Why are ship portholes round?

Portholes on ships are round for the same reason that submarine hatches have round
corners and airliners have rounded windows.  In all these cases, the vehicle is
subject to flexing and mechanical stress.

If the windows had sharp corners, the stresses would concentrate there, resulting
in material fatigue and eventually cracking.  With round or rounded windows, the
stress is evenly distributed.

Large boats like cruise liners can have rectangular picture windows in some places
because the local stresses there are smaller.

Some early commercial airplanes had large, rectangular windows, and the skin of the
planes had a tendency to crack at the corners.

Today's pressurized airliners have much smaller, rounded windows.

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