Why the Sperrbrecher was a difficult limpet-mine target

Sperrbrecher were converted merchant ships intended to lead other vessels through minefields and trigger mines against reinforced hulls. They also served as anti-aircraft and patrol ships, so their crews and weapons added an immediate detection threat inside Bordeaux. The internal strengthening meant that a small limpet charge was less likely to cripple this target than an ordinary freighter. Hasler and Sparks nevertheless attached mines after a sentry's torch swept close to their kayak, illustrating both the craft's concealment and the target's difficulty.