That's possible almost entirely because of shipping – as in the kind with actual ships.

Shipping is responsible for supporting 90% of global trade, from the latest toys to life-saving medicine.

As the world grows increasingly connected, we're all looking to send and receive more stuff than ever before.

The weight of all those seaborne products has grown by more than a quarter just in the last decade.

Although you probably don't give it much thought, getting all those ships safely across the ocean is a big challenge.

Ships have to contend with waves, sea life, corrosion, and more on their journey to deliver what we want, where we need it.

To design vessels capable of weathering everything thrown their way, you'll need marine engineering.

[Theme Music]

You might think that, after looking at nuclear meltdowns, cheese catastrophes, and earthquakes, a bit of water wouldn't be the toughest thing to tackle.

But the ocean isn't just one big, wet blob.

Below its depths lies a world of chemical, biological, and physical complexity that throws up all sorts of challenges.

Which is why marine engineering has very different considerations than the kind on land or in the air.

Marine engineers work on pretty much anything that will be spending time in water.

Ships definitely fall in that category, but so do structures like offshore oil rigs and wind farms.

For all of these things, marine engineers need a broad knowledge of mechanical, electrical and even civil engineering.

They have to bring together fluid mechanics, materials design, power generation, and computer architecture

to build things that are totally independent from the resources available on land.

Although there are some very different challenges for traveling vessels and permanent installations,

they share many of the same problems in coping with the tough environment of the sea.

For starters, as any seasoned sailor can tell you, the surface of the ocean is rarely perfectly calm.

You're dealing with huge swathes of dense liquid colliding with everything you design.

A cubic meter of water weighs a metric ton, which gives crashing waves a force comparable to that of a moving car.

Even when the swells are small, water can easily creep into structures and start causing problems.

Which is why in marine engineering, your first goal is often to make something that's watertight.

As well as being an obvious problem for ships, water leakage can end up short circuiting an undersea cable or the electronics in mining machinery.

The deeper you go, the more water there is pushing down from above, which means more pressure.

That makes the challenge of making submarines mechanically intact and watertight even trickier.

To test the capabilities of something like a submarine hatch, engineers will subject it to trials such as spraying it with a high pressure hose and checking for any leaks.

You can even locate tiny leaks that the hose test can't pick up by placing an ultrasound emitter on one side of the hatch and checking if it can be detected on the other.

By tracing the source of the sound through the edge of the hatch, you can find and fix any small gaps that might allow water into the submarine or air out!

Of course, ships don't just sit in the water – they need to move!

Water is 800 times denser than air and while a ship moves, it's constantly pushing water out of the way.

Fortunately, propellers actually take advantage of that density.

As the propeller spins, the blades are angled to take the water around them and push it out behind the ship.

The more mass the propeller throws out the back, the more it pushes the ship forward.

That's just conservation of momentum in action!

To help with this, a propeller's blades are shaped such that water flows around and past them more efficiently, like the wings of a plane.

Unfortunately, ship propellers face some challenges their airborne cousins don't.

When the blades revolve at high speeds, they essentially boil parts of the surrounding water.

And that's not because the propeller heats the water up, but because of the huge pressure difference on either side of the rotating blades.

That creates tiny bubbles of steam.

And when those bubbles collapse, they send out a shockwave that hits the propeller in a process called cavitation.

It might not seem like bubbles could cause much damage, but cavitation can apply as much force as whacking the blades with a hammer!

Over time, this can cause a serious warp and prevent them from working efficiently.

Cavitation crops up when the blades need to rotate quickly to generate a large propulsive force.

You can slow them down to avoid cavitation, but that lowers the thrust the propeller provides.

Unless you're willing to get creative.

The US Navy's Ohio-class submarines replace the standard three-blade propeller design with one containing seven.

These propellers are shaped to generate more thrust at low speeds, enabling them to provide enough force to move the submarine with much less cavitation.

The Navy considers this such a breakthrough that the exact details of the design have been classified for decades.

The exact details of any propeller depend on the mass of the ship and the speed at which it needs to travel.

Fluid mechanics and thermodynamics are key, since water temperature and pressure will affect performance.

And it's not just the propeller's design that matters, but what it's made of.

