Mk. I-IV 1,500lb and 1,850lbFirst introduced for Bomber Command Operations in April 1940, the Mk. I -IV was sturdily built and designed to withstand drops from aircraftflying at 200 mph at altitudes varying from 100 to 15,000ft. Containingapproximately 750lb of explosives the mine could be detonated usingvarious triggering devices depending on the application required. Thetype along with the Mk. V and VII became the standard mine used by theCommand until being replaced by the Mk. VI in 1944.Mk. V 1,000lbIntroduced into service sometime during 1940-41 this mine was a smallerversion of the Mk. I-IV. Containing between 625lb and 675lb ofexplosives this mine was usually detonated using magnetic triggers,although it could be configured to use our triggering devices.Mk. VI 2,000lbA similar mine to the of the Mk. I-IV in that it could be configured invarious ways to dentate. This mine differed only in that it contained2,000lb of explosives in comparison to the 750lb of the Mk. I-IV.Mk. VII 1,000lbIntroduced in 1944, the Mk.VII was an improved version on the Mk.Valthough no increase in the size of explosive charge was made.Mark VI Airborne DC (1942 onwards)A conversion of the standard Mark VII for aircraft use. Used nose and tail fairings that broke off when the DC hit the water. Could not be used at speeds over 150 knots and altitudes above 150 feet (46 m).Weight 450 lbs. (204 kg) (including fairings), including charge 290 lbs. (132 kg) TNT. Sink rate 3.0 m/s. Set to explode a 25ft depth (7.6m)Mark VIII Airborne DC (1942 onwards)Aircraft dropped DC. Designed to fit standard 250 lbs. (113 kg) bomb racks. Maximum drop limits of 750 feet (229 m) and 173 knots. Had a terminal velocity of 600 fps (183 mps).Weight 246 lbs. (111.6 kg) including charge 170 lbs. (77 kg) Torpex. Set to explode a 25ft depth (7.6m)Mark XI Airborne DC (1942 onwards)An improved Mark VIII with a concave nose to reduce ricochets. Eventual limits were 1,250 feet (381 m) and 250 knots.Weight 246 lbs. (111.6 kg) including charge 170 lbs. (77 kg) Torpex. Set to explode a 25ft depth (7.6m)In the first two years of the war depth charges were mainly set for explosion at a depth of 30/45 metres [this figure having being set years ago and never altered since]. Analysis of pilot reports by ORS showed that in 40% of attacks the U-boat was either still visible or had been submerged less than 15 seconds (these are the U-boats that we would expect to have most chance of killing as we have a good idea of their position). Since the lethal radius of a depth charge was around 5-6 metres it was clear that a shallower setting was necessary. Explosion at a depth of 15 metres was initiated and as new fuses became available at 10 metres and then 8 metres. As mentioned above the standard 250lb depth charge was believed to have a lethal radius of only 5-6 metres. Plainly to increase this radius (within the 250lb limit) the chemical explosive inside the depth charge should be more powerful (e.g. increasing the lethal radius by just 20% increases the lethal volume (sphere) around the depth charge by 72.8%). The best chemical explosive currently available was therefore introduced.Note here that it could be argued (and was) that since a 250lb depth charge had too small a lethal radius a bigger charge (600lb (270kg) was prescribed by the Air Staff) was needed. ORS suggested 100lb (45kg) on the basis that it would be more effective to have many small explosions rather than one large explosion. (As an analogy would you prefer to throw many small balls at a small target or one large ball?). In fact neither alternative ever really preceded past the trial stage due to increasing success with the 250lb depth charge. Spacing between successive depth charges in a stickIn the early part of the war this spacing was specified at 12 metres. ORS calculated that increasing this to 33 metres would increase kills by 35% and this was done. Looking into the historical technical data, we can found in some places the Lethal radiusfor a 420 lbs./136 kg. TNT depth charge was between 5 and 6 meters.In another places do the mention of the early 420 lbs./ 136 kg. TNT depth charges has alethal radius of about 4.1 m.In more detailed level places we can found the Lethal Radius of a Mark III, a 420 lbs.(136 kg. TNT) depth charge has a lethal radius of about 4.3 m.A derivate of this was the Mark VII 420 lbs/130 kg, TNT wich had a lethal radius ofabout 6.1 m.An d the Mark VII Heavy 420 lbs/132 kg TNT with moe sink rate, with a lethal radius of about 6.1 m.When this depth charge was filled with Minol high explosive, it increased its lethal radius to 7.9 m, (according as declared).ohn Campbell, in "Naval Weapons of World War Two", indicates the standard British Mk VII depth charge (containing 132 kg explosive, but cast iron weights were attached to increase sinking velocity) could split a 22 mm pressure hull at a distance of 6 meters with early explosives. More brisant explosives available later in the war theoretically increased this distance to 8 meters. The destructive effect of detonation varied with the position of the detonation with respect to the hull (forward, aft, above, below, etc.), but it was estimated that a submarine would be forced to surface if detonation occurred within twice the above distances.The United States experimented with a magnetic pistol intended to actuate between 6 and 7.5 meters of a submarine hull. This work was probably based on the British estimate, since the standard Mk 3 and Mk 6 US depth charges contained 136 kg explosive. Modern US destroyers also carried larger Mk 4 and Mk 7 depth charges with 272 kg explosive; and the British Mk 10 depth charge fired from a torpedo tube carried 907 kg of explosive. Effective distance would increase as the square root of explosive weight, however, so a pattern of smaller charges might be more effective than a single large charge.