Difference Between MPH And Knots
You’ll typically see ship speeds given in knots (knots-per-hour). We’ve given them here in the more relate-able miles-per-hour.
The average speed of a modern cruise ship is 20 knots (23 miles per hour), with maximum speeds reaching 30 knots (34.5 miles per hour).
In general the top speeds of the large cruise ships carrying a thousand-plus passengers don’t reach much more than 26 knots (30 miles per hour).
The largest passenger ship in the world, Royal Caribbean’s Symphony of the Seas, has a maximum speed of around 25 knots (29 miles per hour).
For large ships that’s actually very reasonable. The most advanced aircraft carrier in the US fleet, the USS Gerald R. Ford, has a speed that tops out starting at around 35 mph. For comparison, Usain Bolt’s human speed record is 27.8 mph.
In the grand scheme the top speed of a cruise ship is mostly irrelevant because they will almost always sail at their cruising speed, usually around 20 mph. Emergencies – a passenger who requires hospitalization, another ship in distress, or an approaching storm – are usually the only reasons a cruise ship will go its expensive-gas-guzzling full-throttle.
On longer cruises the cruising speed might even be lowered to time an arrival so it’s not in the middle of the night.
Knots are derived from nautical miles. Navigation at sea was historically done with trigonometry (it still is but we leave the math to the computers). With the understanding that the earth was a sphere, navigators divided it up into 360 degrees with corresponding latitude and longitude lines. The equator is considered zero degrees and extending out to the north and south poles are 180 and -180 degrees, respectively. Likewise with the prime meridian and the longitudinal lines that extend east and west from it.
A nautical mile is 1/60th of a degree. Why divide by 60? Because there are 60 minutes in an hour. By measuring the speed like this, mariners can express their per-hour speed in terms of their movement along the earth’s lines of latitude and longitude. So if you start at the equator and travel 60 knots per hour due north, that means in one hour you’ll be at a latitude of one degree north.
Generally one nautical mile is equivalent to 1.15 miles.
- A top-speed of 29 mph is 33.4 knots
- An average cruising speed of 20 mph is 23 knots
Why Are Cruise Ships So Slow?
Speed used to be a premium for ocean-going vessels. Owning the fastest ship was something to brag about. In 1952 the fastest record for a passenger ship crossing the Atlantic was set by the liner United States. It covered the 3,521-mile distance between New York City and Le Havre, France in 82 hours and 40 minutes, or three-and-a-half days, clocking in with an average speed of 42.6 mph.
That earned it the Blue Riband, a distinguishment reserved for the fastest Atlantic crossing of a passenger ship in regular service.
Before the advent of airplanes a fast ship meant spending less days (or weeks) on unpleasant and treacherous ocean crossings. Today’s airplanes have made speed an obsolete factor for passenger ships. Perhaps this is best symbolized by the United States; it set the record in 1952 and no one has bothered to challenge it since.
And the faster alternatives that emerged in the middle of the 20th century have only improved going into the 21st.
While part of this has to do with incentive, another part has to do with limits imposed by the laws of nature. Because of the fluid and displacement dynamics of large vessels in water, every extra mile-per-hour gained in speed beyond about 25 mph requires an exponential increase in the amount of power, and by extension fuel.
Because cruise ships are so massive, the only way they can displace enough water to stay afloat is by having a huge amount of their hull – about 30 feet worth – submerged. Having this much of the hull under water also has the advantage of making cruise ships relatively stable in rough seas.
The disadvantage of this design is that ships with significant displacement hulls can’t go very fast. When you see a speedboat shoot by you on the lake, you’ll notice the entire front of its hull is planing above the water. But if you were to dive under a speedboat when it was stopped, you would see that hardly a foot of it is submerged. That’s why speedboats can glide over the water and cruise ships can’t.
And don’t underestimate 20 miles per hour for a vessel that can maintain that speed 24/7. 20 mph over a day is 480 miles. Over a week that’s 3,360 miles.