In fact, what the whole ship is made of.

Since metals are relatively easy to shape and can withstand large amounts of pressure without deforming,

they're the go-to material for constructing the hull of a ship.

But over time, metals and water don't get along so well!

Water contains a lot of oxygen, so submerging a metal can lead to a chemical reaction called oxidation.

You're probably familiar with at least iron oxide, which is more commonly known as rust!

Oxidation corrodes the hull, causing it to lose material and become brittle.

Corrosion is a serious problem for any metal structure in long-term contact with water.

The more they corrode, the weaker they become!

The kind of chemical reaction that marine engineers often worry about is galvanic corrosion.

This happens when parts of an object are made of different metals, such as having an aluminum hull and a stainless steel propeller.

Aluminum is chemically reactive, while steel is relatively inert.

And that difference is the heart of the problem.

Saltwater is really good at conducting electrical currents, which means it can carry a stream of charged particles from the hull to the propeller!

And as the current draws molecules of aluminum away from the hull, it leaves a corroded surface behind.

To prevent this, marine engineers often use sacrificial anodes, which are basically just slabs of zinc.

It's called an anode because it acts like the anode in a battery: it gives up charged particles that create a current between itself and other metal parts of the ship.

Because zinc is much more reactive than aluminum, it releases molecules more easily and undergoes corrosion first.

That's the “sacrifice” it makes to prevent the structural hull from corroding instead.

Another way to stop corrosion is to purposefully put a weak current through the hull that cancels out the one produced by the flowing saltwater.

Now, so far, we've mostly been talking about the problems that water causes.

But the ocean is more than just water, it's an entire ecosystem.

In practice, large animals like sharks and whales aren't what you need to be worried about.

It's the small things like algae, mussels, and barnacles that cause real trouble.

These forms of sea life physically attach to the hulls of ships and build up over time, normally while it's docked or stationary.

Over time, all that living matter creates bumps on the hull's surface that make it less aerodynamic, an effect known as fouling.

Fouling forces the propellers to work harder as the ship travels, burning up to 30% more fuel to compensate for the roughness of the hull slowing it down!

The simplest way of dealing with this is to just regularly clean the hull and remove anything stuck to it.

But in recent years, engineers have come up with more sophisticated solutions to fouling,

like special coatings that make the metal too slippery for barnacles and mussels to attach.

There are problems to think about above the surface, too.

Since ships are totally independent from the infrastructure available on land, marine engineers have to tackle everything themselves.

That means knowing how to supply power, maintain a comfortable temperature onboard, and deal with waste storage.

Big ships are like floating cities, so marine engineers work on everything from fuel cells to navigation systems!

But even after you have a basic ship design, knowing what your ship will need is a very different thing from actually putting it together.

Even without any cargo, container ships can weigh thousands of tonnes!

So you can't usually build one on land, pick it up, and put it into the water.

When it comes to construction, marine engineers have to get creative.

You could build everything on a shipway, essentially a big ramp that lets the ship just slide into the water.

But in the case of a large ship, like an oil tanker, a better option would be to use a dry dock.

Dry docks are basins near the sea that can be sealed off and drained so that ships can be built inside.

There, they can be constructed, made watertight, and tested rigorously without any interference from seawater.

Once engineers have made any final touches, getting the ship into the water is as simple as flooding the dry dock and re-opening the gates into the sea, so it can be towed out.

Similar ideas are also used to build permanent structures like parts of offshore oil platforms,

constructing each piece on a dry dock so they can be carried out to sea and assembled.

So as you can see, it's no easy task putting together ships and getting them into the ocean.

But as more goods are moved around, the world is going to need more ships.

In this episode, we looked at marine engineering.

We saw how ships are designed to handle aquatic environments and challenges of engineering in the ocean, like corrosion and marine life.

We also looked at some solutions engineers employ to overcome them.

Finally, we saw one of the ways large maritime structures can be built on land and eventually transferred into water.

Next time, we'll look at how to take water from the sea and turn it into something we can drink, with the help of engineering design.

Crash Course Engineering is produced in association with PBS Digital Studios.

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Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

And our amazing graphics team is Thought Cafe.