attack submarine vs pigboat

attack submarine and pigboat both are nouns.

Word	Noun	Adjective	Verb	Adverb
attack submarine	Yes	No	No	No
pigboat	Yes	No	No	No

As nouns, pigboat is a hypernym of attack submarine; that is, pigboat is a word with a broader meaning than attack submarine:

attack submarine: a military submarine designed and armed to attack enemy shipping
pigboat: a submersible warship usually armed with torpedoes

Other hypernyms of attack submarine include sub, submarine.

attack submarine (noun)	pigboat (noun)
a military submarine designed and armed to attack enemy shipping	a submersible warship usually armed with torpedoes