Hi all,
I was quoting from memory & made a mistake in the type of DC: the Mk 14 was fitted with an RCA doppler acustic pistol, but was not ready before the end of the war. Its weight of 340 lb (154 kg) with 200 lb (91 kg) charge made it quite dangerous, sinking speed was 23 f/sec (7 m/sec).
The magnetic DC was the Mk 8, first issued in 1943 to the Atlantic Fleet DEs, but was never available in large numbers: the magnetic pistol armed @ 35 ft (11 m) or 200 ft (61 m) & set to explode t 20-25 ft (6-7.5 m) from the sub's hull.
The FIDO was the Mk 24 mine (not 14 as I erroneously said above).
Now, on the subject of the British trials vs submarines, the rtrials were made against HMS Sceptre, Stygian & Spark at 325 ft depth, with charges varying between 200 and 620 lb Torpex (to simulate Squid projectiles). Sceptre suffered slight damage from the detonation at a distance of 100 ft, but the charges were progressively closer to the other 2 subs and Stygian suffered "spectacular damage", while Spark's hull collapsed completely.
The next seeries of trials, at 75 ft depth against Sea Rover, Surf and Shalimar, the hulls of these boats were either welded or riveted, but the detonation of the charges at 20 to 40 ft sank all 3 of them.
A test was carried out against a full size section of an "A" class boat in 11-12/1948, and it was found that a charge of 187 which had caused little damage at an uspecified distance, caused enough damage to sink the boat when moved only 5 ft closer aboard.
The conclusions drawn by these experiments were combined in a formula: D = K W/t, where D is the lethal radius, W the charge's weight, t is the plating thickness and K a constant which reduces with depth, some 30% less at operational depth than on the surface.
Hope that answers the question, regards,
Brian