2009 January 23 Update: Manuals found at hnsa.org for the Mark 6 depth charge and the Type C release track. I'll update the drawings as soon as I can.
2008 November 10 Update: Color images from the forum at ptboats.org show the mechanisms on the ends to be a bronze color.
I am still looking for details on the release track, and release latch mechanism.
The most common depth charge on PT boats was the canister type Mark 6 with 300lbs of cast explosive on a type C release track. The letter designation of the track was unusual for ordnance. Most designations were by Mark. There were four lettered type racks, A and C held one depth charge while B and D held two. All were cable release types usually mounted on smaller vessels. On the PT boat the racks were usually mounted in place of the rear torpedoes or in front of the forward torpedoes. A section of the bow railing was removed when mounted forward. Configurations varied widely, for example the 109 had 2 racks mounted (one port and one starboard, field mounted after production) in front of the forward torpedoes while an image of the 107 shows eight in place of the rear torpedoes. The Elco blueprints I have do not show any depth charge or racks mounted.
Depth charge construction was a drum of explosive cast around a tube that housed the mechanics. The ends of the drum were tapered in so that the firing mechanisms wouldn't stick out past the drum ends. Mounted on one end of the drum was the booster, a safety device that kept the charge from exploding until a minimum depth of water was reached, about 11 to 25 feet. The detonating mechanism, the pistol, was mounted on the other end of the drum. The pistol also had a safety mechanism, a "Safe" depth setting that prevented firing. Both operated on hydrostatic pressure against springs. While on deck a safety fork was locked on to the boosters protruding spindle to prevent its travel inward which detonation required. The fork also protected the boosters water inlet by holding the spindle out slightly which closed it off. Two styles of cap protected the pistols water inlet until launch, a plain one for storage and one with a knob like the safety fork had for attaching a device to pull it off when the depth charge was launched.
The rack's construction was simple. Sideplates were joined by the backplate and the tracks of the adjusting mechanism. The adjusting mechanism track was two U shaped channels that the axles of the adjusting pulleys rode in. Two adjusting knobs at the ends of the pully blocks applied tension to the retaining cable. A release latch on the backplate held a ring that was looped through the eyes on the ends of the cable. Two brackets on the upper rear of the backplate held chains to pull the safety fork and pistol cap off although I think these were usually removed manually before launch.
At the depth charge's minimum depth setting of 30 feet, the boats had to travel top speed to avoid damage from the blast. They also had to be dropped at least 50 feet apart to avoid setting each other off. The depth charges were only mounted when on patrol. No sense in having a 300lb target of high explosive on deck if not needed. Depending on the area of operation and main targets, many images of PT boats in action show that the racks were commonly removed. Mainly used against submarines, depth charges had to explode fairly close to the sub to be effective. This called for great expertise since, unlike ships and airplanes, the target was unseen (except for slight traces if you were lucky) and the depth vector also had to be considered. It was almost like firing the 20mm at a Zero at night during a lightning storm with bullets that only worked if you had set the right distance to the target. Here is the Navy's estimate of distance from the sub's hull / damage for this type of depth charge:
150-90 feet / Negligible
The tracks shown in the references I have vary in small construction differences. :
90-50 feet / Perhaps some
50-30 feet / Moderate
30-10 feet / Probably fatal
Some had rivets holding the drop rails to the side plates while others were (welded?) smooth.
On some the drop rails exited straight while others angled down a bit towards the water.
The back piece was L shaped on some and C shaped on others.
The brackets for the safety fork and pistol cap pulling chains were stamped sheet steel on some and bent rod on others.
The (name?) plate on the tracks side was about 6" x 4" on some and about 3" x 1.5" on others.
Some were mounted to the deck mounts with four bolts per side and others with three wing nuts.
The deck mounting blocks were solid full width wood on some and low pads slightly larger than the track mount flanges on others.
Some tracks had a full width block for suppport between the deck and the exits of the drop rails.
The following dimension images of the Mark 6 depth charge and Type C release track are based on a Higgins smoke generator mounting conversion blueprint, and on images and plans I have in books, bought, and found searching the worlds greatest library, the net. Note that these images are my best interpretation of these resources and not "official" drawings:
Drawings from a depth charge manual with a grid overlay: