It is a design feature seen on ships and planes worldwide: small circular windows, known as portholes, rather than the square windows common in buildings and cars. The reason, as explained in a viral TikTok video, dates back to World War II and the catastrophic failure of Liberty cargo ships.
During the war, 19 Liberty ships were reported to have split in half, leading to significant loss of life. Investigations revealed that the cause was the square hatches on these vessels. The sharp 90-degree corners created stress concentrations, where forces flowing through the steel became turbulent, concentrating stress in one tiny point. This was exacerbated by poor-quality steel containing high levels of sulphur and phosphorus, which became brittle in the icy Atlantic waters.
The same problem emerged in the aviation industry in the 1950s. The De Havilland Comet, the world's first commercial jetliner, suffered three crashes within 12 months—in May 1953, January 1954, and April 1954. The cause was identical: tiny cracks growing from the sharp corners of its square windows, driven by pressure changes during each flight.
Engineering expert Wayne Maltry explained to Reader's Digest that a subsequent crash investigation revealed the fuselage ruptured due to cabin pressurisation applied to fractures that propagated from a front top square window corner. As a result, windows on planes were redesigned to be rounded, reducing stress points at the corners.
Maltry added that airplane windows are round for the same reason space, ocean, and submarine ship hull portals are rounded: to reduce stress concentrations under pressurisation cycles and prevent microscopic cracks from developing into macroscopic fractures.
