nautilus vs pigboat

nautilus and pigboat both are nouns.

Word	Noun	Adjective	Verb	Adverb
nautilus	Yes	No	No	No
pigboat	Yes	No	No	No

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

Other hypernyms of nautilus include sub, submarine.

nautilus (noun)	pigboat (noun)
cephalopod of the Indian and Pacific oceans having a spiral shell with pale pearly partitions	a submersible warship usually armed with torpedoes
cephalopod mollusk of warm seas whose females have delicate papery spiral shells
a submarine that is propelled by nuclear power