towing line vs towline

towing line and towline both are nouns.

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

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

Other synonyms of towing line include towing rope, towrope.

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