Depth Charge: An Early Antisubmarine Warfare Weapon* Part I World War I

John Merrill is a frequent contributor to THE SUBMARINE REVIEW, specializing in the history of sensor and weapons technology. He is retired from the New London Division of the Naval Undersa Warfare Center.

Opening Days

As World War I began, the German U-boat, initially encumbered with existing rules concerning visits and searches of intended targets, was assumed to be an inefficient war weapon. Rules of engagement for the U-boats were limited by the difficult Prize Regulations governing submarine actions against nonmilitary vessels established by eight nations in the 1909 London Declaration. The Regulations were designed to protect neutral nations’ maritime rights and international seaborne commerce in the event of War.**

During these early months of the War, the 1909 agreement significantly hindered effective submarine aggression. “The alternative to sink noncombatant.vessels without warning struck prewar sensibilities as so barbarous that in January 1914 Britain’s.

First Lord of the Admiralty, Winston Churchill, scorned the idea that a civilized power would ever adopt such a policy.” Germany was challenged for the next several years with following the Prize Regulations or departing from their use and adopting unrestricted submarine warfare. By early 1917, three years later, it was the latter. These years of indecision resulted in fewer U-boats and little technological advance in their submarines.2 During these early years of the War, the United States negative view of unrestricted submarine warfare was a significant factor in curtailing Germany’s use of submarines. According to E. B. Potter, throughout the first U-boat campaign from February 22, 1915 to September 20, 1915, the U-boats held back because of the possibility of United States entry into the war.

At first, the small number and size of German submarines also reduced assessment of the potential threat. Some of the German submarines were less than 100 feet long with a crew of 14. Ger-many’s naval intentions in the early stages of the War were directed toward Britain’s Grand Fleet. The German U-boats were relegated to the role of reconnaissance and torpedo support for the German High Sea Fleet in dealing with the Grand Fleet.4 The torpedo as a submarine weapon, although available in small numbers early in the war, was not in general use until March 1917. Prior to that time most sinkings by U-boats were by gunfire.

At first, both sides overestimated the military capabilities of their enemies. Consideration of enemy submarines as potential targets was not in the purview of the designers of the existing destroyers and torpedo boats and their weapons. As submarine warfare developed, it became a close and comparatively intimate encounter of a few hundred yards between enemies while the standard range of engagement for the grand fleets took place at 20,000 yards. A new type of warfare required new tactics and weapons. It turned out that all through the War, the U-boat sea keeping and endurance did exceed expectations.

First Sinkings by U-boats

With few exceptions such as First Sea Lord Sir John Fisher, the menace of the submarine was initially disregarded. On 5 September 1914, a month and a day from the start of the War, U-21 sank the Royal Navy light cruiser PATHFINDER with one torpedo. A blockade was quickly established with more than eighteen cruisers. On 22 September, the German U-9 torpedoed and sunk blockade patrol British heavy cruisers ABOUKIR, HOGUE, and CRESSY. Six weeks into the war, a war zone was in operation with an Admiralty heavy cruiser blockade of the entire North Sea and the waters between Iceland and the Norwegian coast.

These events and later on the increasing and extraordinary loss of merchant shipping from the unrestricted sinkings coming from adoption of a guerre de course strategy by German U-boats pushed the Allies to accelerate development and implementation of antisubmarine weapons to attack and sink the German submarines. As observed by Admiral John Jellicoe of the Admiralty in 1920, British antisubmarine measures were almost nonexistent at the beginning of the war.

Throughout the War antisubmarine weapons improvements followed but the development by scientists and engineers of effective ways to detect submerged submarines did not begin to emerge until the latter part of the War and then only in rudimentary form. The primary Allies’ tactic against the U-boat during the first several years of the War was an offensive approach to search, find and destroy. In large oceans with no practical way to locate the enemy submarine, the success rate was low.

Not long into the War the Royal Navy met with success in blockading the German High Sea Fleet. Lacking capital warships to confront the British but with U-boats prevailing against merchant shipping, the German and Central powers confined their naval effort almost completely to submarines. It was not until mid-I 917 that the Allies initiated broad merchant ship convoying essentially defeating the guerre de course efforts by German submarines. The Allied Antisubmarine Warfare forces (air and sea) included the depth charge in their armament.

Available Weapons

Initially the tools at hand for countering submarines were drift and stationary nets, mines, deck guns and ramming. Mines were widely used but it has been pointed out that as of January I 917, the British did not possess a mine that was satisfactory against submarines ‘ However, a new mine based on a German design developed by the Admiralty was in place and effective against the U-boats by November I 917. A surfaced submarine could be rammed. During the War nineteen U-boats were sunk in this way. Merchant ships also rammed and sunk five U-boats. “As time went on, all the later destroyers were fitted with steel rams at the bottom of the stem, and very efficacious they were as tin-openers”.

