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."Coal Torpedoes", The Times (London), 30 December 1875, page 4. 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.
