Whoever furnishes labor or materials for building a vessel has a lien on it therefor, which may be enforced by attachment thereof within 4 days after it is launched; but if the labor and materials have been so furnished by virtue of a contract not fully completed at the time of the launching of the vessel, the lien may be enforced within 4 days after such contract has been completed. He has a lien on the materials furnished before they become part of the vessel, which may be enforced by attachment. The owners of any dry dock or marine railway used for any vessel have a lien on said vessel for the use of said dock or railway, to be enforced by attachment within 4 days after the last day in which the same is used or occupied by said vessel.