Fleet Boat Speed Control 1: Fairbanks-Morse 38D 8-1/8 9-cyl

CaptBones · 09-12-15, 04:13 PM

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?

fithah4 · 09-12-15, 08:14 PM

........WOW!!

Very detailed information on this. A graph would diffently help in viewing info with each example listed as a color to compare to each other, through the speed and hp / bhp ranges.

Fith

Sailor Steve · 09-12-15, 08:57 PM

Quote:

Originally Posted by CaptBones

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.

Not just for propellers. I was just reading a similar set of equations concerning race cars. One of the big problems is drag. Hydrodynamic drag increases as the cube of the velocity, meaning that it takes 9,261 times as much power to maintain 21 knots as it does to maintain 1 knot. it only takes 1,331 times as much power to maintain 11 knots. That gives almost 7 times as much power needed to maintain full speed as it takes to maintain half speed.

This is true no matter what type of propulsion is being used.

CaptBones · 09-12-15, 10:41 PM

fithah4...Thanks.

Yes, graphs would certainly be helpful, several actually. They do exist and could be extracted/copied from various sources. I'm sure a good search engine would find quite a few useful examples of Propeller Curves with corresponding engine speed/hp curves for a variety of propulsion plant types. In the end, though, I think that is probably far beyond the potential, or even the possibility, of producing a technically complete and correct model to incorporate as a MOD in our beloved subsims. "Tweaking" certain files, as was done in MODs such as "Improved Ship Physics", is probably as close as the game engine itself will allow.

Steve...hello again and yes, you're correct.

Hydrodynamic drag is the biggest factor, but it's not the only one and it is also not "consistent". The drag factor typically used in hull/propulsion plant design is actually the speed-to-length ratio, or speed in knots divided by the square root of waterline length. As that ratio increases, the propeller horsepower required may increase as the 3.5 to 4th power of the speed in knots. Other factors that enter into the problem on a day-to-day basis include bottom fouling, actual displacement vs design displacement, trim, list, sea conditions, head winds, etc.

Also, I referred to the "ideal" propeller curve, which is a cube-law curve. Although we've been coming closer in recent years, there still is no "ideal" propeller; funny things like "slip" keep us from getting there. So, the relationship between a real propeller's rpm and its horsepower to drive the ship, is not an ideal curve. The "ideal" curve (Propeller Law) is used in design work, but the performance characteristics of the ship's propulsion plant aren't even known for certain until they are finally determined at Sea Trials.

AND...if you want to add one more complication directly related to diesel engines, there's BMEP (Brake Mean Effective Pressure). But, I've probably added far too much "confusion" to the basic discussion as it is, so we'll forego that (for now at least).

09-12-15, 04:13 PM	#1
CaptBones The Old Man Join Date: Nov 2010 Location: Rockton, IL Posts: 281 Downloads: 208 Uploads: 0	Helpful (?) hints... 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?

Thread Tools
Show Printable Version Email this Page
Display Modes
Switch to Linear Mode Hybrid Mode Switch to Threaded Mode

09-12-15, 08:14 PM	#2
fithah4 Torpedoman Join Date: Dec 2011 Location: Florida, USA Posts: 116 Downloads: 105 Uploads: 0	........WOW!! Very detailed information on this. A graph would diffently help in viewing info with each example listed as a color to compare to each other, through the speed and hp / bhp ranges. Fith

09-12-15, 10:41 PM	#4
CaptBones The Old Man Join Date: Nov 2010 Location: Rockton, IL Posts: 281 Downloads: 208 Uploads: 0	fithah4...Thanks. Yes, graphs would certainly be helpful, several actually. They do exist and could be extracted/copied from various sources. I'm sure a good search engine would find quite a few useful examples of Propeller Curves with corresponding engine speed/hp curves for a variety of propulsion plant types. In the end, though, I think that is probably far beyond the potential, or even the possibility, of producing a technically complete and correct model to incorporate as a MOD in our beloved subsims. "Tweaking" certain files, as was done in MODs such as "Improved Ship Physics", is probably as close as the game engine itself will allow. Steve...hello again and yes, you're correct. Hydrodynamic drag is the biggest factor, but it's not the only one and it is also not "consistent". The drag factor typically used in hull/propulsion plant design is actually the speed-to-length ratio, or speed in knots divided by the square root of waterline length. As that ratio increases, the propeller horsepower required may increase as the 3.5 to 4th power of the speed in knots. Other factors that enter into the problem on a day-to-day basis include bottom fouling, actual displacement vs design displacement, trim, list, sea conditions, head winds, etc. Also, I referred to the "ideal" propeller curve, which is a cube-law curve. Although we've been coming closer in recent years, there still is no "ideal" propeller; funny things like "slip" keep us from getting there. So, the relationship between a real propeller's rpm and its horsepower to drive the ship, is not an ideal curve. The "ideal" curve (Propeller Law) is used in design work, but the performance characteristics of the ship's propulsion plant aren't even known for certain until they are finally determined at Sea Trials. AND...if you want to add one more complication directly related to diesel engines, there's BMEP (Brake Mean Effective Pressure). But, I've probably added far too much "confusion" to the basic discussion as it is, so we'll forego that (for now at least).