towline vs towing line

towline and towing line both are nouns.

Word	Noun	Adjective	Verb	Adverb
towline	Yes	No	No	No
towing line	Yes	No	No	No

As nouns, towline and towing line are synonyms defined as:

Other synonyms of towline include towing rope, towrope.

towline (noun)	towing line (noun)
(nautical) a rope used in towing	(nautical) a rope used in towing