Deeptanks. In order to increase the water ballast capacity; many older, multi-deck cargo ships are equipped with deeptanks running from the tank top of the double bottom to the lower or upper tweendeck and extending over the entire breadth. As a rule the deeptanks were constructed amidships forward of the engine room or at both ends. The reason for this was to provide capacity for water ballast, thus improving the draught but with hardly any change in trim.
In general cargo vessels deeptanks were also suitable for the carriage of vegetable oil in bulk, e.g. palm oil, coconut oil, etc. By means of oil tight partitions and decks it is possible to carry different kinds of oil in adjacent tanks. Each compartment in the lower holds is separated from an adjacent tank in the tweendecks by tank lids, which can be closed oil tight by means of bolts. The dimensions of these tank lids are such as also permit stowage of dry general cargo in the deeptanks.
The deeptanks are equipped with heating coils in order to enable discharge of edible oil at the required temperature. This was another reason the deeptanks were situated close to the engine room or machinery space: it was relatively easy to supply steam to the heating coils.
Before loading of vegetable oil cargoes, the deeptanks and heating coils were subjected to pressure under supervision of a competent surveyor, in order to trace signs of leakage. If no defects were found, the surveyor issued a certificate of tightness, which is an important document proving, in case of leakage, that carriers have exercised due diligence.