This was actually a qual question my Eng had for me when I was getting qualified for ERS. He had a funny sense of humor.
To understand the meaning you have to go WAY back to the beginnings of steam propulsion. Wayyyyy back then, the boilers were coal fired and it tood time for pressure to go up OR down. Materials were not as robust as they had now do they used steam pressure to control how fast a steam piston operated. The more pressure the faster it ran.
Anyway, the bells were to provide the engineman a standardized guide for operations. Ahead 1/3 would have been 1/3 the maximum normal pressure that the steam plant could produce. 2/3 was 2/3 and full was maximum pressure that it could substain during normal operations. They also had a bell they called maximum turns. It basically was the greatest amount of steam that the engineman thought that the boilers, pipes, and engines could bear. It was also called a 'Battle Bell' or 'Battle Speed'. You were limited by the intestinal fortitude of the engineman on duty.
Things progressed over time materials improved and the technology was more refined. You also had things like steam turbines instead of a reciprocating piston. Anyway, by now the boilers had shifted from a pressure control to a volumn control. They could use throttle valves to regulate the amount of steam going to the engines and auxillary gear. You also have a shifting from coal fired to some oil fired boilers so things could respond a lot quicker to changing situations.
Things like a standard and flank have now entered the vocabulary of the engineman. A Standard Bell is the most economical speed for a ship to move at. It is the greatest range at a given speed . Flank replaced the maximum bell and was a 100% effort of the engineering plant. The older Full Bell was now used to repesent a maximum speed the the FLEET could move at. Thus a full bell could vary depending on the on the vessels you are in convoy with.
Also, with boiler powered ships it was not economical to run all the boiler all the time. Thus, to conserve fuel, some boilers may be shut down and put on standby as a lesser number of boilers could still provide the neccesary steam volumn to drive the ship at the desired bell. When the engineroom is given the order 'make ready to answer all bells' they know that they need to get off their duffs and get the boilers all back online. When the engineroom is ready they would report back to the bridge ' Ready to answer all bells'
On that note, if you have ever watched any of the old wartime movies, you would hear something like: "#2 boiler is flooded, #3 boiler has popped a safety, #1 & #4 boilers are online. propulsion limit is ahead 2/3" during or after a battle. That was the engineroom telling the bridge what was going on and what kind of speed they can make. Later you might have heard, "repairs completed, propulsion limit ahead standard, flank upon request" That meant that all the boiler were not running at that time. They were only running enough boiler to meet the speed requirements at that time but that there was more available it needed.
1/3, 2/3, and standard now are typically a preset numer of revolutions on the screw or a given ships speed depending on the propulsion plant design. When the OOD gives the command for Ahead Standard he knows what the approximate speed the ship will go.
Please understand that this is all greatly simplified. There is so much that goes into steam plant operations that I could write reams of information. I hope this answers some of your questions
edited for spelling
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