Why are portholes on a ship round?

Have you ever wondered why ....

...portholes on a ship round?

The constant up-and-down motion of a ship puts considerable stress on the ship's skin, or outer covering. If portholes were designed

with angles, this stress would tend to concentrate at those points and perhaps crack the skin. With round portholes such stress is evenly

distributed around the holes, making it less likely for such cracks to occur.