Tanker Type Z-ET1-S-C3 The Liberty tanker was proposed in late 1942 as a response to the need to transport more oil (due to the US entry in to the war) and offset the large losses amongst the existing tanker fleet. The initial intention was to both convert existing cargo ships and build specific new tonnage as tankers, however the conversion plan was soon abandoned.
The tanker was very similar to the standard cargo ship, with the same length, breadth and draft, as well as the same propulsion system. It deliberately retained dummy or redundant deck equipment to prevent these ships from being identified as tankers (and so prevent them from being specifically targeted), and the necessary piping was concealed. The deception was further aided by the ability of these ships to carry deck cargo.
Internally, there were also few changes. The major change was the addition of two pump rooms, forward and aft of the machinery space. Pump rooms are used during cargo discharge, to pump the cargo up from the tanks to the shore facility. This reduced the available space in what used to be the number three hold. The two deep tanks under number one hold were removed (adding their volume to the available cargo space), and part of the number three deep tank was used in the forward pump room.
The remaining spaces were provided with additional subdivision. Vertical oil-tight bulkheads were placed at the edges of what used to be the hatches, producing trunks down to the second deck, with bulk of the oil cargo being carried below the second deck. All the pipework, manifolds and valves were placed in to the void spaces created between the second deck and the weather deck, hiding them from view. The previous number one, two, four and five holds were subdivided transversely (ie across the ship) near to the centreline of the previous hatch covers, and all holds were divided longitudinally (forward and aft) down the centreline of the ship.
The resulting tanker had a total of eighteen tanks for carrying oil: tanks number one, two, three, four and five (port and starboard) forward of the accommodation (a total of ten spaces) and tanks six, seven, eight and nine (port and starboard) aft of the accommodation (eight spaces).
Either light fuel oil (petrol) or heavy oil could be carried in the forward tanks, with heavy oil only in the aft tanks. The piping arrangement allowed different cargo to be carried forward and aft, and it was also arranged to allow the aft cargo to be pumped via the forward pump room (and vice-versa) if necessary.
The vents for the tanks were at the top of the masts, and the forward tanks (the light oil tanks) were also provided with pressure relief valves and flame arrestors (to prevent an external flash fire from travelling back down to the tanks through the venting system). Aft tanks did not require this as the heavy oil did not produce high levels of vapour. All tanks had a steam smothering system (to put out fires in the tanks), heating coils (heavy oil does not flow without being heated) and a tank cleaning system (tanks are normally cleaned after discharging every cargo, both to prevent future cargo contamination and to free the vessel of potentially explosive residual gas).
All tankers are very difficult ships to sink. They are designed to carry liquid with a very similar density to water (oil is approximately 98% of the density of water), and a hole in the side (such as one caused by a torpedo) simply replaces one liquid with another. Indeed, as cargo tanks are filled to above the waterline, a hole in the side causes the cargo to flow out and the ship effectively becomes lighter, raising it up out of the water rather than making the ship sit lower in the water. Unladen tankers have such a reserve of buoyancy that they are almost impossible to sink unless structural failure occurs.
For laden tankers the major danger is from fire, as burning petrol (gasoline) leaking out from the side can quickly spread around the ship preventing the crew from escaping (modern lifeboats are now totally enclosed and fitted with a sprinkler system to enable tanker crews to escape if the worst does happen). Heavy oil is difficult to set alight (attempts to burn off the oil using napalm following the loss of the Torrey Canyon in 1967 failed), and does not flow well, and tankers carrying heavy oil were very safe ships to sail on. Ships carrying petrol (gasoline) were, however, extremely vulnerable, and a torpedo hit could easily cause the ship to be trapped in the middle of a burning pool, leaving the crew no way of escape.
A total of 62 tankers were produced, although after the war was over most were converted to dry cargo vessels. The tankers are listed in the 'Production Figures' section.