All but 8 of the U-boat flotillas displayed an emblem. Many of the boats attached to those flotillas then used that same emblem. The most widely used Flotilla emblems seem to have been those of the 7th, 9th, 10th and 11th flotillas.
You can click on the emblems below to get a page showing the larger version and its history.
All emblems here © Guðmundur Helgason 1998-2015