ELi5 Why are most of the boats and ships I see in dock white and not another color?

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Is it a cost issue or does a white watercraft have some advantage over ones that are different colors?

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White is the most reflective colour. It absorbs the least amount of heat allowing ships to use less air conditioning and therefore save on fuel. Military ships on the other hand tend to be painted grey, navy blue, or black depending on the country. This is done for camouflage in/on the water. Many of those ships don’t need to worry about using lots of AC because they are nuclear powered.

White paint reflect UV light better then any other color. And UV light tends to break down plastics and epoxy used in a lot of marine environments. For example fiberglass uses epoxy as binding material so UV light can make the fiberglass disintegrate. And metal plating tends to be covered with epoxy paint to protect it from the salt water so UV light will make the paint disintegrate exposing the bare metal underneath to rust. Using white paint helps reduce the issues of UV light and make the boat last longer.

I’ll be that guy…

Depending on the type of boat, it’s usually not paint. Smaller crafts (often recreational) are often made of fiberglass to keep the weight down for trailering and on-water performance. Fiberglass uses an epoxy resin brushed onto multiple thin pieces of fabric, so white resin is often used because 1) it’s cheaper than adding pigment, 2) it’s actually lighter without pigment, and 3) it streamlines the manufacturing process since the color is integral to the pieces being made.

Even when scratched, there’s no variation in color in fiberglass, because it’s the same color throughout, and not just one layer of color on the outside.

A happy side effect of this is the high visibility (for safety) and the high UV reflectivity (for comfort).

The airline industry follows a similar logic too… passenger jets painted white are notably lighter than colored liveries, thus they get better fuel economy.

Here in the south it’s because any other color will get hot enough to burn your skin in the sunmers.