Mr. Merrill is a retired engineer from the New London Division of the Naval Undersea Warfare Center. Old timers remember that lab as USN/USL. John is a frequent contributor to THE SUBMARINE REVIEW. Part I of this essay appeared in the July issue (p.100).
Depth Charge Effectiveness
With the depth charge, the intention is to use the incompressibility of water to set off an explosion at depth in the vicinity of the enemy submarine and to create a substantial force to damage or destroy the submarine. A significant consideration is that during World War I once the enemy submarine submerged it was lost to the pursuer as underwater detection using sound was still in an embryo stage of development. Even as World War I ended, underwater detection of a U-boat was a low probability.
Dropping the charge where the enemy submarine was thought to be was certainly a step in the right direction for antisubmarine warfare. However the ability to achieve the goal of destroying the U-boat depended upon a number of variables. The amount of explosive in the depth charge, the depth setting for the explosive and the actual proximity of the submarine target to the event were significant factors. Success always required multiple depth charges and prior to 1918, depth charges were a scarce weapon. Nonetheless, even an exploding depth charge, even without damage to the submarine, could be sufficient to rid the area of an enemy submarine.
A variety of distances have been given regarding the separation required for the depth charge explosion to do damage to the target submarine. Admiral Sir John Jellicoe in 1920 recalled a 300 pound depth charge within 14 feet of a submarine hull created serious damage or sinking, and at 28 feet the submarine was disabled sufficiently to force the submarine to surface and be exposed to other weapons.1 Precise distance requirements are difficult to define as the variables are not easily assessed. However, it is interesting that a distance for serious damage to a submarine of 25 feet was identified regarding World War I while a World War II distance of within 23 feet has been cited. Confirming these numbers, a 1993 comment regarding depth charge effectiveness in World War I stated “An underwater explosion twenty-five feet from a U-boat could destroy it and one as near as fifty feet could seriously damage it.”2 Even with an explosion not sinking the submarine, shock waves from the depth charge impacted the submarine’s hull and instrumentation requiring some submarines to immediately surface. On the surface ramming or gunfire could be effective. Admiral Jellicoe referring to the impact of the depth charge noted ” … at distances up to sixty feet the moral effect on the crew would be considerable and might force the submarine to surface. ”
Lieutenant Hersing of U-21 told of a German submarine commander’s depth-charge remembrance, ” … when depth-charged after firing two torpedoes at a convoy off the south-west coast of Ireland. He was forty meters under water, and every ten seconds charges detonated at depths of ten, twenty-five, and fifty meters in all directions … for five hours the Germans in their steel hull could hear the explosions … all round them, and the hollow roaring sound of the destroyers’ propellers overhead.'”‘ The moral and psychological impact on the crew could be significant.
Long-term Depth Charge Problem
Success with the depth charge hinges on the length of time between awareness of the enemy submarine and the arrival of the weapon in the proximity of the target, as the depth charge is a
proximity weapon, not contact. This time is sometimes referred to as blind time. With the early depth charges and their sink rate of the order of 6 feet per second a target at 150 feet requires 26 seconds after launch for the depth charge to be at the point of explosion. With submarines having underwater speeds of the order of l 0 knots, one minute provides about 100 feet of travel. This factor plus other response times by the pursuing vessel did not make for success. Submarine operating depths and speed on the surface and below increased throughout the 201h century. Deeper submarine operations also lessened the depth charge’s effectiveness. Increased sea pressure reduces explosive force. The time required for a surfaced submarine to submerge decreased. Early underwater sound detection devices lost contact with the enemy submarine when close and required increased speed by the targeting vessel to minimize blind time. Some early detection systems required the ASW vessel to be dead in the water. Charges could be in the water after the contact was lost. Mired in these changes, depth charge design and tactics demanded serious attention.
Depth Charge at Sea
Early use of the depth charge did not always insure success as in the case of an action in July 1916 when the patrol craft HMS SALMON attacked the UC-7 with depth charges and the UC-7 escaped. It became clear that large numbers of depth charges were required to raise the probability of damage to a U-boat to better than luck. During World War I, both sides were limited in their antisubmarine efforts by the lack of depth charges in adequate numbers.
The 1915 successful intrusion into the Sea of Marmora via the Dardenelles by a number of British submarines was not marred by the use of depth charges. It is interesting that the Turkish Navy then under the guidance of Germany did~ not use depth charges that were introduced by Germany early in 1915.
However, by 1916 the depth charge was in broad use by Germany, Great Britain with France and Italy introducing the weapon at about the same time. The British submarines operating in the Baltic Sea in the fall of 1917 had to think carefully about Gennan depth charges when challenging Gennan convoys. Throughout World War I, Germany used the float and lanyard triggering type depth charge designated C 15. Failure to explode was about 50% of the time. With a 110-pound charge, a thirty-five-foot destructive radius was expected.”
The first depth charge sinking occurred March 22, 1916. The U-68 attacked HMS FARNBOROUGH a “Q-ship” off the southwest coast of Ireland.7 The submarine’s torpedo missed the surface ship which retaliated with deck gunfire and depth charges sinking the submarine with all hands.
