Many old pictures and prints of wooden ships, show portholes, usually gun ports, that are square or rectangular, but ports are round in steel-hulled ships.
When ships were made of wood, the architectural material was fibrous and fairly flexible, wooden ships really did creak and this was caused by timber flexure from wave action.
However, wood, especially wet wood, is highly resistant to fatigue stress.
Try breaking a piece of wet willow by repeated reverse flexure, and then try the same process with a mild steel bar or rod of similar section.
Ferrous metals, indeed, most metals, are highly prone to crystalline fracture as a result of changes to the grain structure arising from repeated stress reversal. The effect depends upon the section, heat treatment, carbon content and any alloying elements present.
Towards the end of the 19th century, steel hulls became universal for merchant vessels, and subsequently for warships.
Naval architects found out pretty quickly that any rectangular or square hole in a ship, whether on a deck (a hatch) or in the hull (a porthole) was a source of metal fatigue, commencing at the corners.
The hull or deck would literally rip, due to flexure cycles brought about by wave action; the rougher the seas, the greater the magnitude of the stress.
The unlucky sailors found that their ship was most likely to fall apart when weather conditions were at their worst.
Thus, naval architects specified circular portholes, and radiused corners for deck hatches.
This left no sharp corners for stress concentration.
