Corners represent weak points at which cracks can start. Forces, such as those of waves striking the hull, tend to get focussed upon them. Round windows have no corners, so represent less of a weakening in the hull.

1) "Portholes are round holes with doors in the side of a ship, big enough for cannons to be run out and fired. They were invented in England by a French shipbuilder named James Baker, during the realm of Henry VI (in the late fifteenth century). If you just had holes in the side of the ship, too much water would get in during heavy weather; it could fill up the ship with water, and also rust the iron cannons. The word "port" is from French "porte" meaning door. Portholes later came to mean any windows -- round by custom -- in the side of a ship. I have to mention here the great scene in the movie Diamonds are a Girl's Best Friend, where Marilyn Monroe walks into her cabin on the cruise ship, pauses, elegantly points, and coos, "Oh look!!! Round windows!" It is so perfect, because nearly anyone who wasn't a hick and kind of a moron would know that ships have round windows and that they are called portholes -- not really something to remark on! It is a good phrase to say when you want to point out something really obvious." Source and further information: http://everything2.com/title/porthole So they were originally round because cannons are round. 2) "The constant up-and-down motion of a ship puts onsiderable 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. Airplanes have a full frame structure that can absorb the stress." "Aircraft windows are rectangular with rounded corners on some airplanes, but newer ones usually have elliptical windows; with the combined load of internal pressure and fuselage shear, an elliptical window surround structure is lightest. In business jets, the bodies are much shorter, the shear stresses are lower, so a circular window is not much of a weight penalty. I believe ship windows are round because porthole covers can be opened; and like a manhole cover, only a round door cannot fall through the hole (it would be very bad in rough weather to be missing a porthole cover)." Source and further information: http://www.autoqna.com/aircraft/222-aircraft-5.html Further information: http://www.wisegeek.com/what-are-portholes.htm http://en.wikipedia.org/wiki/Porthole

Must be for three reasons. 1. To let the spray, moisture and and rainwater to trickle down to the bottom of the porthole effortlessly where there must me an outlet for the water collected. 2. While sailing in heavy cross winds to reduce the air resistance by diverting the air away by the rounded shape of the porthole which must have rounded rubber framing. For the same reason only the ships must be getting built with a streamlined design unlike barges. 3. Round portholes are aesthetically better looking on a ship. We just can't imagine a ship with rectagular portholes with sharp corners. Is there a 4th reason?

I don't really know. But if I had to guess at it. I'd say maybe because being round & not having not corners in them like other windows. Would be less likely to crack during pressure. Plus it may help prevent erosion from building up & happening by not having corners......Just a guess tho.........Plus 5 for the question.......................M.C.S.

The main reason is pressure and temperature. In rounded glass the pressure and temperature are even distributed therefore less chance of cracking which might results from thermal or pressure shock. The same reasoning can be applied for aircraft and space capsule.