[TorpX]

30 ft./sec. seems too fast to me. That would mean about 6 sec. to reach 200 ft. It would be very hard to dodge charges at that rate.

I know different types had different fall rates. The familiar cylindrical type was suited to the racks, but wasn't stable in the water and did not have an optimum drop rate. The British had a large bomb-shaped type that was designed to reach depth quickly.

The weight of the charges isn't really the critical factor, but rather the density and shape. TNT, or TORPEX isn't all that dense, and the steel casings are thin, so they will not drop as fast as a hunk of steel, or rock would.



Edit:

Here is a little info from Wiki on a few types:

The Royal Navy Type D depth charge was designated the Mark VII by 1939.[9] Initial sinking speed was 7 ft/s (2.1 m/s) with a terminal velocity of 9.9 ft/s (3.0 m/s) at a depth of 250 ft (76 m) if rolled off the stern, or upon water contact from a depth charge thrower.[9] Cast iron weights of 150 lb (68 kg) were attached to the Mark VII at the end of 1940 to increase sinking velocity to 16.8 ft/s (5.1 m/s).[9] New hydrostatic pistols increased the maximum detonation depth to 900 ft (270 m).[9] The Mark VII's 290 lb (130 kg) Amatol charge was estimated capable of splitting a 7⁄8 in (22 mm) submarine pressure hull at a distance of 20 ft (6.1 m), and forcing the submarine to surface at twice that.[9] Change of explosive to Torpex (or Minol) at the end of 1942 was estimated to increase those distances to 26 and 52 ft (7.9 and 15.8 m).[9]
The British Mark X depth charge weighed 3,000 pounds (1,400 kg) and was launched from 21-inch (53 cm) torpedo tubes of older destroyers to achieve a sinking velocity of 21 ft/s (6.4 m/s).[9] The launching ship needed to clear the area at 11 knots to avoid damage, and the charge was seldom used.[9] Only 32 were actually fired, and it was known to be troublesome.[10]
The teardrop-shaped United States Mark 9 depth charge entered service in the spring of 1943.[11] The charge was 200 lb (91 kg) of Torpex with a sinking speed of 14.4 ft/s (4.4 m/s) and depth settings up to 600 ft (180 m).[11] Later versions increased depth to 1,000 ft (300 m) and sinking speed to 22.7 ft/s (6.9 m/s) with increased weight and improved streamlining.[11]
Although the explosions of the standard United States' 600 lb (270 kg) Mark 4 or Mark 7 depth charge used in World War II were nerve-wracking to the target, an undamaged U-boat’s pressure hull would not rupture unless the charge detonated closer than about 15 ft (4.6 m). Placing the weapon within this range was entirely a matter of chance and quite unlikely as the target maneuvered evasively during the attack. Most U-boats sunk by depth charges were destroyed by damage accumulated from a long barrage rather than by a single carefully aimed attack. Many survived hundreds of depth charges over a period of many hours; U-427 survived 678 depth charges in April 1945, though many may have detonated a considerable distance from the target.