Vorpostenboote (patrol boats) were the workhorses in the costal operations of the Kriegsmarine. Used for patrol duty, escort or submarine hunting those smal vessels were used at all costal areas where the Kriegsmarine was operating.
Due to the lack of "real" warships, several kind of existing boats were modified to serve in the role of a Vorpostenboot - harbor boats, yachts and mainly fishing boats. Escepcially fishing boats could be found in huge numbers, either at the German coast or the coasts of occupied territorys. Equipped with 8,8 cm guns and anti-aircraft guns of various kinds and numbers, those boats could harm enemy motor gun boats which they engaged in the British Channel, but were outgunned it they encountered destroyers or bigger warships.
The drawing and the technical data shows a typical Vorpostenboot based on the design of a pre-war fishing boat.