[CaptBones]

Just a couple of comments to help point you in the right direction, I hope. Which is to say I hope this isn't too much "preaching" and doesn’t discourage you from trying to find a better approximation of real engine/ship speed control simulation in the game. I guess I'd also say that using the steam plant operating guide for CV-6 as an indicator of the concept or "philosophy" of plant operations for the "Fleet" submarine is not going to really help at all. That's not even an "apples to oranges" comparison, it's more like a "bicycles to pomegranates" comparison.

OK then, first; remember that the US “Fleet” boats had diesel-electric propulsion plants. Using diesel engine HP and speed (engine RPM) in calculations concerning ship propulsion shaft HP/RPM and ship speed is incorrect. The diesel generators operated on the basis of load-speed settings (e.g., 80-90) that were very different from the shaft RPM/ship speed called for on the Engine Order Telegraph (EOT) and Revolution Indicator. Note that only the EOT is modeled in our submarine simulation games.

The load-speed setting mentioned refers only to the engine-generator sets; as you know, 80-90 is 80% load on the generator (and hence on the engine) at 90% of engine full speed rating (720rpm for the FM OP engines). Bear in mind the main generators were all rated 1100kW at 415V/2650amp and the OP was rated 1600BHP at 720rpm. Thus, 80-90 means running each engine-generator set operating on propulsion at 880kW/650rpm (yes, it was “rounded off” a bit). BTW, you’ll see that the arithmetic to convert kW to HP doesn’t come out exactly right. That’s mostly because the engine rating is in BHP (Brake Horse Power), which is different from the output HP at the engine coupling to the generator. A certain amount of developed HP is absorbed by the engine-driven components and accessories (scavenging blower, water pumps, lube oil pump, fuel oil supply pump, fuel injection pumps, etc.).

Next, add to that the complexities of a DC propulsion plant with several main engine-generators and multiple main motors on separate propulsion shafts. There were two motors on each of the two shafts, driving the main shaft through a reduction gear. That results in “multiple armatures” in a series-loop configuration, a situation that is far beyond the ability of the game engine to even roughly simulate. The utility and flexibility of four main generator armatures that can be connected in different combinations to four main motor armatures is, well, "beyond the scope" of any commercial PC game engine.

Then, in order to accurately model those real-world propulsion plants, you really need to consider Propeller Law and start with a Propeller Curve and multiple engine power output curves on the same graph. As I'm sure you realize, the correlation between propeller RPM and propeller HP (and therefore ship speed) is not a straight-line relation; 282rpm/21kts is not equal to 67rpm/4.99kts. Without an actual propeller curve for the “Fleet” boats, we could just use an “ideal” propeller curve. If you assume that 100% power represents two engine-generator sets on one propeller shaft, the omission of one engine-generator set would mean 50% available power on that shaft, if the remaining engine-generator set could operate at full rated speed. With only one generator operating on propulsion on that shaft at full speed, the total system voltage in a series-loop circuit would be 50%, which would reduce the motor speed to about 50%, since motor speed is approximately proportional to the voltage generated. Using the propeller curve, you would see that 50% shaft speed is equal to 20% of the propeller horsepower demand, which is not equal to the power output available from the engine-generator set on propulsion on that shaft. But, by weakening the motor field, the main motor speed can be readjusted upward along the propeller curve to about 80% shaft speed, where the full capacity of that engine-generator can be absorbed.

Now, since we have a more flexible, and more complicated, plant you could also assume that the Propeller Curve is plotted to represent four engine-generator units. In order to simplify that case to something manageable and easily understandable, we have to assume that both propeller shafts are operated at the same shaft speed at all times. Even so, the following simplified explanation is not entirely correct.

Nevertheless, to illustrate the flexibility of the “Fleet” boat propulsion plant, let’s consider the omission of one generator set on battery charge, which would mean 75% available propulsion power, again with the remaining three units capable of operating at full rated speed. However, with only three generators on propulsion (at full rated speed), only 75% of total system voltage would be generated, assuming a series-loop circuit with three main generators connected to all four main motors. This is also only possible if the design of the generators and motors is correlated so that the sum of the generator voltage rises is equal to the sum of the motor voltage drops. Thus, motor speed would be limited to 75%, since motor speed is approximately proportional to the voltage generated. Using the propeller curve, you would see that 75% shaft speed is equal to 43% of the propeller horsepower demand, which is not equal to the power output available from the three engines running on propulsion. But, by weakening the motor field, the main motor speed can be readjusted upward along the propeller curve to about 91% shaft speed, where the full capacity of the three engine-generators can be absorbed.

Finally, the whole matter is really made more difficult to readily model by the application of the 90% normal speed limitation on the generator sets. This artificiality can be observed in the generator rating; they were designed for a 450 volt output at the terminals (with nominal insulation resistance for 460V) and rated at 415 volts. The FM OPs could, and still can and do today, run constantly at 720rpm for thousands of hours without excessive wear. When “in extremis”, the MoMacs and Enginemen in “Fleet” boats could move, or even remove, the rack-stops and push the OPs to nearly 800rpm. Commercial variants of those very same engines run at 900rpm.

OK…so does that help or hinder your project?