Why Are Ships Painted Red On The Bottom?

It seems a bit peculiar that ships are painted red — or any color — on the bottom since this part of the boat is almost always below the surface of the water, hidden from view. Why do ships need to be a certain color on the bottom? The red color itself is actually not all too important — it's just the natural color of the cuprous oxide found in the paint that's used to coat the bottom of the ship. 

This paint is applied to reduce the amount of underwater growth on the bottom of a ship, like barnacles, worms, and weeds. This process, called anti-fouling, started when ships used to be wooden and slow, the perfect combination for underwater growth to thrive. Back then, worms could damage the wood. Even though many ships are now made with different materials, they can still get damaged from an abundance of underwater growth — and most importantly, the extra weight can cause drag on the ship and make it go slower. These days, any color can be mixed into the paint, but many choose to stick with the red color due to tradition. 

While ships have become faster since the times of wooden boats, there are still many ships that remain slow enough to get affected by underwater growth — that's why the red paint is still needed. Larger ships are slower due to their bigger surface area creating more friction and drag as well as a desire to save on fuel. As an example, bulk carriers reach an average of 13 to 15 knots, which is about 15 to 17 miles per hour. Container ships reach 16 to 24 knots, a maximum of about 28 miles per hour. Cruise ships, which can carry thousands of people, reach an average of 20 to 25 knots, which is almost 29 miles per hour.

These ships are relatively slow-moving, but even fast ships can be impacted by fouling while docked or not in use. Cleaning the hull of a ship can be very difficult, often requiring divers to work in hazardous conditions under the water. More modern cleaning includes the use of drones and remotely controlled vehicles to avoid using divers while keeping the protective paint safe.