A Sperrbrecher was a merchant vessel adapted to clear a passage for other ships by setting off mines. Its reinforced hull was intended to withstand the explosions involved in that work. This made it an unpromising target for the limpet mines carried by the Frankton crews. The attack therefore included a ship specifically strengthened against the kind of underwater damage the raiders were trying to inflict.
Notable Point · 1:50:47-1:51:27
Why a Sperrbrecher was a difficult limpet-mine target
A merchant ship strengthened to survive mine explosions presented the canoe crews with an unusually resistant target.