6.1 Line 16 of the New York Produce form gives the charterers the express right to sub-charter the ship during the currency of the head time charter. A sub-time charter (or a sub-voyage charter) does not of itself create any contractual relationship between the owners and the sub-charterers. The owners may not, therefore, sue the sub-charterers, or be sued by them, under or by virtue of the head charter or the sub-charter. This is in accordance with the principle that a contract cannot, generally, confer rights or impose obligations on persons who are not parties to that contract. (The mere liberty to sub-contract will not engage the Contracts (Rights of Third Parties) Act 1999, so the sub-charterers will not enjoy rights under the head charter by virtue of that Act, nor will the owners under the sub-charter.)