U-boat vs pigboat

U-boat and pigboat both are nouns.

Word	Noun	Adjective	Verb	Adverb
U-boat	Yes	No	No	No
pigboat	Yes	No	No	No

As nouns, pigboat is a hypernym of U-boat; that is, pigboat is a word with a broader meaning than U-boat:

U-boat: military submarine operated by Germany during the First and Second World Wars
pigboat: a submersible warship usually armed with torpedoes

Other hypernyms of U-boat include sub, submarine.

U-boat (noun)	pigboat (noun)
military submarine operated by Germany during the First and Second World Wars	a submersible warship usually armed with torpedoes