In the absence of express stipulations the charterer is under an absolute obligation to provide cargo according to the charterparty, which duty forms the basis of the shipowner’s right to earn freight.
Per Devlin J in Universal Cargo Carriers Corpn v Citati  2 All ER 70 at p.79:
The charterer admits that he was under an obligation to nominate a berth and also to provide cargo, the latter being a separate and distinct obligation from the obligation to load; see Grant & Co. v Coverdale, Todd & Co. (2) (1884), 9 App. Cas. 470. But he submits that he would not be in breach of either of these obligations until after the expiry of the lay days. In my judgment this submission is not good. Since no time is mentioned in the contract within which these obligations have to be fulfilled, the law implies a reasonable time. All these obligations, i.e., the obligation to nominate a berth (and a shipper, if it be a term) and to provide a cargo, are obligations preliminary to loading the cargo. The obligation to load has a time prescribed for it in the charterparty; loading must be completed within the lay days and the charterer is in breach of contract if he fails so to do. The time therefore within which the preliminary duties are to be performed is to be calculated by relation to the time prescribed for the main duty; they need not be performed any earlier than is necessary to enable the main duty to be performed timeously, but they may not be performed any later. The result is that the nomination of the berth and the provision of the cargo must be made in sufficient time to enable the vessel to be completely loaded within the lay days.