Depth Charges against Surface Vessels

Question

Previously, a treaty was signed that effectively banned air-dropped bombs, and military versions of rockets and missiles. That treaty leaves a gap as far as Depth Charges go, and this is not precisely an oversight. The treaty was heavily influcenced by older-fashioned surface fleet Admirals and several centuries of tradition, and two weapons threatened that: Aircraft and submarines. While they didn't want to limit submarines (Since that would limit their own and, at the end of the day, they're still ships), they wanted to leave open methods of dealing with enemy submarines.

This, of course, leaves the opportunity for an enterprising individual to come up with the idea of using depth charges against surface ships, potentially even some designs dedicated specifically to such a device.

In my research, I haven't come across any surface vessels that were sunk by depth charges, although numerous were damaged - Often by anti-submarine craft sinking and having their prepped depth charges go off.

Is it possible for a depth charge (Of any practical size) dropped by an aircraft to sink a significant surface ship, such as a heavy cruiser or battleship? Modifications to enhance weapons specifically for this purpose are allowed and encouraged.

Edit for some clarifications:

A torpedo utilizes active guidance and propulsion. Neither of these are allowed by the treaty.

A mine is often, but not always, tethered to the seabed, and all mines (for the purposes of this definition) explode based on contact or proximity. Mines are not explicitly banned, but their uses are sufficiently different than depth charges that their use by aircraft should fall under a separate question

A depth charge has no active guidance and no propulsion systems. Additionally, for the purposes of the treaty, they are detonated when they reach a specific depth, as opposed to on contact with the ground or a vessel.

Answer 1

You are looking for a bouncing bomb

A bouncing bomb or a skip bomb would fit your criteria. They were dropped from planes flying at low altitude and would 'skip' across the surface of the water and detonate. They were used during World War II to destroy dams and ships.

These bouncing bombs fit under all of your criteria because:

1. They are essentially repackaged depth charges.
2. They have no active guidance or propulsion. They rely on their momentum to move across water
3. They are detonated by a timer, not by impact.

Answer 2

Technically speaking, the battleship Ostfriesland, which was sunk by Billy Mitchell in 1921 in a watershed event, went down due to depth charges. They didn't necessarily plan it that way, but when they were dropping bombs on the battleship, quite a few missed the ship by a few feet and detonated in the water next to it. It was those explosions that punched holes in the hull and eventually sank the battleship. The bombs that hit the upper decks of the Ostfriesland didn't do a great deal of damage due to the battleship's heavy upper deck armor.

Something similar happened in the last case of a battleship sinking another battleship without assistance from other ships or aircraft, when USS Washington sank IJN Kirishima in the Guadalcanal campaign. While Kirishima was greatly damaged by the 16 inch shells that hit the upper decks, it was the underwater hits, shells landing just short of the decks and continued under water to strike the hull, that caused it's capsizing, as those hits tore open the hull under water, unbalancing the ship to a degree that couldn't be addressed with counter flooding. Those underwater hits occurred because Washington had closed to within 8000 yards of Kirishima undetected before opening fire (point blank range for 16 inch naval rifles), so the shells were coming at a fairly flat trajectory. Those shells that landed short just kept on going.

Another form of weapon operated on the same principle as a depth charge, that an explosion in water does a lot more damage than an explosion in air due to the greater density of water: mines. While they were positioned under water to avoid detection, in practice they did far more damage to a hull than a bomb of similar size exploding in air. A lot of ships in war time have been sunk with mines.

Answer 3

Depth charges used by aircraft this way will be called torpedoes

The closer a plane flies to an alerted enemy ship, the more intense the anti-aircraft will be. Pilots don't want to die and planners would like to keep their loses low. You don't want to have to drop explosives directly against the hull or deck of a ship. Having some way to move the explosive charge from further out towards the ship would be really nice.

Such a device will need to have a minimal frontal cross-section to cut down on drag. It will also need to be fairly long to house the detonator, explosives, primitive guidance system and motor. A propellor at the back will provide thrust. Fins at the rear will also provide control surfaces. A device shaped this way will be able to travel a considerable distance before detonating.

The Mark 48 torpedo has all of the above features; packing almost 300kg of explosives. Whether detonating against the hull or against the hull of the target, it will do considerable damage.

Political Considerations

Why on earth would any sane sovereign power give up the ability to use any kind of modern weapon? That kind of a situation seems highly contrived and ripe for cheaters to keep going while those who obey the rule sit back and lose ground.

Answer 4

Depth charges used against surface targets are called mines

There have always been aircraft deployed mines, up to the present day. They principles of operation are similar to a depth charge, except instead of a pressure fuse, they will have a magnetic or contact fuse.

As for whether or not mines can sink large ships...well, Wikipedia has a list for that. Here are four battleships: Alfonso XIII, Bouvet, Hatsue, and Peresvet.

Answer 5

One form of depth charge used during WWII was called the "Hedgehog". Given the rather poor ability of early SONAR (British "ASDIC") to localize the position of a submarine, the Hedgehog projected a pattern of smaller depth charges at the suspected location of an enemy submarine, using a "spigot mortar". The depth charges were fired by contact fuses rather than hydrostatic fuses, with the intention that a strike on an enemy sub wold be far more devastating with the explosion in contact with the hull, rather than the hydrostatic shockwave a conventional depth charge relied upon.

Typical Hedgehog arrangement

Now, while the range of such a device is rather short, the idea of firing a large number of contact fuzed projectiles through the air does allow you to attack a surface ship, with the potential of having explosive damage on the deck and superstructure, the hull above the waterline, and even potentially below the waterline as well. More powerful projectors would allow more range, but also greater dispersion of the pattern.

Showering a large number of similar devices from an aircraft (or even an airship, don't forget blimps were used as convoy escorts during WWII, although mostly to spot enemy U-boats on or just below the surface) in a bombing run provides a better chance at both surviving the attack and damaging the ship. Although each individual bomb in a hedgehog strike is small, a large number of hits spread across the ship could put a lot of systems out of action and overwhelm damage control parties, especially of the attack can be followed up in close order by either more of the same or by more conventional anti ship weapons.

Another means of delivering a large depth charge like projectile (or an actual depth charge if you have the ability to locate a submarine at a distance) would be a Dynamite Gun. Using compressed air or gasses, this was used to launch early high explosive shells with a gentle acceleration, since the violent acceleration of conventional propellant could cause the HE filler of a shell to spontaneously detonate inside the gun. Large versions could launch 15" (381mm) projectiles 2-5000 yards (1800-4600m), which is more useful, although still easily outranged by conventional artillery. The compressors and other equipment make for rather large and unwieldy weapons. A smaller version, using a captive piston and small explosive charge to generate compressed air "on the spot" was developed, but had a correspondingly short range as well.

Example of the Zalinski dynamite gun