Hedgehog

The US used RADAR on everything from PT boats up. The sheer number of signals would have made triangulating on any one rather problematic and even following down a beam a rather unrewarding enterprise. The Germans also seem to have been aware of or at least more worried about the problem than other countries although they still under estimated the British capacity in this regard.

 

A few notes on things in the thread:

Sink rate was critical to any WW 2 ASW weapon. The faster it got to it's depth setting the less time the submarine under attack had to move. Depth charges were first weighted to make them sink faster then streamlined to make them sink even faster. Hedgehog sank at about 24 feet a second and the design was not just made to make it follow a stable flight path, but to sink head first in a stable pattern.

Mousetrap was a variant of Hedgehog that used a rocket motor on the same warhead. It was designed to be used on smaller ships that couldn't take the weight of a Hedgehog installation or the recoil forces of it. It's limitation was the firing ramps were simple trays and not stabilized like Hedgehog was. This made it hard to ensure it fired when the ship was level in pitch and roll. These effected where the rounds would land making it inaccurate. It also used just 4 bombs on each rack not 24 like Hedgehog.

Post war the US made a trainable Hedgehog mount that could be fired ahead or to the side of the ship.

With Hedgehog the bombs landed in the water in a circular pattern that was a bit smaller than a U-boat making it very likely that one of the bombs would catch it and crack the hull open. It had the additional advantage of only going off on contact with the sub's hull. Thus, if one or more bombs in a pattern detonated, the firing ship knew the sub was hit, unlike depth charges.

The RN really didn't like Hedgehog at the design / research level. First there was an objection that unless it scored a hit it had no real value against a sub. Depth charges went off regardless and would have a morale effect on a sub's crew even if they didn't mortally wound the sub. They could also do secondary damage.
There was also a "not invented here" argument going on as the Hedgehog didn't get designed and approved through official RN channels.
This is why post war the RN did away with Hedgehog post war in favor of Limbo / Squid.

 

nicely put and great reading...thanks....wasn't there a greater hit chance with Hedgehog vs Depth Charge?.....

 

http://www.pwencycl.kgbudge.com/D/e/Depth_Charge.htm

The hedgehog had a 37 meter circle pattern. So the depth charge has a greater damage Radius the hedgehog the greater kill radius.

Killing depth charge 9m/ hedgehog 37m.
Damaging depth charge 90m/ hedgehog 37m.

However what was also very important is that the hedgehog gave ships the ability to fire forward of the ship. Greatly reducing the time the sub had to escape the killing radius. So yes there was a greater chance for a killing hit with hedgehogs.

 

The advantage of Hedgehog was in firing ahead the ship didn't lose sonar contact with the target. A second advantage was that if the pattern missed, you knew it for sure and there was no loss of sonar due to noise form the attack.

Depth charges had the advantage of laying a much larger pattern than Hedgehog did. Most Allied ASW ships could lay a 13 + charge size pattern of depth charges. Fleet escorts and small escort ships typically laid 7 to 12 charge patterns. For an ASW ship this meant they had 3 or 4 K guns aboard along with two stern racks. The racks typically held 5 to 7 charges but some ships had ones that were extended to hold 10+ so they could launch more than one attack without reloading.
The RN used hydraulic operated K guns with about half the range of US black powder fired ones. The RN models were adopted because they retained the cradle whereas the US one fired the cradle with the charge. Britain having a steel shortage couldn't waste the steel on expendable cradles.
This means RN depth charge patterns aren't as wide as US ones and that could be a problem in certain approaches on a U-boat.

The way a pattern was fired was to drop a charge off the stern, then fire one set of K guns, drop a charge off the alternate stern rack, repeat. Charges would be alternately set + or - about 50 to 100 feet in depth to form a three dimensional box of explosions to bracket the submarine. The US would vary that some rolling two charges off the stern simultaneously between each pair of K guns firing. This was because their pattern was wider.

Even if the pattern didn't actually take the sub out the explosions could be terrifying for the crew of one. It should be noted that the Germans specifically made their U-boats smaller in part to allow them more maneuverability when submerged to avoid depth charge attacks.

 

Peter Elliott, Allied Escort Ships of World War II, provides some interesting statistics which reflect both the effectiveness of the weapons and the progress of tactics and the proficiency of crews.

Hedgehog
2nd half 1943 - 7 1/2%
1st half 1944 - 15%
2nd half 1944 - 28%

Squid 1944
Castle class (single Squid) - 18%
Loch class (double Squid) - 33%
1945 both classes - 60%

Depth charges "last few months of the war" - 5%

Presumably by the late war months of the war, tactics for depth charges had been fairly well worked out, but it's also true that the top-of-the-line escorts were mainly using Hedgehog and Squid.

Squid equipment included the new Type 147 asdic which for the first time could give an accurate reading of depth. In the twin Squid installations, the two three-charge patterns were 60' apart in depth.

 

Apologies for not picking this up earlier and answering it

No, the KM-converted barges would have been far LESS vulnerable; not only were they less penetrable from below the waterline because of their concrete-filled bilges....part of the conversion was sinking railway rails longitudinally in the concrete as it was poured To provide a long, rigid "bed" for heavy MT and tanks etc. So the Sea Lion barges would have been stronger laterally AND longitudinally than plain unconverted barges....little more than iron-plate boxes....that the RN would have been testing with. And with the extra lateral bracing, they'd have been more resistant to hull plates springing. Might have been a nasty suprise on the day, that...