Damaging or sinking U-boats by shelling was not effective because it was difficult to hit such a small target with normal low free board before it dived. In addition, with intensive submarine crew training the number of seconds required to dive gradually decreased, thereby giving surface ship gun crews less time to hit the target. With regard to diving times during World War I, times ranged from 20 seconds to one minute.7 In 1917, the latter part of the war, U-boat dive times were of the order of 40-45 seconds and by 1918, 30 seconds.

When submarines first started firing torpedoes, ships attempted to use a high-speed zigzagging strategy to avoid being hit. As the speed of submarines improved, ships resorted to other methods. Light steel nets were hung around warships beneath the water line to deflect incoming torpedoes. These nets were ineffective and were soon removed from warships.

Improved skill with the periscope and better submarine construction over time allowed the submarine to be first in becoming aware of its surface enemy and to frequently escape or take aggressive action. A target must be visible for gunfire to be effective. The submarine’s stealth improved. The Royal Navy invoked submarine versus submarine, and eighteen U-boats were torpedoed and destroyed.

Towed explosive sweep was also examined to determine if it would be an effective antisubmarine tool. After testing, the method was found to be inadequate for reasons that included the safety of the pursuing vessel’s equipment handling crew. Another anti submarine weapon-the lance bomb-a hand thrown 7-pound charge contact weapon–proved to be ineffectual even though used in great numbers (20,000) by 1917.

Early antisubmarine efforts by mid September 1914 included the deployment of 250 trawlers and drifters (a boat fitted for drift-net fishing) to support Royal Navy U-boat hunting in spite of the Jack of adequate weaponry if armed. Their role included sweeping mines, antisubmarine patrols and watch for German mine layers. Eventually, auxiliary antisubmarine support vessels included whalers, trawlers, motor launches, motorboats, tugs, yachts, minesweepers, and paddle wheel boats.

British Auxiliary Patrol Vessels

January 1915
January 1916
January 1918

Antisubmarine Weapons

The U-boat losses cited in the tables below are at some variance. However, the overall effectiveness of the antisubmarine arsenal through the World War I years is demonstrated.

U-boat Losses

Against the U-Boat 1914-November 1918

In March 1915, the depth and lance bombs were considered in the experimental stage but by August large orders were placed for depth charges. Slow production created limited availability in 1916. However, damage to the U-boats began with direct or indirect U-boat sinkings from depth charges.

By the end of World War I, the antisubmarine warfare tools included the hydrophone for detection of submerged submarines, the 300 pound TNT-or Amatol-filled depth charge, and mines. These weapons plus the mid-1917 implementation of merchant ship convoying met with success against the U-boats.13 Placing mines at various depths along busy sea-routes also dealt with the U-boats. Estimates as high as 75 U-Boats destroyed have been made. Minefields also blockaded hostile submarine bases. The depth charge, a primitive concept, was eventually adopted by most of the Allies and enemies. “The depth charge was the original dedicated ASW weapon.”14 Airplanes and blimps with depth charges were also added to the enemy submarine hunters.

Antisubmarine weapons and tactics brought to fruition during 1914-1917 included the depth charge that improved the performance of Naval ships and other support vessels and aircraft in their increased and effective escort role for the merchant ship convoys that began in the spring of 1917. The depth charge was prevalent among supporting naval vessels. Requirement for large numbers of depth charges is seen in the number of ships involved in a typical convoy. A convoy of 10-50 merchant ships escort consisted of I cruiser, 6 destroyers, 11-armed trawlers, and 2 torpedo boats each with an aerial balloon. is Depth charges were important weapons for the supporting ships as well as the merchant ships.

Regarding aircraft, dirigible airships and airplanes had important ASW roles. During 1917, airplanes sighted 185 U-boats and attacked 85. Airships located 26 U-boats and attacked I 5. Even the then embryonic underwater detection systems to locate enemy submarines provided a modicum of advantage for the user. As 19 1917 ended, U-boats turned to conducting attacks in the night, whereas during the earlier years daytime attack was the modus operandi.

John Terraine in The U-Boat Wars: 1916-1945 places the depth charge in its historical context” … the weapon that would shortly dominate anti-submarine warfare, and become primary in World War II: the depth charge.” In 1980, J. R. Hill in Antisubmarine Warfare echoes Terraine with “While neither as accurate nor as lethal as expected, the depth charge was stilt the main killing weapon of World War 2.”

Attacking underwater

Underwater bomb explosions to damage enemy shipping received significant attention as a weapon soon after the opening days of World War I. The submarine, as a technological advance, required new counter measures. In the case of the underwater bomb, the above mentioned success of the U-boats in sinking naval vessels as well as merchant shipping created an instant need for some method of countering the German submarines.

