For the U-Boat badge it was 2 war cruises, but for the clasp it gets a bit confusing. In about the middle of 1944 the (bronze) clasp was introduced but with no pre set criteria for awards. It was given for a number of reasons including bravery or other outstanding service, number of missions etc. The only thing of note is that Donitz himself had to approve the award. The silver came out later (1944/45???) In 1944 the situation for the U-Boat crews was not good, so you can imagine that these are quite rare. Al