Ships have a reputation fro being bulky lumps that slowly plot around the world. Even a passage from Southampton to New York on a modern ocean liners now takes almost a week, travelling at an average of 20 knots. In an air liner, you can do the same in a matter of hours. They have a typical cruising speed of 500 knots. Even your own transport have a typical speed of about 60-70 miles an hour. So what is it about ships that makes them so slow?
In terms of physical size, ships’ engines are enormous. They are so big that the larger ones are typically known as Cathedral Engines. The Emma Maersk is powered by one of these. She is an E Class container vessel, 397 metres long and was the largest container ship ever when she was launched back in 2006. Her main engine weighs 2300 tons and pumps out a whopping 109,000 Horsepower (hp).
Now it’s easy to compare that to a car as they all publish the horsepower of their engines. The typical cars you are looking at about 100 horsepower depending on the configuration you choose. At the extreme end, you’ve got F1 cars which can be pushing a 1000 horspower.
It is a little trickier to compare that to an aircraft as they use jet engines which produce a thrust instead of the mechanical horses that we use everywhere else. A typical 747 engines are around the 150,000 hp mark. So, for power it makes sense that the plane is the fastest but the ship comes second, and she is still slower than the car, so there must be more to it than just the horses (Horsepower).
With movement, another factor always makes an appearance and that is “mass.” It is in the energy formula with Kinetic energy (K.E) being half MV squared (K.E= 1/2 mv2) and it forms acceleration force being mass times acceleration (F= ma). So, how can we add mass into this discussion? We can look at horsepower per ton (hp/t) instead.
For the plane we can take the Boeing 747, they come in anywhere between 300-500 tons and are powered by four jet engines, as we discussed earlier, each engine having 150,000 horsepower, resulting in power per ton (t) of about 1,200 hp/t. A car, we said was around 100 horsepower typically and you can safely assume that a normal car weighs between 1 and 2 tons, averaging that out gives you about 75 hp/t. Now, coming back to the ship, obviously she will weigh the most. A fully loaded Emma Maersk will tip the scales at about 200,000 tons. With her engines delivering 109,000 hp, the horsepower per ton will come around 0.5 hp/t. The vehicles are now starting to settle in the correct order. The plane is still out ahead, the car in second and the ship is trailing a long way behind.
But for the ship from here on it only gets worse. Do you remember working out terminal velocity at school? It is when the force produced by an engine matches the resistance force from the medium the object is moving through. For example, you drop a ball and it will fall faster than if you drop a feather, the wind resistance of the feather is much greater so its velocity ends up being slower. The same applies with vehicles. The plane experiences air resistance, the car experiences a combination of air resistance and friction with the ground but the ship is moving through water, a comparatively dense medium. This means she is experiencing the greatest force against her.
The drag equation explains that drag is proportional to a cross-sectional area and the square of the speed (Drag= 1/2 dv2CDA). The cross-section we will get from the breadth of the ship times its draft, which we can assume to be a constant for most of the ships. Of course, if you reduce the draft, like when there’s no cargo onboard, the ship will be able to go faster as there’s less drag.
Otherwise drag is determined by the square of the speed. You double the speed, you quadruple the drag. You can sort of assume the force produced by the engine is constant. There are variations due to the water flow but we can ignore those for now. All the while the engine produces more force than the resistance, the ship will accelerate. As she speeds up, the drag increases according to the square of the speed. Once the drag force matches the engine force, no more acceleration force occurs, the ship has reached the terminal velocity.
For the Emma Maersk, the terminal velocity is around 25 knots and that’s typical for most large ships. For smaller ships this is usually slower and that’s because their engines produce more power, but crucially their cross-section doesn’t reduce in proportion to that change in engine power.
There are ferries that go significantly faster than normal ships. They are often called fast cats (short for fast catamaran). A catamaran is just a boat that has two hulls. Instead of a single box shaped hull the cats have two thin hulls, the separation between them produces the transverse stability that they need and the buoyancy is just produced by the combined underwater volume of both hulls. Clearly, there’s less buoyancy which means less carrying capacity, which is why they are usually only passenger ships. The key here is that you drastically reduce the cross section that produces the underwater drag. Reduce the drag and you increase the theoretical maximum speed, add on a few jet turbines and you’ve got yourself a ferry that’s capable of speeds far higher than a typical ship.
Now what about small speed boats? Well again, they reduce their cross sectional area allowing higher speeds. But rather than changing the shape of the hull, they are designed to rise above the water instead of pushing through it. We call it Planing, above a certain speed the water flow lifts the hull, reducing the cross-section, reducing the drag and increasing the speed.
It is the same theme for every watercraft that’s capable of high speeds, it is all about reducing that cross-section, reducing the drag, and allowing a higher speed.
Here is a very interesting video explaining this complex topic in a very simple manner: