It was not unusual to have a total compliment of 80 or more, whereas now it is not unusual to have a total compliment of about 25. Improved designs (with respect to both safety and cargo carrying capacity) have generally removed the central navigation bridge, and the aircraft has all but eliminated the passenger liner.
1. Tanker:
A tanker can be defined as: "A merchant ship, designed for the specific purpose of transporting liquid cargoes in bulk."
Tankers generally have their machinery spaces aft (at the stern, or back end, of the ship). Forward of this are the cargo tanks. These are numbered from forward to aft, with the number one tank being the furthest forward. Each tank is further divided longitudinally (from fore to aft) by one or more oil-tight bulkheads, so the vessel may have an arrangement such as port number one tank, starboard number one tank, and (perhaps) centre number one tank. This improves stability by preventing liquid sloshing from side to side when the vessel rolls. One or more pump rooms would be provided, and these are used for discharging cargo.
Tanks can be fitted with heating systems (to allow heating of heavy oils to enable them to flow), steam smothering systems (to put out a fire in the tank) and vents (to allow gas to escape). These vents would be fitted with flame arrestors on vessels where light oil cargoes were intended to be carried. At around the mid point of the ship would be the main superstructure, containing accommodation for deck officers, the navigation bridge and the radio room. The accommodation for the engineers would be at the stern, above the engine room.
During the Second World War, tankers were particularly valuable targets. Whilst the loss of general cargo vessels caused much concern, it was the loss of tankers that caused the most anxiety as they were being lost at a much greater rate. As the highest-value ships in a convoy, tankers were normally placed in the inner columns. This shielded them somewhat from attack, although the losses continued to be a problem until the Battle of the Atlantic was finally decided.
Tankers are difficult ships to sink. They are well subdivided in to watertight compartments, and are designed to carry a liquid of approximately the same density as water. If you put a hole in the starboard (right) side of the tanker, the oil will run out and the vessel will list (lean) to port (the left) whilst rising slightly out of the water as the weight of cargo is reduced. The vessel will only sink if structural failure occurs.
The main danger faced by crews of tankers was fire. Whilst crude oil is difficult to set alight (it is thick, often does not flow unless it is heated, and if a lighted match is dropped in to it the match will simply go out), refined products (such as petrol and aviation fuel) can be very flammable.
2. Passenger Liner:
A passenger liner may be defined as: "A merchant vessel designed for the main purpose of carrying passengers."
Passenger liners are usually fast ships, with a speed between 25 and 30 knots. Passenger liners were very different ships from the cruise liners of today. Their main purpose was to transport people between destinations, in much the same way as the airliner does today, rather than to provide a pleasant holiday experience. In the same manner as a modern airliner, passengers were split in to a number of classes. The first class passengers travelled in luxury, whilst the lowest class passengers were packed in as densely as possible.
The most important aspect of a passenger liner was its speed, with faster ships attracting more passengers and higher prices. This was because passengers simply wanted to get to their destination rather than enjoy the process of travelling. This obsession with speed fuelled great rivalry between the great liner companies, particularly on the prestigious North Atlantic route where the Blue Riband trophy was handed to the fastest ship to make the crossing.
The speed of this ships was a great advantage during the Second World War, as it made them almost invulnerable to submarine attack. As they could easily outpace even a surfaced submarine, they could be torpedoed only if the ship accidentally sailed in to range of the submarine. As the torpedo range was limited, the ocean very big and submarines reasonably few in number, passenger liners were unlikely to be attacked even if unescorted.
Passenger liners were essential to moving the large quantities of troops needed around the world. One of the largest ships was the RMS Queen Mary, which carried up to 15,000 troops per trip, and their presence simplified the vast logistical problem facing the Allies. Germany and Italy had less need for such vessels, particularly as their sea lanes were generally less secure than the Allies and the fighting was generally reachable by land, although if they had managed to take Europe as planned they would have been essential to any next steps and their lack was keenly felt in the Norwegian campaign. Japan's problems were more complex. Passenger liners would have made their expansion much easier, however once they began retreating their shipping was not safe and such ships would not have helped. Passenger liners were very vulnerable to surface and air attack and, in areas where such threats were present, needed to be heavily escorted. Submarines, however, posed little threat and liners could operate in waters known to contain submarines provided they had a choice of routes (i.e. were not constrained to going through a particular area).
3. Bulk carrier:
A bulk carrier may be defined as: "A merchant vessel designed for the primary purpose of carrying solid cargo in bulk."
These vessels were developed from a need to be able to handle certain types of cargo more efficiently then could be done in standard general cargo vessels. This included goods such as grain (which could be piped in and out of holds quickly), coal and iron ore (which were easier to handle in bulk rather than bagged on pallets).
Bulk carriers could be extremely vulnerable to submarine attack if they carried very dense cargoes, such as iron ore, scrap metal etc. Such dense cargoes mean that holds contain a lot of empty space to provide the necessary buoyancy. If the ship is torpedoed, water rushes in to the empty space, rapidly destroying the ship's reserve of buoyancy and often causing structural failure. It was not unusual for scrap metal or iron ore carriers to sink within two minutes of being torpedoed, and they were not popular assignments for merchant crews because of this.
4. General Cargo Vessel:
A general cargo vessel may be defined as: "A merchant vessel designed for the primary purpose of carrying break-bulk dry cargo."
These ships generally had a central engine room, with cargo holds forward and aft of this. The accommodation and navigating bridge was located above the engine room. Accommodation could also be provided at the stern of the vessel (in the poop) and at the bows (in the forecastle). These ships generally had their own cargo-handling equipment (booms) so that they could handle cargo without having to rely on shore facilities. During the Second World War, guns were fitted for defensive purposes. The speed of these vessels varied considerably, from only five or six knots for old tramp steamers up to over 15 knots for fast cargo liners.
Although a rare sight nowadays, the general cargo vessel has a long history, stretching back for as long as cargo was carried by sea. This type of vessel usually carries cargo stowed on pallets, lifted in and out of the hold by cranes and stowed within the hold by the ship's crew. Cargo handling operations were labour-intensive, and it was for this reason that large crews were carried.
