[gap]

Originally Posted by gap

According to John Campbell (Naval Weapons of World War Two, 1985, p. 94):

"The original pistol setting of 50ft (15m) was too deep for aircraft attack on a surfaced U-boat and it was reduced to 25ft (7.6m) in 1942"

This is referred to the British Mk. VII airborne DC, but it probably applies to other aircraft depth charges used ny the British. An user in navweaps' discussion board quotes another source (a link to is provided, but it is no longer available online):

"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."

This is probably referred to US depth bombs. The two pamphlets by Navy Bomb Disposal School I mentioned above (dated summer 1944 and autumn 1945) specify for all the hydrostatic fuzes used with air ordnance depth settings in steps of 25ft, from 25ft (7.6m) to 125ft (38.1m), but I think only the 25ft setting was used in practice.

Quick update from the USNBD Pamphlet "British Bombs and Fuzes, Pyrotechnics, Detonators", November, 1944 Ed.
Apparently, the hydrostatic pistols that could be used with British a/c depth charge were as follows:

Pistol Mk. X** (obsolete by the date of publication):
Depth settings: 50, 100 and 150 ft (15.2, 30.5 and 45.7 m respectively)
DC's used on: Mk. VII and VIII

Pistols Mk. XIV and XIV* (in service):
Fixed depth setting: 14-18 ft (4.3-5.5 m)
DC's used on: Mk. VII, VIII, XI and XI*

Pistols Mk. XVI and XVI* (in service):
Fixed depth setting: 20-24 ft (6.1-7.3 m)
DC's used on: as above

Pistol Mk. XIX (in service):
Fixed depth setting: 20-24 ft (6.1-7.3 m)
DC's used on: Mk. XI and XI* with Mk. IV tail

Pistol Mk. XX (in service):
Fixed depth setting: 14-18 ft (4.3-5.5 m)
DC's used on: as above

Summing up: after all, unlike stated by post-war sources, British a/c depth charges could be set to detonate at depths shallower than 25ft. Indeed, we don't know how reliable/effective were the shallowest pistols (Mk. XIV and XX, 14-18 ft), and how common they were.

Incidentally: happy Easter guys