After the war, the idea of the coal torpedo caught the public imagination in England in a way it never did in the States. Conan Doyle even mentions it casually in one of his stories, "That Little Square Box." There was some discussion about whether such devices were proper methods of warfare. One article mentions the allegation that the Union used them too, but it doesn't give any specifics. If you e-mail me privately I can look up the article.