Why are the Bottoms of Ships Painted Red?

If you have a special interest in ships, or have watched the movie “Titanic” carefully enough, you might have noticed that the bottoms of ships are almost always painted red.

While at first glance this might seem like a coincidence or a way to make the ship easier to see underwater, the actual reason is more interesting, and also more confusing. This blog post will handle this question in two parts: 1) Why are the bottoms of ships painted? 2) Why are they painted red?

Section 1: Why are the bottoms of ships painted?

The whole point of the pain is to actually protect the bottom (or hull in marine terminology) from unwanted marine organisms such as sea worms, sea weeds, barnacles etc. However, this paint is definitely not the same today as it was back in the 19th century.

You see, back when ships were made out of wood without any engines, it was difficult to get the ship to accelerate, thus captains tried to remove every unnecessary thing that was weighing the ship down. These marine organisms were one of the “unnecessary” things, and particularly stubborn ones too. They would stick to the hulls of the ship, and the ships had to be regularly cleaned. Some of them would also eat the wood, creating holes in the ship - which is something no captain would want. [1]

One solution was to cover the ships with copper, which at first kept the ship safe from sea worms, but rapidly gathered dust by oxidizing with the oxygen in the sea water. [2] Even when people began to make their ships out of iron, they could not stop the build up of marine organisms.

That was when anti-fouling paints became the answer. The first paint formulas included some very heavy ingredients such as zinc oxide, mercury oxide and arsenic [3]. All three of these substances are incredibly harmful to human beings: arsenic in its inorganic form is highly toxic and can lead to skin cancer, lung cancer and even fatality [4]. Mercury oxide on the other hand is a relatively stable compound, yet when heated it turns into mercury vapour which can damage the eyes, lungs, kidneys as well as the nervous, digestive and immune systems. Marine bacteria can also transform mercury into methylmercury which bioaccumulates in their bodies. (It is important to note that when these ingredients were used ships were made out of metals which are great conductors of heat) [5]

With all the toxicity packed in, it was found that this new paint was highly effective to remove marine organisms’ build up because it simply poisoned them.

However despite being highly effective at getting rid of marine organisms, it was found that paints with arsenic and mercury oxide had to be constantly reapplied because arsenic leached out into water and disappeared from the paint, thus the paint lost its properties. [6]

Towards the end of 1960s, another highly toxic chemical, TBT (Tributyltin) became a popular ingredient for anti-fouling paints. [7] The answer to why TBT was favoured over other ingredients is a bit confusing: because it had a “self-polishing copolymer” structure. [8]

I know it sounds overwhelming, but let me explain:

All anti-fouling paints are mixtures of a wide range of substances, however, the main ingredient of the paint is a polymer. A polymer, in simple terms, is a very large substance that is made of repeating chemical units. [9] (Keep the “repeating part” in mind here.)


The polymer in the paint is bound to an ester, which is bound to TBT. Normally, an ester has this structure:

R–COO–R’

However in our case, this ester looks like this:

Polymer–COO–TBT


When touched with water, the ester bond hydrolyzes (breaks into its components with water) and the TBT is cut loose.

Polymer–COO–TBT + H2​O → Polymer–COOH + TBT’–OH

Now there is a carboxylic acid group on the polymer ( R-COOH is the formula of carboxylic acids - I highlighted it on the equation.)

Carboxylic acids are polar, and because they are polar they can form hydrogen bonds with water and are hydrophilic (water attracting). As they attract water easily, the seawater easily erodes the surface of the paint, though this is not exactly as bad as it sounds.

Remember the definition of a polymer? A polymer is made out of smaller, repeating components, and therefore, when one layer erodes, a new and reactive layer of paint appears - keeping the paint safe and reactive. This feature of TBT is referred to as the “self-polishing copolymer”.


Although TBT was much more useful in keeping the hulls away from marine organisms, it had some very interesting effects on marine life.

