Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
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Post by Al Johnston on Aug 27, 2005 9:42:12 GMT -4
The Liberty Ship problem was investigated at the time, and found to be due to crack propagation from stress concentrators such as steps in the hull profile and rectangular corners on the hatch openings.
Crack propagation requires a brittle material (hence the importance of the transition temperature) but otherwise depends on how sharp the crack tip is. In a rivetted ship, a crack will soon run into either a free plate edge or a rivet hole, either of which will serve to blunt the crack tip and stop the crack from growing further. Conversely, a welded ship can give a continuous path for a crack to grow, particularly through the weld metal and the surrounding Heat Affected Zone, which will be more brittle than the substrate.
Of course, as you say, the efforts of the Unterseebootswaffe made ease and speed of construction a much more important design priority than durability, and the Liberty Ship was a fine solution to a problem that might otherwise have seen Britain starve and the war effort of the Soviet Union seriously degraded.
The ET does indeed present a nasty design problem, I'm glad it wasn't my responsibility ;D
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Post by gezalenko on Aug 29, 2005 5:49:47 GMT -4
JayUtah wrote - "the Liberty Ships were not necessarily designed to survive their maiden voyages"
Hi Jay. What did you mean here ? Did you mean that they were specifically designed to FAIL on their maiden voyages ? Or did you mean that the need to survive the maiden voyage was not a (significant) design consideration ? Or something else ?
I appreciate that the German navy were placing "unusual" demands on the design. But even if the designers couldn't do anything about that, surely they must have intended the ships to be at least reasonably seaworthy ? If not, what was the point of making them ? Perhaps there was a propaganda value in having the shipyards be seen to turn out ships at a very fast rate. But was that their only intended purpose ? Am I missing something ?
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Post by JayUtah on Aug 29, 2005 9:12:07 GMT -4
They were not necessarily expected to survive being sunk by the German navy on their maiden voyage. The purpose in making them was to make them in numbers large enough for that unpleasant possibility not to matter. The ships were not expected to have long lifetimes and so they were not designed and built with longevity as a primary design concern.
Sorry if that confused you -- the accidental failure of the ships was of great concern. The remedy was doublers placed in key positions. But even then, the goal was to make the largest number of ships given a certain amount of money and shipbuilding capacity. It was not to make the best individual ships.
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Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
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Post by Al Johnston on Aug 29, 2005 9:24:26 GMT -4
But even then, the goal was to make the largest number of ships given a certain amount of money and shipbuilding capacity. It was not to make the best individual ships. This was indeed the key concern: before the introduction of Liberty Ships, the Kriegsmarine were sinking merchant vessels faster than they could be built.
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Post by gezalenko on Aug 29, 2005 12:37:44 GMT -4
Thanks Jay and Al, I see what you were getting at.
So they came up with a design that 1) enabled ships to be built very quickly, 2) would be good enough for a few trips in normal circumstances 3) would not necessarily last as long as normal ships in normal circumstances 4) would not necessarily be able to cope with attacks by the German navy, whether on the maiden voyage or later, but there would be so many of them that losing a few wouldn't matter.
Maybe going off topic now but sticking with WW2, it occurs to me that that design spec was a bit like that of the Sherman tank - build them quickly in huge numbers, not the best possible design, but there'd be so many of them that losing a few wouldn't matter too much. It probably applied to many other pieces of WW2 kit.
It also occurs to me that that design spec was almost the exact opposite of Apollo (and the Shuttle) - build them one at a time, in an enormously complicated process, and the loss of a single one was considered disastrous.
I think this links back to one of the most interesting features of the Apollo programme (for me) which was the design and development process. It's not just about saying "let's put a man on the moon" it's also about identifying many many more mission requirements, design, production and cost constraints, and tying them all into a project that delivered a workable solution. When you start reading about how they went about it, it becomes apparent that, even if they faked it, the planning, preparation and design work was so thorough that they could have actually gone to the moon using the methods they developed.
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Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
|
Post by Al Johnston on Aug 29, 2005 14:03:26 GMT -4
That's pretty much it.
It's fair to say that only one method has been found effective in preventing surface vessels being sunk by submarine-launched torpedoes:
Sink the submarine first.
(The Japanese can be said to have come close through their construction methods, in that both Yamato and Musashi were hit by upwards of a dozen (air-launched) torpedoes, but they still sank...)
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