"Effectiveness of One Ton Depth Charge" Topic

It is frequently mentioned that RN destroyers used as convoy escorts had their torpedo tubes retained or refitted in order to fire the "One Ton" or Mk X depth charge. Plenty of information is available about the sinking rates and the need for speed to keep clear of the resulting detonation, but I not seen anything to give a measure of the effectiveness of these beasts, particularly in terms of percentage chance of damaging a submarine.

I realise they were not frequently used but can anyone point me in the right direction.

Given it cntained 2000lb of explosive, and that some other DC figures are known, perhaps some calculation is in order?

If (and it's a big if……I'm guessing here) damage is proportional to the inverse cube of the distance to target, and the 290lb of Amatol in the Mk VII could split 7/8" skin at 20 feet (from wiki – link ), then I'd expect 2000lb to do so at 38 feet (6.8 times the explosive, cube root of that is 1.9 – so 20ft * 1.9 = 38), and force a sub to the surfact at twice that.

Less technically, in some first person accounts there are mentions of the Mk X – IIRC, it was considered to be much more effective! It was easier to hit – or at least discourage – submarines because they didn't have to be as exact in their placement of the DC to get a result.

Further in the wiki article it mentions:
The killing radius of a depth charge depends on the payload of the depth charge and the size and strength of the submarine hull. A depth charge of approximately 100 kg of TNT (4 MJ) would normally have a killing radius (hull breach) of only 3–4 meters (10–13 ft) against a conventional 1,000-long-ton (1,000 t) submarine, while the disablement radius (where the submarine is not sunk but put out of commission) would be approximately 8–10 meters (26–33 ft). A higher payload only increases the radius by a few meters due to the fact that the effect of an underwater explosion decreases with the distance cubed. The killing range would be greater against a larger submarine and shorter against a smaller submarine. It is doubtful if the hull of a midget submarine with a titanium hull could be sunk by a depth charge by anything less than a direct hit, even though it could be decommissioned with less.

The article mentions that kills with a single hit are rare, submarines are usually sunk after multiple,if not hundreds of compression/expansion cycles weaken the structure enough to force the crew to choose to surface. So percentage wise you are talking less than 1% to kill, but may have done 1% cumulative damage to the target.

-Binhan

From my collection of weapon effectiveness snippets:

ADM 219/564, Evaluation of ship-borne anti-submarine weapons
This is a copy of the USN ASWORG memorandum no. 39, 
published 30 Jun 1943.
It calculates the theoretical probability of inflicting 
lethal or surfacing damage on a U-boat with various weapons.
The submarine is assumed to have a pressure hull measuring 
200 by 16 feet, moving at an indeterminate speed between 0 
and 6 knots, and to have a tactical diameter of 300 yards.  
The method used is to calculate the volume the U-boat may 
be in when the dead time has elapsed, and divide the lethal 
or damaging volume of the charge pattern by this probable
volume.
Weapons considered are British and American depth-charges 
with depth-setting or proximity (magnetic gradiometer) 
fuzes, and the ahead-thrown weapons Hedgehog and Mousetrap 
(a further weapon called "Mortar B", apparently theoretical, 
is also mentioned; this may be double Squid).
The following performance is given for depth-charges with 
depth-setting fuzes.  The US Mk9 has a "teardrop" shape; 
the others are "ashcan" shape.  The British Mk7h (heavy) 
is weighted with 150lb of lead.
Weapon	       US Mk6	        US Mk7	        US Mk9
Filling	       300lb TNT	600lb TNT	200lb TNT
Sink rate	8 ft/sec	9 ft/sec	14 ft/sec
Effectiveness with a 10-charge pattern, 
normal depth range (50-250 ft)
P (damage)	13%	        36%	        10%	 
P (lethal)	7.6%	        14%	         7.6%	 
Effectiveness with a 10-charge pattern, 
deep depth range (300-500 ft)
P (damage)	5%	        14%	         5%
P (lethal)	3%	         6%	         4%	 
Weapon	       UK Mk7	        UK Mk7m	        UK Mk7h
Filling	       290lb Amatol	290lb Minol	290lb Amatol
Sink rate	8 ft/sec	8 ft/sec	16 ft/sec
Effectiveness with a 10-charge pattern, 
normal depth range (50-250 ft)
P (damage)	13%	        26%	        18%
P (lethal)	7.6%	        10%	        10%
Effectiveness with a 10-charge pattern, 
deep depth range (300-500 ft)
P (damage)	5%	        10%	        10%
P (lethal)	3%	         4%	         6%
A pattern of 6 US Mk6 or 4 US Mk7 charges is stated to give 
a 10% chance of lethal damage and 22% surfacing damage in 
the normal depth range,  and 4% lethal damage and 9% 
surfacing damage in the deep depth range.
It is stated that British figures give lethal probabilities 
of 6.5% and 17% respectively for patterns of 5 and 14 Mk7s.
Also mentioned is the British Mk10, a very heavy charge 
carrying 2000lb of Amatol and fired from the torpedo tubes 
of destroyers.  It sinks at 6.5 ft/sec to a fixed detonation 
depth of 220ft, or 640ft for the Mk10*.  Its effectiveness 
is quoted per 1000lb total weight; as its all-up weight 
is 3050lbs, the effectiveness per charge would be as follows:
Charge	P(damage)	P(lethal)
Mk10	   22%	            4%
Mk10*	   24%	            4%
Charge with proximity (magnetic gradiometer) pistols are 
not in service at the time of this memo, but their 
performance per pattern of 10 charges is calculated as:
Weapon	     US Mk8	UK Mk9
Filling	     285lb TNT	290lb Amatol
Effectiveness with a 10-charge pattern, 150 feet
P(lethal)    36-48%	40%
Effectiveness with a 10-charge pattern, 400 feet
P(lethal)    17-23%	22%
Only lethal damage is considered as the fuze should 
only actuate when the U-boat is inside the lethal radius.  
The US charge is weighted with 150lb of lead, and the 
UK Mk9 is also weighted with lead.

Hope that helps,

All the best,

John.