The depth charge (underwater bomb) quickly evolved in Great Britain, Germany, France, and Italy at about the same time and the weapon and its implementation gradually improved. The concept was to provide a way to damage the enemy submarine that did not require contact with the target. The depth charge and the compres-sive forces of the water in the vicinity of the underwater explosion could damage at a distance from the target. For the next thirty years, the device continued as one of the important tools for antisubmarine warfare (ASW).

Use of exploding powder charges underwater to damage enemy naval vessels and shipping saw increased activity during most of the 19111 century. In 1843, American revolver-inventor Samuel Colt and others in Europe independently proposed an electrically command-detonating of explosives under water. Colt’ s concept of a submarine battery did not receive government support to proceed. In the early years of the Civil War, Confederate Navy Commander Matthew Fontaine Maury (fonnerly USN and an outstanding nautical scientist of the 19111 century) with others successfully initiated electrical triggering underwater explosives to destroy Union naval and merchant shipping underwater. This technique was successful and has been credited more than any other weapon for damaging or sinking Union naval vessels during the entire Civil War. White-head’s 1860s epic invention, the modern torpedo, became the major undersea weapon by the end of WW II. After successful use of the depth charge during the War the status of the depth charge changed, “The US abandoned depth charges after World War II, preferring torpedos and ahead-thrown proximity contact weapons.”

Addressing a Need

In Great Britain, the original idea of a dropping mine or depth bomb dates to 1911. How to combat the new naval warfare weapons submarines and aircraft slowly emerged. In a 2002 historical paper, “Anglo-American Naval Inventors 1890-1914”, speaking to the invention of the depth charge, the author said “The history of the depth charge is still mysterious.” Admiral Jellicoe referring to the origins of the depth charge regarding the real inventor said “No man in particular, .. .It came into existence almost spontaneously, in response to a pressing need.”

However, several Royal Navy Officers at about the same time during 1914 perceived that a depth bomb type weapon could be used to counter the submarine. Officers cited are Admiral Percy Scott, Admiral Sir Charles Madden, and Captain P.H. Colomb. Further, Royal Navy Commander in Chief, Sir George Callaghan in October 1914 asked for a depth charge.Development started in November.

Early Depth Charges (Surface and Air)

Admiral Percy Scott a long time Royal Navy technical innovator of improved naval gunnery director and range finding systems recognized the impact that submarines and aircraft would have on naval warfare prior to the start of WWI. Late in the fall of I 914, he was appointed head of the Anti-Submarine Department of the Admiralty. Scott designed a bomb that could be dropped by ships on a submarine if it was on or near the surface. He suggested that aircraft could also drop charges. Scott’s advocacy also included the opinion that depth charges could be thrown from ships.

At first, depth charges were triggered using a float and a lanyard of fixed length sometimes, referred to as float and line. These English and German depth charges used the float and line for detonation in the early designs. Failure to explode was frequent.

Initial United States Navy Bureau of Ordinance depth charge designs immediately prior to entry World War I in April 1917 also used the float and line technique. In October 1914, Captain P.H., Colomb and Admiral Sir Charles Madden independently developed a depth bomb actuated by a hydrostatic valve adjusted to explode at a preset depth. It has been noted that Colomb’s and Madden’s schemes were sent to Scott but were somehow delayed.

British Depth Charge Status

Depth charge work started December 1914 with four primitive models including an aerial model. The following December, eight depth charge configurations were issued or under development in England.21 As the depth charge was essentially a proximity weapon and the location of the enemy submarine underwater was imprecise, many depth charges were required for success. Having a sufficient number of depth charges at hand was an unending problem for the remainder of World War I. Early charges used gun cotton as the explosive. Gradually, later models used TNT or Amatol as the explosive of choice, with Amatol used primarily by the British to conserve TNT. Quantities of these explosives varied from 32 to 35 pounds in the early models up to 250 and 300 pound charges later. Typical firing depth settings were in the 40 to 80 foot range. As mentioned previously, the hydrostatic valve for triggering the explosive eventually replaced the mechanical triggering devices of the earlier charges.

A 1915 British designed and developed depth charge designated Type D met with success. However, production only gradually increased. The canister {18 inches in diameter and 28 inches long, similar to a fifty-five gallon oil drum) contained either 120 or 300 pounds of TNT with a dropping rate in water of 6 feet per second. Because of its shape and size, the depth charges took on the name ash cans.

The hydrostatic pistol or detonator was inside a hollow chamber in the middle of the depth charge and fired when the chamber filled with water. The inlet for the seawater to the chamber could be switched with a tool to s ix holes of different size, smaller holes causing firing at greater depths (100 to 600 feet). This was an improvement over the float and lanyard technique but with limitations. It should be noted that later in 1917 soon after the United States joined the Allies, an engineer of the United States Bureau of Ordnance after examining the British design invented and patented an improved detonating mechanism. Further comments regarding this are discussed below.

