The \'Fido\' acoustic homing torpedo was quickly developed and effectively used by the US during WWII. It sank a large number of U-boats, and was highly effective - more so than the T-5, and later, German acoustic torpedoes.

But this is not a fair comparison.

The Fido was meant to hunt a submerged U-boat. Submerged subs of WWII vintage were noisy beasts, especially if they were travelling \'quickly\', which means 5 to 10 knots, as they likely were after a crash dive, trying to get to depth as fast as possible. The U-boat would be submerged in the more silent depths of the ocean, removed from the background noise of surface waves. The speed of a U-boat going at \'high speed\' submerged is only 10 knots or so, slower than a slow escort (~15 knots or so).

So an acoustic torpedo hunting a submerged U-boat is hunting a noisy, slow target far removed from any background noise. This is an easier problem than escort shooting in every aspect. As such, you are right in that the Fido could be designed for a relatively slow max speed, below 20 knots I believe, as higher speeds caused too much noise over the micropohones.

Much more limited use of the sub version of the Fido by US subs in the Pacific also enjoyed a good hit ratio against Japanese ASW vessels. Whether this was too small a sample to be valid, or in easier environments (shallow water with smaller waves, less background noise?) than the Atlantic, or whether the Fido was better than German acoustic torpedoes, I don\'t know. Supposedly a captutured T-5 was examined by Westinghouse (that made the Fido) and they couldn\'t figure out how it could work - whether they were right, or just didn\'t understand the design, I also don\'t know.

For more history of the Fido development, try \'Hellions of the Deep\'.