How Do Ships And Submarines Get Drinking Water While At Sea?

Water is a key component of life. While humans can go much longer without food, we can't survive much more than about three days without water. Naval submarine deployments typically last three to six months, most of which the submarines spend underwater. Meanwhile, naval surface ships can be at sea for up to nine months at a time. And the massive cargo ships that travel to destinations across the globe (with relatively slow top speeds for container-ship travel to reduce fuel use and emissions) can be out in the open water for weeks, depending on their final destination. 

Huge cruise ships are generally out to sea for shorter periods, but they're faced with volume over duration as the average capacity of a cruise ship is approximately 3,000 passengers, and many holding thousands more. So where are they getting their much-needed allotment of clean drinking water for the crew and passengers while surrounded by an ocean filled with salt water? Some cruise ships (rubber ducks and all) can store water in huge tanks as big as 500,000 gallons, but other types of ships don't have that kind of storage space.

For instance, submarines with a crew of just 130 have limited storage capacity and use every square inch of available space to store supplies. Thus, they have no practical way to carry all the water needed. So they depend entirely on a shipboard desalination/distillation plant that takes in ocean water, removes the salt, and churns out clean drinking water.

Ships and submarines use a few methods to extract salt from ocean water, but advanced reverse osmosis is typically the most reliable. This consists of a pressurized membrane through which seawater is pushed to filter out the salt and turn it into fresh water. It's the same process many home filter systems use, like the Waterdrop Filter reverse osmosis systems. It also uses less energy by replacing the standard pump-and-motor system with a device that captures energy from the high-pressure discharge water created during the process. Additionally, most reverse-osmosis units are smaller than those used in the form of distillation known as steam evaporation.

Steam evaporation often uses the ship's hot engines to facilitate the process, passing water directly over them, which causes the water to boil and turn into vapor. A piece of plastic above the steam collects the now-clean water, which runs through a pipe into storage tanks. Other times, a separate distillation unit is used to heat the seawater, and then the desalinated steam cools as fresh water in a collection tank.

When used inside a submarine, this process can make as much as 40,000 gallons of fresh water daily, while those used aboard surface ships can process up to about 100,000 gallons a day. All of this water is then used to cool computers and navigation equipment as well as for drinking water, cooking, and personal hygiene.