Why Do Most Cruise Ships Have Diesel Engines?

Diesel has been the preferred fuel for maritime use for many decades now. Water is denser and more viscous than air, so there's more drag, and more grunt is necessary to over come it. Apart from the friction drag, marine vessels experience something else called pressure drag, which creates a high-pressure area in front of the vessel as it moves through the medium. This, alongside a slew of other drag forces, puts up resistance the vessel's forward propulsion. To overcome these, cruise ships and other massive marine vessels, which, are often about as aerodynamic as a brick, require a lot of power — or, more specifically, torque — while still ensuring fuel consumption remains reasonable. Ships have to travel long distances after all, so diesel engines make perfect sense.

While traditional cruise liners rely on diesel alone, modern ones employ a diesel-electric hybrid, much like what you'd find in a diesel locomotive. In this setup, the diesel engine(s) act as a generator, feeding power to the electric motors that spin the propeller. Typical cruise ships have anywhere between four and eight engines, of which there are main propulsion units and auxiliary engines that are often used exclusively for the ship's many electrical needs. Unlike the slow-speed two-stroke engines typical in bulk carriers, cruise ships often use medium-speed four-stroke diesel engines, primarily because of emissions.

Modern emissions regulations are getting much tighter for the shipping industry, because cruise ships do not run on the same diesel we use in trucks on the road. While some cruise ships use heavy fuel oil (HFO), most of them run marine diesel oil (MDO), which is also called very-low-sulfur oil or VLSFO.

In the quest for lower emissions and less environmental impact, the shipping industry is switching to alternative engine/fuel solutions like marine gas turbines and dual-fuel gas-diesel setups. One cargo ship is even being built to run on ammonia. 

In a gas turbine propulsion system, the turbines power the generators that then supply the energy to the ship's propellers. In a dual-fuel arrangement, however, liquid natural gas (LNG) is used, which, according to Wartsila — one of the leading marine engine manufacturers — offers a seamless switch between diesel and LNG whenever the ship transitions to heavily regulated areas like the North Sea. LNG offers several environmental benefits in the form of reduced NOx, SOx, and other particulate emissions when compared to HFOs. But they tend to be more expensive.

It's worth pointing out that while diesel won't likely go away anytime soon, at least in the maritime context, companies haven't shied away from adopting modern technologies when there's a monetary incentive. Take the now-retired Queen Elizabeth 2, for example. In 1986, parent company Cunard Line decided to switch the QE2 from traditional steam turbines to run on diesel power.

This was a diesel-electric system, specifically, nine medium-speed engines from MAN B&W and two GEC propulsion motors. Not only was the ship faster, but the cruise liner was reportedly more efficient, saving up to $6.5 million in annual fuel costs according to company estimates. The $162 million gamble proved fairly successful, as the ship managed to extend its service life 20 more years, finally retiring in 2008. So if there's enough of a breakthrough, we might finally see modern cruise and merchant fleets shifting away from diesel propulsion entirely.