21-in D.R. Torpedo Tube (UK)

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21-in D.R. Torpedo Mounting Mark II[1]
First used in 1913's Arethusa class light cruisers. Mark I and Mark III mountings were similar, but would lack the guard rails when fitted on the centreline of a ship such as a destroyer.

The 21-in D.R. Torpedo Tube was the Royal Navy's first dual torpedo mounting, and was employed in destroyers and light cruisers designed shortly before and during the Great War.

Prior to the D.R. Mark I of 1913 which first appeared on 1913's Laforey class destroyers, single mountings had limited the British destroyer to a stingy torpedo outfit of two ready torpedoes in two single revolving above-water tubes. The D.R. design series evolved through the World War I period to at least the Mark IV model.

D.R. Mark I

Firing Mechanism[2]
Training Brake ball on right, Firing Ball on left secured by safety pin "D". The safe arc interlock is illustrated on right side. This is the modified design fitted to the aft mounting in Laurel.

The D.R. Mark I mounting was designed for the "L" class destroyers of the 1912-13 Programme, and it offered them four ready torpedoes rather than the maximum of two found in previous British destroyers.

The mounting was designed to be trained by two men standing on the platforms marked "H" in the first image, each working a large cross-crank to effect the worm-screw engaging the circular training rack. The Mark I mounting had just a single trigger for both tubes. This deficiency was worked around by keeping the safety pin for one tube in place when firing a single torpedo.

The two tubes, being offset to either side of the axis of training, required the mounting to be fixed in train before firing to avoid stress and yaw errors upon firing. Despite this, it was also desireable for destroyers to be able to fire their torpedoes at any moment. The solution to these offsetting needs was accomplished in the Mark I by a cleverly designed firing system: the hand trigger would cause two braking balls (actually weighted cylinders, it appears) to drop; their fall would lock the mount in train and precipitate the release of firing balls whose fall would fire the unsafed tube. Safety gear prevented the firing balls from falling when the tube was on an unsafe bearing, the safe angles being indicated by a cutaway in a ring at the base of the central spindle. While this solved the larger problem, it resulted in slow firing action which aroused complaint. As a proper fix for the slow firing was sought, Laurel's aft mount had a trial modification applied which caused the braking and firing balls to fall simultaneously, as illustrated in Plate 16 above. This variant also gave two triggers, either of which dropped both braking balls but only one firing ball.

The earliest-built Mark I tubes had their firing cartridge pocket on their back door, and this was found to make them prone to damage while training the tubes. Later articles had these placed on the side of the tubes at the back, a change that also produced a better ejection pressure of 49 rather than 29 psi on average using the standard 10oz service cartridge, though no one could account for the difference in performance. The central spindle also contained concentric steam pipes to feed warming coils to keep the mount ready in freezing weather. The weight of the trainable tubes was borne by eight rollers on the roller path.[3]

In 1914 firing trials in Liberty, it was found that the torpedoes from the aft torpedo mount struck the ship's deck when fired on extreme bearings. While investigations were undertaken, the expedient precaution was taken of restricting fire to 10 degrees to either side of the beam. By fitting plugs of some kind, it was eventually determined that the full 50 degree arc could be used and the torpedo would always clear the gunwale by 19 inches.[4]

The new double tubes were found satisfactory in the end, though were slow to train and the slow action of the firing lever would make firing when under helm worrisome. Laurel's tubes were found to require four men working the training wheel and four more pushing the ends of the tubes around. Lysander reported better, if still abysmal results: two men could barely train the tubes when the tubes were loaded.[5] Testing revealed that the design was sensitive to alignment during erection, and alterations improved things to a point that a training speed of 8° per second was achievable with two men. The firing timing in Laurel was measured as:[6]

Mounting Timing in Seconds (after trigger press)
Striker fell Torpedo Moves Air Lever hit
by Tripper
Gyro released
stock Mark I mounting 0.497 0.62 0.747 1.077
modified Mark I mounting 0.283 0.35 0.431 0.761
earlier, single tubes N/A 0.144 0.474

D.R. Mark II

The D.R. Mark II mounting was designed for the Arethusa class light cruisers of 1913.

The Mark II Mounting, unlike the Mark I, bore the weight of the rotating tubes on the centre pivot, and not on the circular racer, as the sturdier decks of a light cruiser could better take the strain at a single point. It also used a locking bolt in place of the Mark I's training brake, effectively sacrificing the ability to fire at any given moment: the tubes had to be locked in train before firing, and this necessitated some teamwork and delay. Two triggers were fitted in this model, one for each tube, and a spring mechanism cocked by a a lever was used for firing rather than the weighted balls on the destroyer mounts. A training index was fitted to the centre pivot under the director stand where is was easily viewable by the man in the seat.[7]

The weighted balls of the Mark I design were replaced here by springs, which it was hoped would be quicker in function. However, these were withdrawn some time before or during 1917.[8]

In 1917, it was discovered that Mark II and III mountings could be trained much more rapidly if the radius of the handle was reduced by 5.5 inches.[9]

