Modern day cruise ships are almost like floating cities: they have seemingly endless amenities and dining options while giving passengers little reason to ever even want to go into port when the ship docks. Cruise ships are luxury vacation spots for people around the world because they’re typically all inclusive and provide worry free days at sea where you can relax and escape the worries of every day life. Have you ever wondered how these massive floating cities have come to be, and how they continue to stay afloat when it seems like there’s so much aboard they appear as though they should struggle to stay above water.

How did they come to be?

During the great depression there were a couple shipping companies that were really struggling to keep afloat because the economy was just sinking and the business wasn’t there. From this the idea of transporting  people across great bodies of water instead of just goods was born, and with it came ships like The Queen Elizabeth 2 or the QE2. These ships made popular the idea of luxury sea travel instead of just storing away among some shipping containers.

There are many cruise lines that offer popular destinations around the Caribbean, the Mediterranean and even Alaska, but that one shipping line that started it all still offers the trans-Atlantic voyage that crowds of people originally signed up for.

Keeping Everyone Above Water

The size of cruise lines today will lead anyone to question how exactly they manage to stay above water. A ship built for extensive travel needs to have specific engineering to be able to withstand the weight brought on by passengers, crew members, luggage, food and supplies. In order to stay above water ships use density and buoyancy principles to their advantage. How this works is ships will displace an equal amount of water when they push down against the water, while that same amount of water pushes back up against the ship to make sure it stays afloat.

When looking at the size of ship engineers are much more worried about displacement rather than weight. So in order to make sure it doesn’t sink, the ship has to displace its weight before it’s completely submerged under the water. In simple terms what happens is the ship has to be less dense than the water underneath it. So it’s almost like comparing a bowling ball and a beach ball: a bowling ball can’t displace the amount of water needed for it to float before it is completely submerged so it sinks. However a beach ball is much less dense than the water underneath it so it will float.

Underneath the Surface

There’s a lot going on under the water with the ship to make sure the rest of it stays above the water. Large ships – like cruise ships – often use something called a displacement hull which is actually used to push water out of the way and under the ship to help the ship stay afloat. Often this hull has rounded edges, rather than sharp ones. This helps to minimize force against the ship ultimately allowing for the ship to move along smoothly and without issue. Round bottom hulls, unlike their V shaped counterparts, move through the water rather seamlessly so it makes them extremely stable. This means that passengers on these kind of ships do not often feel any kind of rocking or the typical side to side movement thought of with ships.

The downside to round bottom hulls are that they are typically very slow moving, but when it comes to a luxury vacation cruise stability and smooth sailing is definitely more important than speed so that is why this type of hull is the preferred choice of many luxury cruise lines.

 Under the Water, and Inside the Ship

While the hull helps to provide stability for the ship while it’s traveling, it also gives some protection for the ship from debris in the ocean that can tear apart fiberglass, or even steel. In order to prevent massive damage, or sinking, shipbuilders will usually build something called a double hull in the ship. This is typically built of extra strength steel and is built on the inside of the outer hull. Further they will also install items called bulkheads. Should any damage happen to the hull these bulkheads are vertical dividers that will seal off portions of the hull so that water doesn’t continue to move through the ship and cause it to sink.

Getting from Place to Place

Since these ships are basically floating cities it seems logical that it takes quite a bit of energy to move them from port to port and across bodies of water. Older ships typically used diesel as it was the main source of fuel energy however technology has propelled us forward into other options. Currently cruise ships will use gas turbine or diesel electric engines to keep them moving and for their navigation systems. Most cruise ships use one or the other, however some of the larger ones will use one source for their navigation and systems, while using the other source of energy to propel them along.

Gas turbine engines generate heat which is converted from mechanical energy to electrical energy when compressed air is ignited in a combustion chamber. The hot exhaust is then forced over turbine, which spins to drive the shaft of the ship propelling it forward. Diesel electric engines work in a very similar fashion, except they don’t make use of a turbine but rather use a direct driving system.

All cruise ships – no matter what source of energy is used – rely heavily on propellers to move them forward and backward. Propellers have come a long way: modern day versions are so much better than they were in the days of the Titanic with some of the rotating a complete 360 degrees to make these huge floating cities as maneuverable as possible.

Modern day cruise ships are a true engineering marvel, and it’s really surprising as to how they stay afloat. With technology developments we are moving forward and are well on our way to ensuring luxury sea travel continues to keep its head way above water and come up with new attractions all the time.