How do cruise ships work?
Have you ever been on a cruise ship? It can be a very relaxing experience - a smooth ride, good food, drinks, and lots of activities. Between swimming, sightseeing, gambling, and eating too much, it’s easy to forget that cruise ships are a marvel of engineering with impressive power systems. The skyscraper-sized vessels carry thousands of people over hundreds (or thousands) of miles across the ocean, with relatively little fanfare. Let’s take a look at how the power systems work on cruise ships and what happens if the power goes out. We’ll also look at the largest cruise ships in the world and how cruise companies make money - spoiler alert, it’s not ticket sales!
Royal Caribbean Grandeur of the Seas. Image from Ship Technology.
Cruise ship propulsion
On cruise ships, there is an army of computers and engineers keeping everything running: lights are on, AC keeps everything comfortable, and the toilets flush (more on that later). Part of the reason no one sees the engine room is its location: engines are usually mounted as low as possible to keep weight in the bottom of the vessel. This means the lowest decks of the ship, right on the keel, is where the engines are located. Engine rooms can be more than three decks tall, but are split into smaller compartments for safety. Engines and other equipment are required to be duplicated in case of an emergency like an impact or fire. Smaller compartments help to keep flood and fire contained, and redundancies help to keep vessels running if one or more systems go down.
There are two types of drive systems on cruise ships. With direct drive systems, the diesel engines are connected to the propeller shaft. On many vessels, especially newer vessels with dynamic positioning systems, propulsion is diesel-electric, meaning diesel generators power electric motors that drive the ship. To tell the difference, in short, engines use fuel and motors use electricity. Engines generate power while electric motors may drive the propellers. The engines and generators themselves work pretty much the same way, however. Fuel combusts, pushing a piston up, turning the crankshaft for power. Propellers spin relatively slowly compared to engines, so with direct drive setups, gearboxes can be used to slow the rotation and get the correct speed, just like in your everyday vehicle.
Cruise ship engine room. Image from Cruise Mapper.
Why diesel-electric power?
The majority of vessels built today are diesel-electric and power generators. This allows the engines to run at or near optimal efficiency no matter the speed of the vessel. At low speeds in port, only one engine may be needed to supply power for auxiliary and propulsion, but once at sea, multiple engines can be started to increase power and speed. While the propellers or thrusters might run on high voltage, that power can be easily converted to lower voltage like 110V for your room. According to Cruise Critic, hundreds of miles of cable runs through cruise ships supplying power to every light and outlet. Redundancies are also in place to ensure if there is an issue, power can still get to critical systems. In port, the main generators can be taken offline while smaller generators produce electricity for the “hotel” load. Propulsion takes up approximately 85% of the power needs, so these smaller generators are plenty to keep everyone comfortable. This also means the outlets and air conditioning will usually continue to work even if the ship loses propulsion. Many ports and ships are also being equipped with the ability to plug in at port so they don’t have to run generators at all, reducing fuel consumption and emissions. According to Cruise Critic, there are multiple new cruise ships being built that will run on LNG, reducing emissions over traditional diesel power.
Emergency generators are located away from the main engines for safety. They only power the required functions like lights, fuel pumps, and navigation systems. According to Cruise Critic, emergency lighting on a cruise ship is usually very good; emergency lights have little red dots that show which lights will be illuminated in an emergency. “On your next cruise, take a look as you walk down the passageways and look for the red dots -- you might be surprised how many lights are powered in an emergency.” Should all of the above fail, ships actually have a battery backup that is required to power essential equipment only for 24 hours.
Safe-to-port regulations are designed to ensure cruise ships have redundancies to keep passengers safe for longer periods onboard. “International Maritime Organization regulations entitled "Safe Return to Port'' came into effect for all passenger ships longer than 120 meters (393 feet) built after 2010,” according to Cruise Critic. Full redundancies are required including two engine rooms and double cables and electrical systems. Each engine room must have the power to move the ship on its own. Basic services like a working toilet for every 50 people are also mandated.
Azipod propulsion system. Image from Cruise Mapper.
What about dynamic positioning?
