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How fast Can Cruise Ships Go?

Unlike cars, ships (and boats) have a natural speed limit. This has to do with the way the water interacts with the hull of the ship. As a ship goes faster, it generates more of a wake (at both the bow or front and the stern or back). This takes energy away from the ship, so the faster the ship goes, the more fuel it takes to go the same distance.

When a ship reaches it's so called "hull speed", the bow and stern waves combine to make a hole in the water with the ship inside the hole. To go faster, the ship has to climb out of this hole. In a small speed boat, you can feel this. At low speeds, the boat is level.  As the boat goes faster, the front tilts up as it climbs out of its hole. As the boat goes even faster, it levels out again and the hole is now behind the boat. The boat is said to be planing.

Unfortunately, planing takes a lot of power, so ships never, ever plane. There is a magic formula for the hull speed (in knots): 1.34 times the square root of the length of the ship at the waterline (in feet). A 15 foot speed boat has a hull speed of about 5 knots, so it has to plane to get anywhere in a reasonable amount of time. Cruise ships, though, are typically around a 1000 feet long (somewhat less at the waterline because of the angle of the bow). This gives a hull speed of 42 knots!

In practice, cruise ships never get close to this speed. Most of them have a top speed less than 25 knots. This is determined by the power of the engines and the drag (due to the wake). You may have noticed that it usually takes a cruise ship overnight to get anywhere new. This has more to do with tides, traffic, and the availability of pilots than the ship's engines. At top speed, the ship can cover more than 550 nautical miles in a (24-hour) day. If the distance is less, the ship simply goes slower to save fuel (or just enough faster to keep the ship from rolling too much).