This post will be one of several that introduce non-conventional weapons used during the Civil War that many of you may not have known about (and there was a lot of them!). This one will deal with Coal Torpedoes, which were an old-fashioned IED used by the Confederates.
"The coal torpedo was a hollow iron casting filled with explosives and covered in coal dust, deployed by the Confederate Secret Service during the American Civil War, and intended for doing harm to Union steam transportation. When shoveled into the firebox amongst the coal, the resulting explosion would at the very least damage the boiler and render the engines inoperable, and at most cause a catastrophic boiler explosion that would kill crewmen and be likely to start a fire that would sink the vessel."
"The torpedoes were manufactured at the 7th Avenue Artillery shop (across the street from Tredegar Iron Works) in Richmond, Virginia, in January 1864. The manufacturing process was similar to that used for artillery shells, except that actual pieces of coal were used as patterns for iron castings. The walls of the coal shell were about 3/8 inch thick, creating a hollow space inside sufficient to hold 3–4 ounces of gunpowder. After filling, the shell was closed with a threaded plug, then dipped in melted beeswax and rolled in coal dust, creating the appearance of a lump of coal. Finished coal torpedoes were about 4 inches (10 cm) on a side and weighed 3–4 lb (1.5–2 kg). "
"The size and powder charge of the coal torpedo was similar to a 6 pound Shrapnel shell (a hollow, four-inch cannonball containing gunpowder and 24 musket balls as shrapnel) or the equivalent of three Civil War-era hand grenades. Even so, the explosion of a coal torpedo under a ship's boiler would not by itself be sufficient to sink the vessel. The purpose of the coal torpedo was to burst the pressurized steam boiler, which had the potential to cause a tremendous secondary explosion. Boiler explosions were not uncommon in the early years of steam transportation, and often resulted in the complete destruction of the vessel by fire. In action, the coal torpedo would leave little evidence that a boiler explosion was due to sabotage."
Due to the nature of such a weapon, it's hard to tell just how effective these really were. During the beginning of steam powered transportation, boiler explosions were common as the technology was just in it's infancy. Also, when a boiler exploded it usually resulted in a fire that completely destroyed the ship so evidence was hard to come by.