Why Are Big Ships Faster Than Small Ships?

In a very general sense, when you take two similar ships just one being larger than the other, you'll usually find that the bigger one is faster. For example, these 400 metre container ships typically have a speed of 25 knots yet, these feeder-style container ships might only have a typical speed of 15 knots. It's odd because it goes against common sense when you think that something bigger, weighing significantly more, will be faster.

一般来说，当你比较两艘相似的船，只是其中一艘更大时，通常会发现较大的那艘速度更快。比如，这些400米长的集装箱船通常速度为25节，而那些支线型集装箱船的典型速度可能只有15节。这有点奇怪，因为我们直观上会觉得更大的东西、重量更重的东西，应该更慢才对。

So why are big ships faster than small ships? Well, firstly, we need to remember that a ship is just a hull displacing water. When you move a hull in the water, it generates waves which we see as the wash. At the bow, the waves are generated by the hull pushing water out of the way. At the stern, similar waves are generated as the water rushes to fill the void left as the hull moves forwards. From the side, it looks as if there's one wave system generated with its first peak at the bow, and a second wave system generated with its first trough at the stern.

那么，为什么大船比小船更快呢？首先，我们需要记住，船只是一个在水中排开水的船体。当船体在水中移动时，会产生我们看到的波浪。在船头，船体将水推出，从而形成波浪。在船尾，当船体前进时留下空隙，水流回时也会产生类似的波浪。从侧面看，这就像一个波浪系统，在船头形成第一个峰，另一个波浪系统在船尾形成第一个谷。

Now, it gets complicated fairly quickly if we have two wave systems, so let's just focus on the bow to begin with. At slow speed, you might get a wave profile like this. The wavelength is shortened, the wave speed is, well, it's the same as the ship's speed. As the ship speeds up, the whole thing stretches out, increasing the wavelength. This immediately tells us that the wavelength is proportional to the wave speed, which is directly linked to the speed of the ship.

现在，如果我们有两个波系统，情况会很快变得复杂，所以让我们先专注于船首。在低速行驶时，你可能会看到这样的波形。波长变短，波速，其实和船速是一样的。当船加速时，整个波形会拉长，波长增大。这马上告诉我们，波长与波速成正比，而波速又直接与船速相关。

The higher the ship speed, the higher the wave speed, and the longer the wavelength. As the ship's speed increases, you'll eventually get to a point where the wave length is two-thirds of the ship's length. Your peaks are here and here, and your troughs are here and here. Remember though, we said that there are two wave systems generated, the second being at the stern, starting with a trough. The troughs from the stern system and the bow system are in the same place, so we get constructive interference.

船速越快，波速就越快，同时波长也越长。当船速增加到一定程度时，波长会达到船长的三分之二。波峰在这里和这里，而波谷在这里和这里。不过请记住，我们说过有两个生成的波浪系统，第二个是从船尾开始的，起始点为波谷。来自船尾系统和船首系统的波谷在同一个地方相遇，因此产生了叠加效应。

Behind the ship, a massive wash is generated as the bow waves and stern waves add together. While that doesn't help you on a ship, if you're on a small boat, pulling a wakeboarder for example, that might be what you need. Just find the speed that corresponds with the wavelength two-thirds of your boat's length, and you'll have the best water sports business in town.

在船后面，当船头波和船尾波相互叠加时，会产生巨大的浪花。虽然这对大船没有帮助，但如果你在一个小船上，比如拖动一个滑水板时，这可能正是你需要的。只需要找到一个速度，使得波长等于小船长度的三分之二，你就会拥有这个镇上最好的水上运动生意。

Anyway, back to ships. Let's see what happens as you further increase the speed. You'll get to a point where the wavelength is the same as your ship's length. Now, your stern wave and bow wave will destructively interfere, leaving minimal wash, but take a look at your hull. You have two peaks and only one trough. At this speed, the wetted surface of your hull and corresponding resistance are as high as they can be. This is known as your hull speed, which is the least efficient speed for a displacement vessel.

好的，回到船只这个话题。我们来看看当你进一步提高速度会发生什么。当船速达到一个程度时，波长会与船的长度相同。此时，船尾波和船首波会相互干扰，使得水面波动最小。但注意你的船体，这时会出现两个波峰和一个波谷。在这个速度下，船体浸入水中的表面积和相应的阻力达到最大值。这就是所谓的船体速度，它是对排水型船只来说效率最低的速度。

You can approximate it using the formula, speed in knots, is equal to 1.34 times the square root of your waterline length in feet. For example, Queen Mary 2, with a length of 1132 feet has a hull speed of 45 knots. A 300-foot coaster will have a hull speed of 23 knots, an 80-foot small commercial boat will have a hull speed of 12 knots and a 20-foot small pleasure boat will have a hull speed of only 6 knots.

