How Cruise Ships Work

Everyone likes to go on steam engine, a revolutionary way to harness kinetic energy and convert it to power.

As the steam engine evolved, so did its uses, until in 1819 the first American ship aided by a steam engine crossed the Atlantic. The S.S. Savannah left from the U.S. city bearing its name on May 22, 1819, and arrived in Liverpool, England, 29 days later. While the Savannah only used its steam engine for approximately 85 hours (roughly 12 percent of the trip), the voyage made history, and the era of the steamship began [source: Columbia Encyclopedia].

The passenger ship industry flourished in the late 1800s and early 1900s, courtesy of the steam engine and of the growing number of American immigrants crowding ocean liners. Trans-Atlantic passage was still primarily one way, although affluent passengers traveled back and forth between England and New York for business or holiday.

Ambitious British shipping company White Star Lines began aggressively building the first fleet of ocean liners in 1849 and would revolutionize trans-Atlantic passage over the next 60 years. White Star set records in size and grandeur, building, among others, three large ships dubbed Olympic Class liners. The Olympic, Britannic and Titanic broke the mold of traditional ocean liners, and their speed and interior features made other ships look obsolete. But ocean travel was on the verge of changing.

The popularity of trans-Atlantic sea passage gradually declined with the arrival of the airplane. People could fly to more destinations in a fraction of the time it took on an ocean liner, so shipping companies changed their business model to focus on tourism instead of passenger transportation. In 1900, the American-Hamburg Company built the first ship specifically designed for cruises. The Prinzessin Victoria Luise measured 406 feet (124 meters) long by 52 feet (16 meters) wide, and was 4,409 gross tons [source: Norway-Heritage]. (In nautical terms a ton, typically called a gross register ton or gross ton, does not measure weight; it represents 100 cubic feet of internal capacity.)

During the early 1930s, Nazi leader Adolf Hitler boosted the fledgling cruise industry by offering holiday packages to German workers as part of a state-sponsored effort to unite the nation. Hitler eventually commissioned several new ships for service, making the Nazi Party early pioneers of the cruise ship industry.

Since the early days, cruise ships have shared one universal goal: Don't sink! Learn how they try to avoid that fate on the next page.

In order for ships to journey across the open sea, they must withstand the tremendous burden, or weight of the ship, along with the crew, luggage, supplies and passengers. They do that with a little help from the principles of density and buoyancy. Cruise ships can weigh upwards of 71,500 tons (65,000 tonnes). They displace the equivalent amount of water when they press down on the ocean, which meanwhile pushes up and keeps the ship afloat, or buoyant.

That's why when engineers talk about how heavy a ship is, you'll hear them talk about displacement instead of weight. To keep from sinking, the cruise ship has to displace its weight in water before it's submerged. That's a lot easier to do if the cruise ship is constructed in a way so that it's less dense than the water below it. Think of it as the difference between dropping a bowling ball in the water and trying to submerge a beach ball. The bowling ball can't displace enough water before it's submerged, so it sinks. The beach ball does the opposite and floats.

Engineers help ships to achieve buoyancy by choosing lightweight, sturdy materials and dispersing the weight of the ship across the hull. The hull, or body of the ship below the main deck, is typically very wide and has a deep base line, or bottom. Large ships such as freighters, naval vessels and transport and cruise ships commonly utilize displacement hulls, or hulls that push water out of the way, to stay afloat.

A round-bottom displacement hull looks like a large rectangle with rounded edges to dissipate drag, or the force exerted against a moving object. The rounded edges minimize the force of the water against the hull, allowing large, heavy ships to move smoothly along. If you somehow hoisted a cruise ship out of the water and looked at it standing a few hundred feet away, the hull would look like a huge capital letter "U," depending on the size of the keel. The keel runs from the bow to the stern and acts as the backbone of the ship.

Like just about everything in our lives, round-bottom displacement hulls have advantages and disadvantages. Unlike a boat with a v-hull, which rises out of the water and s­kirts the waves, round-bottom hulls move through the water, making them extremely stable and seaworthy. Passengers on these ships rarely feel any rocking or side-to-side movement.

