Higgins 21" Torpedo Tube
The Higgins 21" torpedo tubes used compressed air to launch the torpedo rather than a small explosive charge, this reduced the visibility of the craft, from a distance, as they produced no smoke, nor could the grease in the torpedo tubes catch fire as sometimes happened to the Elco's torpedo tubes.
The Higgins 21" torpedo tubes were fixed in their firing position and angle and were not "trainable" like the Elco tubes and this meant that they were always "ready" as there was not as much set up time required before a torpedo could be launched. One disadvantage of this approach is that the torpedo tubes and torpedoes rode higher than the Elco so as to make sure they cleared the deck when launched.
I believe the construction of the tubes were somewhat lighter and simpler than the Elco (and other tubes using an explosive charge) as they would not need to cope with the instantaneous blast forces of an explosive type charge. When one reads the specifications of the various torpedoes used in WW2, there was a specified "maximum afterbody pressure" that could be applied to each model and this varied. This leads me to believe that an air fired torpedo tube would be able to use a larger range of torpedoes than that which could be used with an explosive charge, however this is only a speculative opinion on my part.
The long sausage shaped tube adjacent to the main tube is the compressed air cylinder and had a charging valve fitted at the front end, the air cylinder was charged with a compressor prior to launching the torpedo but this could have also been done at the beginning of the day, so that the torpedoes were always ready to be fired electrically from the bridge, or in the event of an issue they could also be fired manually from the valve at the rear of the air cylinder.
I have shown a late model Mk 8 Torpedo as a size comparison, the earlier variants of the Mk 8 had a shorter (and before that an even shorter) warhead and would have easily fitted fully inside the torpedo tube, the later Mk 8 torpedo shown would have just fitted with the nose slightly proud of the tube.
The webbed structures shown at the tops of the mounting bases were to strengthen the main tube at the points of maximum stress. The torpedoes used weighed several tons and PT Boats were easily tossed around by heavy seas, it would not be a good thing for the tube to break free.
Note: There was a later 22.5" torpedo tube fitted to late war Higgins boats to launch Aircraft style Mk13 torpedoes, these tubes can be seen in a number of photographs and they can be distinguised by being shorter and having two straight air cylinders, one mounted on each side of the main tube.
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