The German submarine UB-26 was sunk near Le Havre from a depth charge fired from the French destroyer TROMBE on April 5, 1916. The same month unsuccessful depth charge attacks on two U-boats operating in the British Isles alerted Germany to the introduction of the new weapon.
Two U-boat losses by depth charge occurred later on December 4, 1916, UC-19 in the Dover Straits and on December 6, UB-29 in the English Channel, by HMS destroyers LLEWELLYN and ARIEL. On December 13, 1916 two depth charges from HMS LANDRAIL operating in the Straits of Dover sank the UB-29.
On 8 February 1917, destroyer HMS THRASHER operating off Flamborough Head, at 53.56 N 00.05 E, observed the minelayer UC-39 sinking a ship. As the submarine dived, a depth charge from the destroyer burst in the UC-39’s conning tower, flooding it and the control room. Forced to the surface, the submarine was sunk by the destroyer’s gunfire.8 In 1918, seventeen of the U-boat depth charge sinkings occurred around the British Coast.
According to Messimer,9 in October 1916, the Austrian U-16 sank the Allies’ Italian destroyer NEMBO. As the depth of the sinking destroyer increased, its depth charges exploded and sank the U-16. Both the Italians and the Japanese operating in the Mediterranean in the later years of war made effective use of depth charges in defeating U-boats.
Although in short supply by 1915, Allied ships began using depth charges. These waterproof bombs exploded at a chosen depth. At first, these were not very effective and between 1915 and the end of 1917, depth charges accounted for only nine U-boats. By 1918, they were improved. With more depth charges available, twenty-two U-boats were destroyed. Improvement included a hydrostatic trigger with a dial for depth providing settings between 50 and 200 feet.
WWI Monthly Depth Charge Use
Orders for improved depth charges were 10,000 in July 1917, with 20,000 ordered January 1918.11 It has been estimated that as many as l,745 per month were expended during the later part of 1918.12 The total number of depth charges expended during WWI has been estimated at 16,500. Significantly higher numbers have been reported.
United States and the Depth Charge WW I
Frequently, details about submarines and associated systems are under the heading of secret. Depth charges were no exception. Countries using depth charges placed the construction and methods of exploding them in the secret realm. It follows that United States, a neutral nation, was not fully aware of depth charge developments and progress until the declaration of war in April 1917. Prior to that time, some initiatives were taken.
Before United States entered the War and recognizing the need for the new weapon, the United States Bureau of Ordnance (February 1917) selected a depth charge design with 50 pounds of explosive that used the float and line trigger mechanism and a depth capability of 25 to l 00 feet. Designated as MK I, an order for I 0,000 was placed and they were available upon entry into the War in April 1917 at a time when the U-boats chose unrestricted warfare. With the MK l, a speed of 5 knots or greater was specified for the depth charging vessel.
The limitations of the float and line trigger mechanism brought attention to the British hydrostatic technique that replaced that method. The United States Navy was not comfortable with the British designed depth charge hydrostatic trigger. Their method was found to detonate prematurely in the water and the exposed external firing device that protruded several inches beyond the head of the cylindrical depth charge container could fire while handling. Detonation during transportation was another consideration.
Critical of the safety and effectiveness of the British hydrostatic depth charge trigger, a careful examination of the British depth charge was undertaken. With safety and reliability a priority, the Bureau of Ordnance tested different ways to detonate. “Various means of effecting this explosion were tested, including slow-burning time trains, buoys paying out wire, and hydrostatic pressure devices.”13 This effort led to a new development.
Chester T. Minkler
One of the investigators working at the Naval Torpedo Station in Newport, Rhode Island, developed a new device to detonate the charges. The investigator was Chester T. Minkler, a young and experienced Bureau of Ordnance engineer of mines and explosives at the Naval Torpedo Station. He devised a new hydrostatic trigger that corrected the shortcomings. The new device also allowed greater depth settings and included an external control for setting the desired depth for explosion. 14 Minkler received his patent in August 1917 and turned it over to the United States Government. It should be noted that in October l 929 the British unsuccessfully challenged Minkler’s patent rights.
When the United States entered the war, an exchange of information with the British made it clear that 50 pounds of explosive was not effective. The 300 pound charge being used by the British was adopted. New and stronger submarines mandated a larger charge. In 1940, the U. S. Navy ran depth charge tests against an operational submarine (for most of the test, moored underwater without crew), and determined that 300 pounds of TNT was not very effective; the explosive charge was doubled to 600 pounds.
The American version of the Newport designed depth charge with the newly patented detonator was designated Mark Il. An initial contract was placed in July 1917 for the manufacture of 10,000 with first deliveries in the fall of that year. The British government adopted the MKII in 1918 and placed a request to the Bureau of Ordnance to contract 15,000 depth charges. •s With some modifications, an additional U.S. Navy order for 20,000 Mark II depth charges was placed in the spring of 1918. The United States during World War I let contracts for a total of72,000 depth charges. With the end of the war, unfulfilled contracts were closed where feasible.
