shape to minimize wave generation. A long, thin hull with a sharp bow is designed to push the water aside more gently, which can reduce resistance and aid in exceeding hump speed. Such adaptations explain how some smaller vessels, such as a 300-foot ferry, can reach significantly high speeds despite having lower theoretical hull speeds. In conclusion, large ships are not inherently faster but they possess a naturally higher hull speed due to their greater waterline length. While it's possible for smaller ships to achieve high speeds through specialized hull designs and powerful engines, in the commercial world, fuel efficiency and operational costs are paramount. Therefore, large ships are often designed and operated to leverage their higher hull speeds, which are a consequence of their size. In essence, it's "easier" for larger ships to maintain faster speeds because they encounter prohibitively high drag due to water line length.