“Then why don’t the Soviets copy our screw designs?” “Several reasons, probably. You design a screw for a specific hull and engine combination, so copying ours wouldn’t automatically work for them. A lot of this work is still empirical, too. There’s a lot of trial and error in this. It’s a lot harder, say, than designing an airfoil, because the blade cross-section changes radically from one point to another. I suppose another reason is that their metallurgical technology isn’t as good as ours—same reason that their jet and rocket engines are less efficient. These new designs place great value
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