The Type D became a prototype for other improvements and adap.tations. Twenty-eight years later in 1943 in World War 11, it was still standing the test of time as an important weapon. In the ten months from August 1942 to May 1943, of 150 U-boat sinking, 120 resulted from the depth charge (about 85 percent).

Depth charges typically kept in racks on the stem of the vessel could be released from the bridge and rolled overboard. A weapon in the water behind the ship presented a problem. If the ship’s speed was too slow and the depth charge set for a shallow depth the danger of an early explosion was significant.

The nature of this uncomplicated way of introducing the weapon into the water behind the ship presented a problem. Upon locating an enemy submarine, the usual practice was to drop a series of depth charges at intervals of 10 or 15 seconds depending upon the de-stroyer’s speed.23 The attacking ship’s speed required that it not be in harm’s way when the explosion took place. Fast ships used the Type D with a 300-pound charge; slower vessels D* with a 120-pound charge. Depth charges with adjustable depth setting and greater depth capability allowed either fast or slow vessels to use the 300-pound charges.

Some Royal Navy antisubmarine vessels were equipped with as few as two depth charges in early 1916 and four charges by the end of the year. The allotment for some old destroyers was one depth charge. Depth charges also became included as part of the armament of the Allies merchant ships along with deck guns. Production numbers for the last half of 1917 show a depth charge production of 140/week in July, 500/week in October and 7800/week in December. By early 1918, a destroyer’s allotment increased to 30 to 40. Success with depth charges comes about with a saturation approach therefore large numbers are required.24 One weapon source during the period 1914-18, the Standard Ironworks located in Colchester, England, manufactured 20,000 depth charges.

Depth Charge Throwers

Early experience with the depth charge developed a new requirement. An enemy submarine, by turning perpendicular to the pursuing vessel’s track and increasing speed, might evade and escape. This created a need to be capable of throwing a depth charge along a perpendicular to the centerline of the ship to widen the attack pattern by providing a type of barrage. Coupled with the stem dropped charges the throwers completed a semicircular pattern. During the War, two throwers evolved the British K-gun and the American Y-gun. In parallel with the introduction of the K-gun in July 1917 and its 75yard range, the Howitzer (a direct hit-projecting weapon with a range of 1200 to 2600 yards) became available at that time. The depth charges are a proximity weapon. These weapons introduced late in the war and in small quantity proved effective and were the vanguard of highly effective World War II depth charge throwing or projecting systems (throwing ahead) such as Hedgehog, Squid, and Mousetrap.

British K-gun

The thrower or projector, a type of mortar, was at sea by mid-1917 but, like the basic depth charge, was in short supply. The above mentioned Standard Ironworks manufactured 264 depth charge throwers (projectors) capable of hand) ing 300 and 400 pound loads, between September and the end of November in 1917. In mid-1917, roughly 300 depth charge throwers were delivered to the Royal Navy.

The basic design of the K-gun was the Thomycroft single barrel design fitted with an arbor or stem. A standard British depth charge was secured to the stem’s outer end by lashings or adjustable clips. Throwers were mounted in pairs with one set on each side of the ship, and firing was either manual by percussion or electrically by remote control from the bridge. Initially, the total weight of the depth charge and the firing stem to which it was attached was large and overall awkward to handle restricting the K-gun rate of fire. Later improvements in weapon handling and delivery to the gun overcame this limitation. By 1918, a destroyer equipped with four K-guns and stem racks could drop a pattern of depth charges to bracket the presumed position of a U-boat.

Y-gun

In 1917, the United States Navy Bureau of Ordnance was provided with photographs and designs of the Thomycroft depth-charge thrower. Considerations included the design of a gun that would reduce the impact of recoil on smaller vessels and easier to produce than the K-gun. Suitability for destroyer installation was an additional criterion. The General Ordnance Company of Groton, Connecticut made the calculations needed fo r the working design and undertook the production and test of the first gun. The order was placed with the New London Ship & Engine Company a manufacturer of submarine engines located in Groton.

A gun was devised to simultaneously throw two depth charges, one to port and one to starboard. The Y-gun with its simultaneous delivery of depth charges resolved the recoil problem of the single thrower. The gun shaped like a Y consisted of two barrels at an angle of 45 ° from the vertical capable of throwing two 300-pound depth charges. Ranges of 50, 66, and 80 yards made it possible to have a wide pattern barrage. Interestingly, on destroyers and submarine chasers the Y-gun required a limited commodity-a centerline location competing with deck guns for coveted space.

General Ordnance Company received a contract for the Y -gun 8 December 1917 and because of preliminary work in November, deliveries were made 10 December 1917. As a result of this contract, 947 Y-guns were installed on destroyers and submarine chasers.