您可以使用这个公式进行近似计算：速度（节）等于1.34乘以您的船水线长度（英尺）的平方根。例如，伊丽莎白女王二号邮轮的长度为1132英尺，其船体速度为45节。一艘300英尺长的近海货轮的船体速度为23节，一艘80英尺长的小型商船的船体速度为12节，而一艘20英尺长的小型游船船体速度仅为6节。

Of course, you can push past your hull speed. As you get faster though, your wavelength will continue to increase until you hit the point where it's one and a half times your boat's length. Your boat's stern will sink, increasing your trim and creating the feeling of continuously running uphill, earning it the name Hump Speed. With the trough at the stern, again, you get constructive interference with the stern wave generating a massive wash.

当然，你可以超过你的船体速度。然而，当你速度加快时，波长会不断增加，直到达到船身长度的1.5倍。这时，船尾会下沉，增加船的倾斜度，让你有一种不断爬坡的感觉。这种现象也被称为“驼峰速度”。由于船尾处在波谷的位置，船尾的波浪会产生强烈的干涉效应，形成巨大的水流波动。

This really is the worst speed to run at, but once you get over the hump, there is the potential to reach really high speeds. This is known as Planing, where the wavelength can be many times your own boat's length. The thing is, a displacement hull like you find on most cargo ships will never be able to generate enough energy to get past its hump speed so they can never plane. You can only reach a plane with a hull designed for it. Small speed boats, ribs and things like that.

这实际上是运行速度中最糟糕的一种，但一旦你克服了这个速度的瓶颈，就有可能达到很高的速度。这被称为滑行，在这种状态下，波长可以是你的船自身长度的好几倍。问题在于，像大多数货船那样的排水船型永远无法产生足够的能量来突破这道速度障碍，所以它们永远无法滑行。只有设计为滑行的船体才能做到这一点，诸如小型快艇、充气救生艇之类的船只。

Anyway, combining all of today's examples together, we can produce a nice little graph with the hull's resistance, caused by the wave pattern, at different speeds. Notice how it's not a nice smooth curve. There are Humps and Bumps causing little sweet spots at different speeds. As the length of the shipping increases, the speed at which all these Humps and Bumps occur changes.

总之，将今天所有的例子结合在一起，我们可以绘制出一张关于不同速度下船体因波浪形态所产生的阻力的图表。注意，这不是一条平滑的曲线，而是有凸起和凹陷，导致在不同速度下出现一些理想的“甜点”。同时，随着船体长度的增加，这些凸起和凹陷出现的速度也会发生变化。

Longer ships experience sweet spots at higher speeds, meaning bigger ships can be naturally faster than smaller ships, but of course that isn't a whole story. You can reduce resistance from the bow wave using a bulbous bow. It generates a second bow wave, designed to destructively interfere with the first, reducing all the effects that we've just covered. Alternatively, you can adapt the shape of your hull to reduce the waves generated.

船只越长，在较高速度下会遇到“甜点”，这意味着大型船只可以自然地比小型船只更快。但当然，这并不是全部的情况。您可以通过使用球鼻艏来减少由船头波引起的阻力。球鼻艏会产生第二个船头波，这个波与第一个船头波相互干扰，从而减少我们刚才提到的所有影响。或者，您也可以调整船体形状，以减少产生的波浪。

A final line like a long thin hull with a sharp bow will need to push the water far more gently, reducing resistance and helping the ship to get over its hump speed. This is how things like this 300-foot ferry can travel at 40 knots, despite having a hull speed of only 23 knots. Of course, if you're not limited by power, you can always just strap on a massive engine and not worry about the effects of resistance and go at whatever speed you like.

一条末端呈长而薄且船首尖锐的船体线条，能够更加柔和地推动水面，从而减少阻力，并帮助船越过其“阻力速度”。正因为如此，即使这艘300英尺长的渡轮船体速度只有23节，也能以40节的速度行驶。当然，如果不受动力限制，你可以选择安装一个强大的引擎，无需担心阻力的影响，以任意速度行驶。

Unfortunately though, particularly in the commercial world, that just isn't an option which conveniently brings us back to our original question. Why are big ships faster than small ships? Well, they're not necessarily, but it is, in a way, easier for bigger ships to travel faster, as they experience prohibitively high drag at higher speeds due to their greater water line length, giving them a naturally higher hull speed.

不幸的是，尤其是在商业领域，这并不是一个可行的选择，这就让我们回到了最初的问题：为什么大船比小船快？其实，大船并不一定比小船快，但从某种程度上说，大船更容易跑得更快。这是因为大船的水线长度较长，在高速行驶时会遇到更大的阻力，这使得它们的船体速度更高。