If you’ve ever salvaged a sea vessel, you might have noticed that ship hulls are often red. If you haven’t dealt with a shipwreck—and chances are you haven’t—you may have still seen a red hull in pictures or in partial view at a shipyard. Since that portion of the ship is below the waterline, it seems strange to opt for a specific color.
The reason is tradition. And worms.
In a piece for Jalopnik, Andrew P. Collins explains that early sailing ships protected themselves against barnacles and wood-eating worms by covering their hulls in a copper or copper oxide paint that acted as a biocide. The copper gave the paint a red tint. By reducing the muck that naturally collects on the hull, ships can maintain their structural integrity and avoid being weighed down by gunk like seaweed that would reduce drag.
These days, biocides can be mixed with virtually any color of paint. But the hulls are often painted red to maintain a nautical tradition. Collins also points out that the red may help observers gauge the load of a ship’s cargo. The more weight on board, the lower in the water it will be. That's why you often see numbers positioned vertically on the side of the hull.
No matter what’s covering the hull, it’s never going to completely eliminate growth. Often, ports will prohibit ship owners from scraping hulls while docked, since ships traveling in outside waters might have picked up a non-native species of weed that could prove problematic in a new environment.