A submarine chaser dropping MKl charges with 50 pounds of explosive specified at least a 25-foot depth setting and ship’s speed of 7 knots. The MKII with the 300 pounds of explosive making a total weight of 420 pounds and a dropping rate of 6 feet per second had a 50-foot depth limitation for detonation and a required speed of 15 knots. A 200-foot maximum depth was another parameter.
An order for 20,000 MKIII with a 300-foot depth setting capability was placed in July 1917. At about the same time,1000 MK IV with a 600 pound charge and a weight of 745 pounds were ordered and available overseas in September 1918.
New Convoying Initiatives
By May 8, 1917 (about a month after United States entered World War I), the first six of 36 US destroyers arrived and were ported at Queenstown in southern Ireland for duty. At the same time as the arrival of the destroyers, the Allies began a significant push to convoy merchant ships with naval escorts as a means of countering the U-boats. Successful convoying required a multitude of escort ships, and the destroyers were available to escort convoys and to aid merchant ships shelled or damaged by U-boats.
The first Gennan submarine sunk by the U.S. Navy in World War I was the U-58. Commissioned in 1916, U-58 was 219.8 feet long with a submerged speed of 8 knots and 14 on the surface and a maximum operating depth of 164 feet. It was the first U-boat kill of the war by American destroyers. On November 17, 191 7, as the USS FANNING (DD 3 7) patrolled in the eastern Atlantic in the company of other destroyers, Fanning’s lookouts sighted a periscope.
FANNING attacked and the first depth charge pattern scored a hit. NICHOLSON (DD 52 accompanying the FANNING made a depth charge pass. The U-58 broke the surface. It has been inferred that the explosions jammed the submarine’s diving gear and the U-boat plunged towards the bottom and that at about 300 feet, the submarine blew ballast and shot toward the surface. When the U-boat broke the surface the destroyers shelled. The submarine crew came out on deck with hands raised in surrender. FANNING maneuvered to pick up survivors as the submarine sunk. Forty survivors were taken prisoner. Two different locations are mentioned regarding the location of the engagement. One site is near the Hebrides, the other some distance away from the Hebrides off Milford Haven, Wales at 5132N 0521W.17 in the Bristol Channel. This was the first of two U-boats sunk by US Navy destroyers in World War I.
World War I Ends
By mid-1918 and during the closing months of the War, improving success of merchant ship convoying and the enhanced performance of depth charges on the destroyers with stem racks, K-guns, and Y-guns, the life expectancy of a U-boat was six combat patrols. Further, U-boat attacks were beginning to be limited to nighttime.
October 21, 19 l 8 three weeks before the Armistice, the British ex-cargo vessel PRIVET operating as a “Q” ship encountered the U-34 in the Straits of Gibraltar, attempting to leave the Mediterranean. PRIVET’s depth charges and gunfire sinking the submarine made the U-34 the last U-boat casualty of the War.
The U-34 was observed leaving a trail of light in the water as it was exiting the Mediterranean. PRIVET tracked down and destroyed the submarine. Later it was suggested that a possible source of the aforementioned light was the bioluminescent glow resulting from the disturbance of the plankton by the motion of the submarine.
Even closer to the Armistice on November I 0, 1918, (the eve of the Armistice), the minelayer HMS ASCOT was torpedoed on the northeast coast of England. The central role of the U-boats during the entire five years of World War I persisted until the end.
A mid-1960s appraisal of the depth charge as the War closed is appropriate. “The weapon with the greatest future was the depth charge independent of geography wherever and whenever U-boats made attack on shipping. ”
World War II Comment
“At the start of the Second World War the stern-released depth charge was the only viable AIS weapon.”
Entering World War n, the available depth charge capability heavily reflected the status at the end of World War I. Five years of the new World War saw significant changes in the depth charges and their tactical use. Early depth charges were still primarily rolled over the stem of antisubmarine craft or flung out to the side of the pursuing craft using the K-gun or the Y-gun.
Features of the wartime depth charge developments included the ability to fire ahead of the vessel pursuing the submarine and deliver a wide semicircular pattern of charges. This capability, associated with much-improved underwater detection reduced the blind time between enemy submarine detection and weapon delivery. In some instances, wartime systems were implemented that coordinated depth charge firing with the sonar system’s enemy submarine detection. During the entire World War U, the generic depth charge, (“ashcan”), underwent improvement and refinement. In a timely fashion, United States and Great Britain through research and speedy development produced various new antisubmarine warfare weapon systems of the firing ahead type augmenting the basic depth charge. At the same time, improving sonar systems enhanced the effectiveness of depth charges systems with their ability to locate and track the enemy submarines. The significant changes came in the firing ahead capability.
Reviewing U-boat losses for the period August 1942 to May 1943 cited by Tarrant, demonstrates the extensive use and effectiveness of the depth charge.
During that IO-month period, 150 U-boats were sunk with 127 or about 85% of the sinking a result of depth charging