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#1
Old 01-05-2003, 08:25 PM
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why are ships portholes round?

Just wondering, from ancient ships to spaceships, seems most are round.
It seem it would be harder to cut round holes.

dcurt
#2
Old 01-05-2003, 08:30 PM
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But it easier to make a good seal if you don't have sharp corners.
How old do you consider "ancient"?

(FWIW, I believe the first "port holes" were square, as the "ports" were the gun ports.)
#3
Old 01-05-2003, 08:36 PM
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Putting a round hole in something actually makes it stronger, while a square hole makes it weaker.

Too large & (rounded off) square windows is what brought down the Comets, the first jet airliners.
#4
Old 01-05-2003, 08:41 PM
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Portholes are only round in metal skinned craft.

Square holes concentrate stress fractures at the corners.
#5
Old 01-06-2003, 12:18 AM
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To expand on Hail Ants' and Desmostylus' comments, round holes are preferred when the structure you're cutting a hole in is under a lot of stress.

The fuselage skins of pressurized aircraft take an enormous amount of stress, mostly from pressurization. If you cut a hole in such a structure, the load that the material you cut out was supposed to carry must be carried by the surrounding skin. So you must strengthen the frame around the hole to make up for the lost material. But it's not a simple case of just putting back the material you took away. The load must be re-routed around the hole, and that takes extra material. The most efficient shape for a cutout in this situation is round.

In most large cruise ships, the windows in the lower hull are round while those on the upper decks are square. I'd imagine that's because the lower hull must withstand the enormous stress of waves crashing against it. The superstructure doesn't have this requirement.

Speaking of spacecraft, the two main windows in the Apollo Lunar Excursion Module were triangular.

But then again, the LEM never really went into space, now did it?
#6
Old 01-06-2003, 12:27 AM
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Quote:
Originally posted by Kamandi

In most large cruise ships, the windows in the lower hull are round while those on the upper decks are square. I'd imagine that's because the lower hull must withstand the enormous stress of waves crashing against it. The superstructure doesn't have this requirement.
I suspect that the fact that the hull does a lot of bending and twisting has a lot to do with it. I remember on the Ile de France there were foot wide finger joints at two or three places in the decks. As the ship moved over the waves you could see the joints opening and closing, especially in rough weather
#7
Old 01-06-2003, 01:42 AM
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Quote:
Originally posted by Kamandi
So you must strengthen the frame around the hole to make up for the lost material. But it's not a simple case of just putting back the material you took away. The load must be re-routed around the hole, and that takes extra material.
And just to expand on your post a bit, that extra material is called a "rip-stop doubler".
#8
Old 01-06-2003, 06:04 AM
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Putting a round hole in something actually makes it stronger, while a square hole makes it weaker.

Too large & (rounded off) square windows is what brought down the Comets, the first jet airliners.


New research suggests that the fuselage was subject to premature, and chronic, metal fatigue, due to being way too thin.
#9
Old 01-06-2003, 12:56 PM
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Quote:
Originally posted by Hail Ants
Putting a round hole in something actually makes it stronger, while a square hole makes it weaker.
I forgot to address this in my previous post.

Cutting any kind of hole will always weaken a structure. You're removing material that load formerly passed through. That load doesn't just disappear: it must now pass through the structure around the hole, putting the remaining structure under greater stress than if the hole wasn't there. (Think of the stress in a material roughly as load divided by area. With equal load, a smaller structure means greater stress. Very roughly. Please don't hurt me. My stress analysis training is eleven years old and getting foggier by the day.)

The trick is to design the hole such that a minimum of material (and effort) is put back into the structure to make up for the loss of strength (or other mechanical properties). As I mentioned before, usually the most efficient hole shape is round.
#10
Old 01-06-2003, 06:44 PM
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Quote:
Originally posted by Kamandi
To expand on Hail Ants' and Desmostylus' comments, round holes are preferred when the structure you're cutting a hole in is under a lot of stress.
I was taught in a science class that they're round so that the window itself withstands the presure better, and is less likely to be blown in by waves during a storm. I can see how what I was taught fits, but the explaination I was given is really lacking, isn't it?
#11
Old 01-06-2003, 08:34 PM
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It's certainly true that a round window is able to withstand pressure better than any other shape (everything else being equal). Part of the reason is that since the perimeter of a circle is the shortest of any shape with equal area, the seal is of minimal length.

My discussion previously about skin loads assumes the most common type of cutout - one in which the window does not take the skin load (as mentioned above, the skin load must be routed around the window).

Some cutouts, however, are designed such that the window or other cover does take the skin load. For example, if you look at the wing of an airliner (especially the lower surface) you'll see quite a few small doors ringed with dozens of fasteners. These doors are maintenance or inspection ports - rarely opened - and are designed so that the skin load can go right through them, transmitted by all those fasteners.

This is still less efficient than a continuous structure, so some extra thickness at the edges is required. Unscrewing all those fasteners to get into the opening sucks, too.
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