From: astride Mason and Dixon's Line
First, if destroyers were intended as anti-torpedo boat weapons then why did some of them carry so many torpedos themselves? For, surely a high rate of fire deck gun would be far more effective against lightly armored fast moving targets. I do seem to recall hearing about how destroyers were used in that capacity, but the placement of so many torpedo tubes on destroyers would suggest that they had another purpose.
It is quite simple. It was recognized early on that in carrying out their mission of destroying torpedo boats before they could reach torpedo launch positions (somewhere in between opposing battle lines), they would themselves have attained good positions to carry out torpedo attacks of their own. Because of this, "pure" torpedo boats (fast, fragile craft with scarcely any guns) became obsolete, and the two types merged together into the destroyer.
Actually the torpedo boat did not become obsolete; rather it became smaller and faster, cheaper to build, harder to hit. It became the MTB/PT boat.
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