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What is the minimum safe speed when dropping depth charges - especially if they are set for shallow depth ?

I wonder if a slow merchant man really can use depth charges in a safe manner i.e. without risking a fair damage to itself.

The wikipedia article abouth depth charges says (for WW I): deeper than 30m and with speeds larger than 10kn.

With regards to SS Medoc : is the effect of the triple expansion engine (only 155 hp) really plausible ? I ask because the machine i happen to have to take care of has 250 hp and that it is a much smaller ship and we with that power only does 11 knots or so.

wrecksite.eu on the Medoc gives a number of 9 knots for SS Medoc so i think droping deep charges behind you which detonates to early is more dangerous for the ship than the sub...

  • Have you read the Wikipedia article on Depth Charges? The section on Delivery mechanisms may be of interest. Further, the merchant vessel Medoc was certainly fitted with depth charges in 1940. – sempaiscuba Jan 18 at 13:02
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Question:
What is the minimum safe speed when dropping depth charges?

Short Answer:
The Depth charge in WWI was the same design used to begin WWII, modifications made during the war to make them more effective would not have significantly changed the answer to your question. For Depth's as shallow as 40 feet, your answer is: Six Knots well below the typical speed of a WWII merchant ship of 11-17 knots.


Detailed Answer:
In WWII depth charges had a 300lb warhead with a delayed fuze and they were "shot" off ships by hydraulic bellows, which would shoot them into the air form the side and aft. The charges would then deploy a parachute before hitting the water. The parachute and bellows however only delayed the water impact by a few seconds. Not particularly meaningful for a 300 lb warhead detonating when you are on a ship traveling at 6 knots.

The United States began WWII with the same design of depth charges used in WWI. During WWII the big innovations were changing the shapes of the DC to get it to sink faster, and extending the depth for it's use to up to 1000 feet.

Depth Charges
At the start of WWII, depth charges were essentially the same weapon as from the end of WWI. Development concentrated on increasing the depth at which a submarine might be successfully attacked and improvements to the sinking speed of the depth charges.

During WWI testing was done to determine the safe speed to lay such depth charges and it was determined for shallow depths, (set to 40 ft) the minimum safe speed for the surface ship to use them was 6 knots.

Depth Charges: Mines, Depth Charges and Underwater Weapons, 1914-1945 page 52-53
( Trials carried out December 13, 1915)
Next day the commandeer of the HMS Vernon, together with Alban Gwynne, Commander Holbrook and Lieutenant Commander Sherman, carried out four runs on HMS Wizard to determin the fireing capabilitis of the D-type depth charge. Two runs were made at various speeds between 6 and 20 knots, and two more at steady speed at 20 and 10 knots, the depth was set for 40ft, whereas on the other runs it was set at 80 ft. The two charges dropped at 40ft, wheras on the other runs it was set at 80ft. The two charges dropped at 40ft both had parachutes attached. In all cases the charges went off 11-15 seconds after they had been dropped. The observes commented: The Shock appeared to be more severe when the setting was set for 80'. This setting was therefore taken for the slowest speed. It was concluded that the depth charge was completely acceptable and that the minimum speed for laying was 6 knots.

Merchant men in WWII could do about 11-17 knots knots.

And yes Merchant men did use Depth Charges.

Anti Submarine Warfare
The first recorded sinking of a submarine by depth charge was U-68, sunk by Q-ship (an armed merchant ship) HMS Farnborough off Kerry, Ireland 22 March 1916.

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In practice, however, the concept of a retarding parachute went away very early in the employment of either a single launcher or a Y launcher . . . see

https://maritime.org/doc/destroyer/depthprojector1/index.htm

And

https://maritime.org/doc/depthcharge6/index.htm

and was not used, at least by the USN, in WWII. The ballistics of a launcher were pretty straight forward and could by slightly modified by selection of charge size used in the launcher.

Further, a depth charge fired from a launcher presents little hazard to the ship as the charge is “thrown,” as they say, off to the side of the vessel to create a larger danger area, or pattern, for the submarine as is shown in the first reference above. This pattern is to take into account possible attempts to maneuver by the submarine. But if you are using, solely, a launcher, be it one or two barrels, the speed of the vessel is relatively immaterial in terms of potential damage from your own charges. Where speed becomes important using launchers is if you have more than one, the speed of the ship, too fast, can lessen the density of your pattern relative to your rate of fire.

A charge rolled off the stern could certainly present a problem if one’s speed is insufficient to clear the blast area. That is a calculation one has to make . . . for what depth is the charge set . . . on average, how long for the depth charge to reach that depth . . . how fast should the ship be going so as to be, oh, say, 3 times the blast radius, or about a minimum of 75 yards from the point of the explosion. So, if the depth charge drops off the stern and takes 10 seconds to reach its discharge depth - not very deep if the sink rate was about 8 feet per second as was typical for the US Mk II type - it might be a good idea to be pushing about 13 knots, 14 would be better. Not all that hard to figure out once you know the depth charge sink rate, the effective blast radius and how much of a risk you're willing to take. Your results may vary.

  • The Medoc according to a source (wrecksite.eu/wreck.aspx?9718) didn't go faster than 9 knots. Which makes me wonder – Stefan Skoglund Jan 21 at 12:04
  • A Q-ship, did all of them used a launcher/thrower or a ramp simply in the stern ? – Stefan Skoglund Jan 21 at 12:05
  • The parachute was probably a big hazzle/danger. Could they easily fail and so throwing of the calculations? – Stefan Skoglund Jan 21 at 12:06

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