Dynamic positioning is a computer operated system that keeps a vessel on station without human intervention. It uses thrusters underneath the ship combined with wind, wave, and GPS data to keep a vessel almost completely still in the water. Cruise ships use DP systems to better navigate in port as well as have the ability to “anchor” offshore without actually dropping an anchor. In this case, DP systems can keep the ship locked into a position without disturbing marine wildlife with anchors. According to Ship Technology, Royal Caribbean’s Grandeur of the Seas made its maiden voyage in December 1996, the first cruise ship to be equipped with dynamic positioning. “The Grandeur is powered by two diesel electric propulsion units generating an output of 17MW. The total on-board power generated is 50,400kW.” Here at OneStep Power we test DP2 and DP3 dynamic positioning systems for electrical fault ride through - testing if they survive a short in the system and still function. DP2 systems have redundant systems while DP3 vessels are fully redundant. You can learn more about how dynamic positioning systems work, the difference between DP1, DP2, and DP3, and 22 types of DP vessels in our other articles.
What is the largest cruise ship?
According to Reference, the average cruise ship weighs about 20,000 to 60,000 tons, but most are weighted at gross tonnage, with the average GRT being around 100,000 GRT. GRT is 100 cubic feet of enclosed revenue-earning space in the vessel. The largest ships in the world are Royal Caribbean’s Allure of The Seas and Oasis of The Seas, which weigh 252,282 GRT. The engines are similarly massive - according to cruise1st, “Oasis of the Seas, the world’s second largest cruise ship, has three 16-cylinder engines and three 12-cylinder engines. The 16-cylinder engines consume a staggering 1,377 US gallons of fuel per hour each at full power, and the 12-cylinder engines consume a further 1,033 US gallons of fuel per hour each at full power.” On longer journeys, ships have to stop at ports along the way to refuel, although it depends on the ship and how it’s been outfitted. All those engines power three azipods of 20,000 kw each and four bow thrusters at 5,500 kw each for docking. The azipods can be rotated any direction and have huge propellers that are 20 feet in diameter. These are the final link in the dynamic positioning chain that help these massive vessels keep station.
Royal Caribbean Oasis class vessel Wonder of the Seas. Image from the Royal Caribbean blog.
That’s it for most of the propulsion side of cruise ships, but we had a couple of other questions we wanted answers to.
How do cruise ships make money?
According to the very interesting video on how cruise ships work from PolyMatter (seen below), cruise ships don’t actually make any money from ticket sales. If you’ve ever been on a cruise, you’ve probably been surprised to find tickets as low as $60-$70 per day, with total prices averaging less than $1300. With tickets as low as $60 per day, the flight to get to the cruise location can be more expensive than the cruise itself. If no one spent any money on board, the cruise line would lose money on each ticket. But, the average person spends $429 on board gambling, drinking, going on excursions, or just buying swag. This is how the cruise lines stay in business while keeping ticket prices low.
Video from PolyMatter.
So where does the waste go?
You might have also wondered, given the thousands of people on board and limited space, where does the waste go? There’s leftover food, wastewater, trash, and recycling that just seems to disappear. According to the Royal Caribbean blog, the International Maritime Organization, or IMO, sets strict guidelines on waste. Their Symphony of the Seas is a zero-landfill ship, so they recycle glass, cardboard, plastic, and metal right on board. It has an incinerator, a compactor for water bottles and “Once the ship returns to port, it can then transport plastic, aluminum, paper, and glass for recycling through a third party vendor.” They also incinerate leftover food after meals to minimize space and weight of trash. Without getting into too many details about the wastewater, Royal Caribbean has systems to handle gray water from sinks and drains, blackwater from galleys and toilets, and bilgewater from the bottom of the vessel. According to Royal Caribbean, “Wastewater is run through the advanced wastewater-purification plant on the ship, which is above the US federal standard for purified water.” Don’t worry, this water isn’t fully recycled back to the tap - treated water is then discharged in approved areas.
Recycling can. Image from Royal Caribbean blog.
Next time you’re on a cruise ship, take a look around, there’s a lot of amazing engineering that went into building that floating city. From the immensely powerful engines to the ticket prices, everything has been designed to function just right. If you happen to own a cruise ship, get the dynamic positioning systems tested with non-destructive, repeatable testing with OneStep Power’s patented technology.