The Engineering Department of a naval steam warship had, at a minimum, these people:
Chief Engineer: The Chief Engineer, or CHENG, or Engineer, was a department head. On a destroyer or frigate, the CHENG was usually a lieutenant. On a large guided missile destroyer or a cruiser, the CHENG was a lieutenant commander on a second department head tour. Aircraft carrier CHENGs were commanders or captains who had previously commanded their own ships.
MPA: Main Propulsion Assistant. The MPA oversaw B and M divisions (boilers and main machinery). On a smaller ship, the MPA was the division officer for those divisions. On a larger ship, the MPA had division officers working for him/her. MPA on a carrier was a second-tour department head job.
DCA: Damage Control Assistant. The DCA ran R division (HTs) and oversaw the ship's damage control program. Each division in a ship had one or more Damage Control Petty Officers, who maintained the DC gear of their own division. While still a part of their own division, the DCPOs were responsible to the DCA. E (EM, IC) and A (EN) may be under the DCA on a small ship. The DCA on a carrier was a second-tour department head.
After that, if there were more officers, you could have a division officer or officers for each of the engineering divisions. The guiding rule on a ship was if there was a piece of gear and nobody knew who was responsible for it, it belonged to the engineers.
Sailors (enlisted) in the Navy have rates. A "rate" is the specialty that the individual is trained in. In common usage, the abbreviations were used, with some exceptions. These are the rates that existed in the last decades of the use of oil-fired steam-driven warships. If you combined a sailor's rate and paygrade, that was the "rating."
BT- Boiler Technician. The key rate of the steam era, for it was the BTs who operated and maintained the ship's boilers. Everything that happened on a naval ship happened because the BTs were doing their jobs. The firerooms (never "boiler room") were hot places to work and, if there were some steam leaks, it was humid as well. The Oil King and Water King were BTs.
EM- Electrician's Mate. They ran the switchboards and maintained the electrical distribution network. They are the "3-wire electricians."
EN- Enginemen. They operated and maintained the auxiliary diesels and most of the environmental equipment (HVAC gear). Because they also maintained and manned the engines of the ship's boats, they were referred to as "fresh air snipes." They were the core rate for "A Gang."
HT- Hull Technician. HTs were the experts in maintaining damage control equipment and they were the core of the damage control parties. HTs were also the ship's welders. They maintained the sanitary system on the ships; they were sometimes referred to as "shitter techs."
IC- Interior Communications Technician. IC men maintained the interior communications systems: Dial telephones on larger ships, sound-powered telephones, intercoms and announcing systems. They are the "2-wire electricians."
MM- Machinist's Mate. MMs operated and maintained the engine rooms, containing the main engines. They also operated and maintained the steam-powered turbogenerators, though the maintenance of the electrical side fell to the EMs.
MR- Machinery Repairman. MRs were the ship's machinists. If something had to be made from metal stock, MRs made it. A destroyer or frigate may only have one or two MRs.
Oil King: The Oil King tested, kept track of, and generally pumped fuel oil from storage tanks to settling tanks and then to service tanks. The Oil King directly controlled the distribution of fuel oil during refueling.
Water King: The Water King was in charge of maintaining the chemistry of the ship's boilers. At the working temperatures and pressures of naval boilers, if the chemistry was out of whack, corrosion would occur and very rapidly. "Salting up" a boiler was a casualty to be dreaded.
The Oil and Water Kings were BTs.
The divisions and their rates:
A Division (A-Gang)- EN
B Division- BT
E Division- EM and IC
M Division- MM
R Division- HT and MR
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