Ships with round-bottom hulls move fluidly, but the resistance of the water makes them extremely slow. They can only go so fast before the addition of more engine power reaches a point of diminishing returns. Nevertheless, the need for stability and a smooth ride outweigh overall speed, thus making the round-bottom displacement hull a good fit for cruise ships.

The hull serves not only as stability but also as protection. Reefs, sandbars and icebergs can tear apart fiberglass, composite materials and even steel. To prevent catastrophic damage, shipbuilders typically construct cruise ships using extra-strength steel and insert double hulls as an extra precaution. A double-hull design is a hull within a hull, like a tire with an inner tube.

Unfortunately, accidents do happen. In order to prevent cruise ships from sinking should something penetrate the first two lines of defense, vertical watertight dividers known as bulkheads are installed throughout the interior of the hull. These dividers keep damaged ships afloat by containing incoming water into a compartment or compartments, thereby preventing the whole ship from flooding.

Now that we've learned how these massive ships float, let's look at the various propulsion systems that propel them from port to port.

Without a source of power, these mammoth diesel engines to generate power for propulsion. The power of the engine is fed through a transmission to propeller shafts. Transmissions determine the revolutions of the propellers, much like the transmission transfers the engine RPM into manageable speeds to power the rear wheels in our cars. Modern cruise ships use either gas turbine or diesel electric engines as their power source for propulsion, as well as for the ship's systems. The larger the cruise ship, the greater the demand for electrical power. Some larger ships rely on two different power sources: one for propulsion and one exclusively for electrical power.

Aero derivative gas turbine engines generate heat that is converted from mechanical energy into electrical power. To achieve this, compressed air is ignited in a combustion chamber. The hot exhaust is forced over a turbine that spins to mechanically drive a shaft. This power can then be used to spin electrical generators. Diesel-electric engines work much the same way, yet use a direct drive system rather than a turbine. Output shafts are connected to electrical generators to produce electrical power.

Both engine types require fuel, and lots of it. For example, the QE2 consumes about 380 tons of fuel daily if it's traveling at a speed of 28.5 knots and carries enough fuel to sail nonstop for 12 days [source: Warwick]. Cruise ships usually fill up at various ports, using fueling barges like floating gas stations. They use a lower-grade diesel that tends not to burn as cleanly as road-going diesel-powered vehicles. No doubt when prices rise at the pump, cruise ships also feel the pinch.

All cruise ships rely on propellers to push them through water. Propellers, commonly referred to as screws, cut through the water and provide forward or reverse motion. Unlike airplanes, which require tremendous propeller speeds to provide the forward motion needed for flight, cruise ship propellers do not need to turn as fast. They rely on torque, or brute power, over RPM, or high speed. Therefore, cruise ships travel slowly, rarely topping 30 knots.

Newer, cutting-edge cruise ships, like the QM2, use azimuth thrusters, which are pods housing propellers that can rotate 360 degrees and provide optimum maneuverability. These thrusters replace rudders and are thought to have several benefits over conventional screw-type propeller systems, such as decreased stopping distance and greater fuel efficiency [source: AP]. They can be used with either gas turbine or diesel electric engines.

If the Titanic had been equipped with azimuth thrusters, things might have turned out differently. One of the Titanic's downfalls was its inability to turn quickly to avoid the iceberg that ripped the bow open. Traditionally, cruise ships rely on rudders and differentiating propeller speed to turn, but ships equipped with azimuth thrusters turn much more quickly, so more cruise ships are incorporating this technology.

Cruise ships are designed with serio­us comfort in mind, so they have a lot of amenitie­s and a lot of staff to oversee those amenities for the as many as 3,000 passengers. These thousands of passengers are spread out over the ship's multiple levels called decks. For instance, the QM2 has 13 separate decks and carries up to 3,056 passengers and a crew of 1,253. If you're on deck 13, you would be standing on the highest level of the ship, whereas if you ventured down to the first deck, you'd be closer to the bowels of the ship.

