Section 21. If a contractor or sub-contractor unreasonably neglects or refuses to pay for labor procured by him to be performed in constructing, repairing or launching a vessel upon which a lien exists therefor and the owner or other person who made the agreement with such contractor or sub-contractor pays the debt secured by the lien, he shall have the same claim against such contractor or sub-contractor as if the lien had been enforced by judgment.