How Does A Cruise Ship Float?

With some mega liners boasting huge swimming pools, colossal entertainment complexes, 6000 passenger occupancy and over 225,000 gross tonnes of weight, it makes you wonder how these floating resorts manage to keep their decks above the water. At the end of the day it all comes down to the two key principles of density and buoyancy.

To float or not to float

Put simply, as the huge weight of the ship pushes down on the ocean, the same amount of water is displaced which pushes the vessel upwards and keeps it afloat. It’s because of this effect that when talking about the heaviness of a ship, engineers will often use the term ‘displacement’ instead of ‘weight.’

In order to prevent the ship from sinking, engineers must ensure the vessel can displace its weight in water before it is submerged. For example, if you dropped a bowling ball into the water it would sink before it could displace the water needed to stay afloat. In comparison, if you dropped a basketball it would be light enough to displace the surrounding water and stay buoyant. Essentially, it all depends on the ship’s density and whether or not it can displace enough water to keep it afloat.

Using the right materials and designs

One of the most effective ways of maximising buoyancy is to build ships using lightweight yet sturdy materials. Evenly dispersing the weight of the ship across the hull is also a popular construction method used to increase buoyancy and stability.

As such, most cruise ships are designed with a round bottom displacement hull that is wide and features a deep base line. This ultimately pushes water out the way and helps to keep the ship afloat.

The rounded edges work to minimise drag and will also dissipate the impact of the water as it hits the hull which helps to keep the ship smoothly sailing. This makes them stable, seaworthy and perfect for minimising sea sickness.

Extra protection in case of emergencies

As well as keeping the ship floating and stable, the hull also offers ample protection against reefs, sandbars and icebergs that can jeopardise the ship’s buoyancy. To prevent any catastrophic Titanic style damage, engineers generally design cruise ships using industrial strength steel. Double hulls featuring tyre like inner tubes are also used for extra protection.

Should something manage to penetrate both layers, the ship has one last line of defence to keep it afloat. A series of vertical watertight dividers are fitted across the hull’s interior which are activated in case of emergency. Known as bulkheads, the contraptions keep the ship above water by containing the incoming flow of water in compartments which keeps the entire ship afloat.

Image Source

• Diamond Princess