If you booked passage on the QM2, you would likely find yourself staying in one of the Britannia staterooms (or cabins) located in the middle decks. The smallest of these rooms (typically 155 square feet to 250 square feet) contain a king-size bed, closet, television, a phone and a desk. Go with money to blow, and you could find yourself rubbing elbows with the other high rollers on a separate deck where the deluxe apartments and suites are. One of these rooms can be as big as 2,250 square feet. That's as large as a four-bedroom house. Whatever you need -- whether it's an Xbox or a bottle of bubbly -- these are the accommodations where you'll get it fast.

After a good night's sleep, you would probably head to the upper decks to eat breakfast, choosing from one of the many restaurants. Rather than returning to your stateroom after breakfast, you might hang poolside, shoot some hoops, shop, check out the golf simulator or gamble. Of course, you could always venture off the ship if you're at a port of call.

After a packed day, nightfall brings with it more food. You may want to try a formal restaurant or grab something quick at one of the grills. With dinner done, the nightlife begins. If you're in the mood for the opera, you'll need to book a reservation at the Royal Court Theatre. Perhaps you just want to hang out, have a few drinks and dance at the Q32 Nightclub.

Wherever you are on the cruise ship and whatever you're doing, it's likely that a crew member isn't too far away since cruise ships employ very large crews, well over 1,000 people. Here's a list of some of the more nautically inclined crew members and some of their responsibilities.

In addition, some­ of the staff focuses less on how the boat is running and more on how you're enjoying yourself. Since cruise ships truly are hotels on the sea, some of the folks you may encounter include bartenders, plumbers, electricians and ushers. Working on a cruise ship can be rewarding and financially appealing. Employees can earn up to five times what they might make on land, depending on which ship and company they work for.

But not everything is fun and games when you're sailing along with thousands of strangers over the open seas. The next section points out some of the drawbacks of cruise ships.

A floating resort packed with thousands of tourists and staff is bound to run into some trouble. The cruise industry has drawn the ire of many public health and enviro­nmental critics, not to mention law enforcement and legislators. Let's start with the environmental criticism first.

As you may expect, cruise ships generate several forms of pollution as they lumber through the seas.

[source: Herz, IICRC]

An average cruise ship creates an estimated 90,000 gallons to 255,000 gallons (340,687 liters to 965,280 liters) of gray water, 30,000 gallons (113,562 liters) of black water and 37,000 gallons (140,060 liters) of bilge water daily [source: Herz]. Loose environmental requirements allow cruise ships to dump everything overboard except untreated and solid waste. Bilge water, gray water, as well as treated sewage and incinerated solid waste can be, and regularly is, dumped directly into the sea, so long as it is not within 3 miles (4.8 km) of the shore.

Ballast water taken on to stabilize the ship can also disrupt ecosystems. Ships inevitably fill their ballast tanks in one location and purge them in a different area, thus introducing new species of marine organisms into different environments. Much like kudzu when it was introduced to the U.S., foreign microorganisms can infect or kill native plankton, sea plants, coral, and fish.

From a public health perspective, disease outbreaks on ships are also a concern. Most of the major cruise lines -- Carnival Cruise Lines, Norwegian Cruise Lines, Celebrity Cruises and Princess Cruises among others -- have ­reported outbreaks, according to the U.S. Centers for Disease Control. The organism responsible for getting passengers sick usually is the norovirus, a common virus linked to gastroenteritis (the "stomach flu"). Norovirus infections spread quickly on a cruise ship due to the close quarters. To see the cruise ship inspections for a particular ship, visit the CDC Vessel Sanitation program Web site, which posts inspection reports.

Probably one of the most frustrating aspects of traveling on a cruise ship is the lawlessness. Several critics point out that cruise companies are more concerned with protecting themselves from liability. Cruise companies are hesitant to have legitimate security forces onboard ships for fear of lawsuits from the public.

Overcrowding also has become a deterrent for people taking cruises, especially as companies like Royal Caribbean, Cunard and Carnival push the limits of occupancy. Lines to participate in the onboard attractions keep growing as ships like the Genesis hit the water. Slated to arrive sometime in 2009, the Genesis, with a price tag of $1.4 billion, will be 1,180 feet (360 meters) long and will accommodate 5,400 passengers at double occupancy [source: AP]. These mobs of passengers can also overwhelm a port call destination. But cruise ship companies show no signs of stopping their quest to build the biggest ship.