Ships under these arrangements can be bareboat chartered (where the charterer is responsible for crewing, maintenance, etc.) or time chartered where the actual owner performs these functions. Other companies will time charter suitable tonnage from other shipowning companies. Time charters are also used to acquire tonnage to meet short term commitments or fluctuations in the fleet, perhaps to replace tonnage during a dry dock programme or to meet a seasonal high level of demand. Ships require constant supervision of their structure and their machinery, much of which need a regular programme of maintenance. A merchant ship which is not kept in a seaworthy condition will be unemployable. Seaworthiness does not just mean that there is no danger of the ship sinking although that is a vital element, the term can also be considered as also meaning cargo-worthiness. No matter how sound the hull of the ship is against springing a leak, and how good the engine is to propel the ship to her destination, if the hatches let water into the holds, or the ventilation is inadequate so that cargo becomes damaged, then a merchant ship is considered unseaworthy. Looking after the physical structure of the ship falls neatly into two distinct sections which are usually referred to as deck and engine-room. The term engine-room is easily understood as there is no difficulty in visualising the compartment of the ship which contains the main engine plus auxiliary machinery such as electricity generators, pumps etc. It does, of course, extend a little further than the actual engine room as the term naturally includes the propeller shaft and the propeller at the end of it. It is perhaps better to think in terms of ‘deck’ as meaning all the rest of the ship which is not covered by the expression ‘engine-room’ because that is the responsibility of the deck department.