In some species of gastropodos (a class of animals that include snails and slugs that live in saltwaters, freshwaters and on land) female species developed male reproductive organs, and vice versa. Referred to as imposex or intersex, the situation was first observed in the southwest of England in a population called dog whelk, the phenomenon was reported all around the world. Some cases of imposex could even lead to the premature death of the females. [10]

With this event and many other instances, TBT was deemed very dangerous to marine organisms as it not only damaged their body and systems but also accumulated in their bodies over time. It was eventually banned in 2001 by the International Maritime Organization. [11]

Nowadays, producers have moved onto “biocides” as the best option. [12] The word biocide comes from two parts: bio- which means relating to biology and -cide, which mean “referring to substances that kill a particular thing”. Biocides are therefore, chemical substances, mixtures or microorganisms that control harmful organisms through a chemical or biological reaction.

Some of the most popular examples of biocides in anti-fouling paints are cuprious oxide (Cu2O) and zinc pyrithione. Cuprous oxide releases copper ions (Cu⁺) into the water, which bind into the enzymes and amino acids of marine organisms. This disrupts the cellular processes and kills the organism. Similarly, zinc pyrithione interacts with the organisms’ enzymes. These ingredients also share the “self-polishing” feature of TBT, with water eroding the coating and exposing the new layer of toxic substances underneath. [13]


And although biocides are still more eco-friendly than TBT, they still include non biodegradable compounds that bioaccumulate in marine organisms. Scientists today are trying to find even better alternatives with nano antifouling paints and biometric anti-fouling paints. The biometric anti-fouling paints, in particular, mimic marine organisms such as sharks, dolphins and shellfish to decrease the adhesion of marine organisms on the hulls. [14] So exciting news is expected for the future of anti-fouling paints.

Section 2: Why are these paints red?

Well, there is a simpler answer to that part of the question.


The first examples of anti-fouling paint used a substance called venetian red. Venetian red is made out of iron oxide (Fe2O3) obtained from red hematite. This red color is naturally occurring and has been used all around the world all the way from ancient cave paintings to dying the coats of the British Army since the 1600s. And it is also a plus that the paint is very durable in all conditions.

One of the first formulas of anti-fouling paints, the ones we talked about all the way in the beginning that included arsenic and mercury oxide, used Venetian Red, and it has become a tradition that still goes on today! [15]

So now you know the long history of why the hulls of ships are painted, and more importantly, why they are red.

Thanks for reading :)



Citations:

1, 2 & 15) Oceanliner Designs. (2024, January 3). Why Are Ships Painted Red? YouTube; Oceanliner Designs. https://www.youtube.com/watch?v=05NL_yKpVcU

3) Admin. (2023, April 30). The history of antifouling and the evolution of ultrasonic antifouling technology. EFC - Electronic Fouling Control. https://www.electronic-fouling-control.com/the-history-of-antifouling-and-the-evolution-of-ultrasonic-antifouling-technology/
4) World Health Organization. (2022, December 7). Arsenic. World Health Organization; World Health Organization: WHO. https://www.who.int/news-room/fact-sheets/detail/arsenic

5) World Health Organization. (2024b, October 24). Mercury and Health. Who; World Health Organization: WHO. https://www.who.int/news-room/fact-sheets/detail/mercury-and-health

6) Kopylov, N. I., Kaplin, Yu. M., Litvinov, V. P., & Kaminskii, Yu. D. (2007). Large-scale use of arsenic in the production of antifouling coatings. Theoretical Foundations of Chemical Engineering, 41(5), 780–785. https://doi.org/10.1134/s0040579507050582

7) Admin. (2023, April 30). The history of antifouling and the evolution of ultrasonic antifouling technology - EFC - Electronic Fouling Control. EFC - Electronic Fouling Control. https://www.electronic-fouling-control.com/the-history-of-antifouling-and-the-evolution-of-ultrasonic-antifouling-technology/

8) Acock, G. P. (1966). The protection of ships’ bottoms—a review. British Corrosion Journal, 1(4), 129–133. https://doi.org/10.1179/000705966798327911

9) Britannica. (2024). polymer. In Encyclopædia Britannica. https://www.britannica.com/science/polymer

10) Veiga, Joana M (2024): TBT and Imposex. Available from http://www.coastalwiki.org/wiki/TBT_and_Imposex [accessed on 7-07-2025]


11, 12, 13 & 14) Job Dronkers (2024): Antifouling paints. Available from http://www.coastalwiki.org/wiki/Antifouling_paints [accessed on 7-07-2025]