D.R. Mark II*

These were made for what was described in 1917 as the "repeat 'C'" class cruisers, whichever seem here to mean the '"Capetown class. This design replaced the 4.5 foot lip of the Mark II tube with one seven feet in length in order to reduce the angle at which the torpedoes entered the water. Additionally, the 15 ounce firing cartridge of the Mark II was replaced by the new 17 ounce cartridge and the impulse pocket was moved forward to be two feet ahead of the back door, in order that baffles to protect the torpedo's propeller from the blast could be omitted.[10]

D.R. Mark III

The D.R. Mark III mounting was designed for the "M" class destroyers. The mount's design was still being finalised in 1913, when it was hoped that it might retain the training brake of the Mark I mountings rather than the locking system of the Mark II mounts to preserve the ability to fire the torpedoes at any opportune moment. It was also hoped that the Mark II's quicker spring-actuated firing system could also be employed.[11]

Mark IV

The D.R. Mark IV tubes were developed in parallel with the triple 21-in T.R. Mark I torpedo tube and was nearly identical except it lacked the third tube sitting atop the first pair. While spring-actuated firing had been generally unsuccessful, the old ball drop gear caused issues in early testing, and in 1917 it was hoped that Electro-Pneumatic Firing Gear would eventually be used when available.[12]

The Mark IV featured the same innovations as the T.R. tubes. They employed slack fit construction to simplify repair and adjustment while reducing weight and had special access doors to expose principal adjustment controls for parameters such as gyro angle, running speed, etc and holes for air service and other supplies, single ball firing gear with local hand-releasing gear, and more powerful impulse charge of 17 ounces (rather than 10 ounces as in previous tubes), providing a pressure of 65 psi and torpedo speed of 32 feet per second on firing.[13]

The torpedo layer sat on the right-hand tube, with a director stand and instrument panel in front of him and handles for local firing below. The tubes could be locked to either 90° Green or Red, as well as (presumably) right ahead or aft. The torpedo within the tube was supported on two side-bearing bars, and entered the water at an angle of about two degrees. The outer diameter of the tube was 23 inches. The large hinged access doors were sealed by india rubber pads on the inside and were secured by four large locking levers.[14]

Length of cut-away part of lip 7' 9""
Length of circular part of lip 3' 9""
Lip length overall 11' 6"
Length of rear portion 11' 6"
Total length 23'
Radius, lip to pivot 12' 11.5"
Radius, rear end to pivot 11'
Diameter of tube 23"
Distance, face of door to pivot 10' 3"
Weight, complete with racer and pivot 10,080 pounds

These tubes proved satisfactory in testing, though the torpedoes left at lower velocity than anticipated (varying between 31.2 and 34.5 fps with two models of torpedo under different conditions) due to friction which was attended to by alterations. The new access doors proved sufficiently tight to not compromise operation in firing.[15]

Common Maturation and Improvements

In 1917, it was discovered that D.R. Mark II and III mountings could be trained much more rapidly if the radius of the training handles was reduced by 5.5 inches. In Noble, this alteration reduced the time required to train on the beam from fore-and-aft from 27 to 18.5 seconds. In Marvel, these figures were 25 and 17 seconds.[16]

In 1917, it was ordered that all torpedo tubes should be modified during their next refit to have access doors similar to those which appeared in the T.R. torpedo mounts, so that settings for speed, gyro angle and perhaps depth could be altered without "launching back" the torpedo through the loading hatch to expose the requisite hardware.[17]

In 1918, extensive experiments were conducted in pursuit of means to increase torpedo discharge velocities from existing tubes, as this would reduce angles of entry as the torpedo hit the water. Methods explored were cascade firing of multiple impulse charges, use of slow-burning powders to better maintain pressure while firing, and use of cordite charges in an explosion vessel. Despite initial doubts, a variation of the cascade firing method was ordered implemented in A.W. tubes for Arethusa, Caroline, Cambrian, Caledon, Ceres and Capetown classes. Tubes that had their impulse pockets situated well forward of the rear door, such as the D.R. Mark II*, or those with slack-fit construction such as the Mark IV were not amenable to this alteration, however.[18]

See Also

  1. Annual Report of the Torpedo School, 1913. Plate15.
  2. Annual Report of the Torpedo School, 1913. Plate16.
  3. Annual Report of the Torpedo School, 1913. pp. 36, 37, 38.
  4. Annual Report of the Torpedo School, 1914. pp. 39-40. (T.O. No. 61 of July 1914. G. 16299/14).
  5. March. British Destroyers. pp. 138-9.
  6. Annual Report of the Torpedo School, 1913. p. 37. This source described the D.R. mountings as being based on the Mark II, which I feel is a likely error.
  7. Annual Report of the Torpedo School, 1913. p. 37.
  8. Annual Report of the Torpedo School, 1917. p. 89.
  9. Annual Report of the Torpedo School, 1917. p. 89.
  10. Annual Report of the Torpedo School, 1918. p. 84.
  11. Annual Report of the Torpedo School, 1913. p. 37.
  12. Annual Report of the Torpedo School, 1917. p. 76.
  13. Annual Report of the Torpedo School, 1917. p. 76, 77.
  14. Annual Report of the Torpedo School, 1917. pp. 76, 77.
  15. Annual Report of the Torpedo School, 1917. p. 78.
  16. Annual Report of the Torpedo School, 1917. p. 89.
  17. Annual Report of the Torpedo School, 1917. p. 89.
  18. Annual Report of the Torpedo School, 1918. pp. 81, 82.