With the increased threat of terrorist attacks against docked naval and commercial vessels, the use of magnetically attached limpet mines or explosives is of major concern. Detecting limpet mines, or verification of their absence, using magnetic field sensors could be a quick and inexpensive method to determine if a vessel is under attack. Before the design of such a magnetic detection system can begin, the mines' field strength must first be quantified. The field distribution in the proximity of a small, medium, and large limpet mine was measured initially in a magnetically clean laboratory environment. The experiment was then repeated with the mines attached to the steel hull of a destroyer, while measuring the resultant field strengths on the opposite side of the hull. These data can be used in the design of limpet mine detection systems.