Why Are Ship Hulls Painted Red?

Image of container ship CMA CGM Benjamin Franklin by ship spotter Claus_Gaser has its hull painted red.

Any avid ship spotter or someone on board a commercial ship knows that these are often painted red below the waterline. Since the ship hulls mostly remain underwater, one question that might be asked is ‘Why is red the color of choice?’ The reason lies simply in shipping tradition – Oh, and worms!

Up until the 19th century, all ships were made out of wood. The latter half of the 1800s saw unprecedented growth in the iron and steel industry, with iron and steel making its way into the shipbuilding industry. Steam engines replaced sails. The red-colored hull traces its story back to the time before the industrial revolution.

Being an organic compound and having a porous structure, wooden hulls were constantly subjugated to decay, with wood-eating worms, barnacles, and seaweed fast-tracking the process.

On top of that, slow ship speeds and rough hulls proved it to be an ideal accelerator for the process.

The speed of the ship got considerably affected. Damages suffered by the hull due to the growth of marine organisms increased the ship's weight, thereby increasing drag. As a result, the ship’s speed was also got affected considerably.

To counter the unprecedented growth of marine organisms on the hull, the shipbuilders required something that could ward off the immense outgrowth of marine life at the bottom of the hull. Here is where antifouling came into play. Antifouling has emerged as one of the most important research areas for modern shipbuilders and maritime bodies worldwide.

The scientific method of designing structures, coatings, and materials to ward off marine growth on any submerged structure is called antifouling.

Some of the earliest methods of antifouling comprised placing copper sheets on the vessel's hull. The copper sheet served as a barrier from the incoming marine organisms, largely worms, from reaching the wooden hull. The copper sheets that were used were red. This is what gave the hull its original reddish color.

The advent of iron and steel reached the shipping industry, and things did change forever. Wooden hulls are now antiquated, with iron ships being the bread and butter of the global supply chain. Nevertheless, the issue of drag being caused by marine life remains. With companies increasingly pressing on cost-effectiveness to maximize profit, antifouling has emerged as an extremely significant topic for the maritime sector.

These days, ships’ hulls are painted with copper-oxide-bearing paints known as ‘antifouling paint’— none of which need to be red. But again, the shipping industry is unlike any other industry around. We believe in honoring nautical traditions, so we choose red.

Copper oxide has a reddish tinge, thus giving the paint its famous red color. That is why ships are painted red below the hull.

As the primary biocide, Tri-Butyl Tin (TBT) was a deterrent against the growth of marine organisms on the ship’s hull even a few years back. But researchers showed the adverse effects TBT had on the marine ecosystem.

These days, self-eroding paints or self-polishing polymers serve this purpose. These are environment-friendly paints, which are engineered in such a way that it gets removed by water flow. Ship movement washes one layer of the self-eroding paint after another, preventing the outgrowth of any marine life on the surface of the ship’s hull.

Another reason can be traced in the contrast of the red hull to the seawater, which demonstrates if a load of cargo is overweight: The more cargo a ship is carrying, the deeper it enters the water. In the same context of contrast, the red color at sea can be very easily captured by passing-by helicopters in case of an emergency.

Author: Ankur Kundu