Allowance for propelling machinery space. In the middle of the 19th century steam engines were replacing sails as the main means of propulsion.
Ships were no longer at the mercy of the weather; in particular, the wind, and engines could generally be used to get a ship out of danger. Therefore, from a safety viewpoint, authorities encouraged owners to fit engines to their ships. Early engines were steam reciprocating and occupied a large space in relation to the power they produced. Modern engines are reduced in size for the same power. However, an effect of the large early engines and also of the authorities’ attempts to encourage the fitting of engines was that the machinery space was allowed to be deducted from gross tonnage according to a sliding scale formula which advantaged ships with larger propelling machinery spaces.
The consequence is that owners have ships designed with large spaces reserved for propelling machinery, much larger than would be required for the main propulsion system. This has led to some anomalies where unnecessarily large propelling machinery space is provided for small engines so that the gross tonnage can be greatly reduced. The large machinery spaces could be a disadvantage to the vessel’s reserve buoyancy and could cause the vessel to sink after damage. In Hong Kong, for example, a ferry sank in 1977 after a collision. The subsequent marine inquiry established that the cause of the sinking was the large open spaces, although the engine was small. The allowance is determined on a formula.