Real-Life Modern-Day Speeds
Here is a look at some real cruise itineraries to demonstrate how fast cruise ships are sailing.
Norwegian Cruise Line has a route that goes from New York City to Bermuda to Port Canaveral in Florida. Eager guests sail aboard the 2,394-maximum-passenger-capacity Norwegian Pearl, built in 2006 complete with two main propeller engines and three bow thrusters.
It departs New York at 8pm and arrives two days later at 9am in Bermuda’s Royal Naval Dockyard, a 36-hour voyage. Passengers set sail from Bermuda the next day at 12:30pm and arrive at Port Canaveral two days later at 7am, a 43.5-hour voyage.
That works out to:
- New York City to Bermuda (771 miles) – 21.4 mph average speed
- Bermuda to Port Canaveral, Florida (979 miles) – 22.5 mph average speed
Princess Cruises offers a 111-day round-the-world cruise beginning and ending in Fort Lauderdale aboard its Pacific Princess. Built in 1999 she can hold a maximum of 826 passengers and is driven by two enormous propellers.
While circumnavigating the globe the ship has a Pacific crossing from Los Angeles to Honolulu – more than five days at 133 hours – and an Atlantic crossing from Walvis Bay, Namibia to Natal, Brazil, which takes over eight days at 200 hours.
This comes out to:
- LA to Hawaii (2,559 miles) – 19.2 mph average speed
- Namibia to Brazil (3,515 miles) – 17.6 mph average speed
Engines That Pack a Punch
Cruise ship engines are massive. The standard design is two or even three main engines at the rear of the ship. Many ships have additional bow thrusters that help the front of the ship move left or right. An increasingly standard main-engine thrust design is to have propellers capable of rotating 360 degrees to give ships extra maneuverability.
Just one of the main propeller engines of a cruise ship can weigh 250 tons. That’s how much each weighs on Royal Caribbean’s Symphony of the Seas, the largest cruise ship ever built. And each of those five-blade propellers puts out 20 megawatts or brute force. That’s enough energy to power a medium-sized city, or if you prefer truck analogies, 26,820 horsepower in a single engine.
You might be surprised to learn that right now most of the largest cruise ships are propelled by electric engines, including the Symphony of the Seas. But don’t get the idea these are ocean-going Teslas.
These engines are what’s known as diesel-electric; the electricity that powers them is made on-board with diesel-powered generators. Symphony of the Seas’ 16-cylinder diesel generators are each four-stories high. If they’re at full power, together they can burn through 66,096 gallons of fuel per day. That’s why the gas tanks on the biggest cruise ships can carry up to two million gallons.
Environmental Pressures on Cruise Ship Engine Design
Environmental issues are increasingly at the forefront of policy discussion. When you buy airplane, train, or bus tickets today you’ll often see a box to check so you can pay a few extra dollars and offset your carbon footprint.
The cruise industry has not been spared from the spotlight on this issue either, especially its diesel-hungry luxury liners. Some cities in Europe are already laying down plans to ban diesel cars for their offending sulfur dioxide and particulate matter, and it doesn’t help that the kind of diesel used in shipping is a worse on these fronts than automobile diesel. This has caused some previously out-of-favor ship engines like gas turbines to start making a comeback.
Gas turbine engines are efficient. They benefit from the power they produce as internal combustion engines, and create additional power by recycling hot post-combustion gasses for additional kinetic and thermal energy.
The kinetic energy of the internal combustion portion of the engine can be used to turn the ship’s propellers. The captured vent gas is channeled through additional turbines that spin to generate electricity. And finally when the hot exhaust leaves the internal portions of the engine it can be used in steam-powered drive shafts for additional mechanical energy.
Both types of engines – diesel-electric and gas turbines – can generate comparable amounts of horsepower. While gas turbine engines may be more complicated to maintain and costly, market trends favoring the environment are making the future prospects for gas turbines more realistic.
Some companies are already starting to market combination engine packages for cruise ships, where the ship’s propellers are driven by a diesel-electric motor, and its electricity is generated by gas and steam turbines.