SUBSIM Radio Room Forums - View Single Post

Chromatix · 06-06-16, 08:15 PM

First, a note about units of speed, distance and power:

1 nautical mile (nm) = 1.15 English miles = 1.852 km.
1 knot (1 nm/hr) = 1.15 mph = 1.852 km/h = 0.5144 m/s.
1 fathom = 6 feet = 2 yards = 1.8288 m.
1 horsepower (hp) = 745.7 W.

Power = force * distance
Power = torque * rotation speed
Power = volts * amps
Power = energy ÷ time

Now, ignoring nuclear subs and rare oddities, there are two main arrangements for submarine propulsion machinery: "direct drive" and "diesel-electric". The two are very different in practical operations.

The direct-drive layout is the older type, and was used on all the early-war U-boats and the smaller British subs during WW2. There are normally two propeller shafts, with one diesel engine and one electric motor attached to each shaft as follows:

[ENGINE]---{clutch}---[MOTOR]---E{clutch}E---§propeller§
[ENGINE]---{clutch}---[MOTOR]---E{clutch}E---§propeller§

The tail clutch is marked with Es to emphasise that it is a "dog clutch", and cannot be engaged while in motion (if you try, it'll be like a failed gear change with a car's manual transmission). A dog clutch is however very reliable and does not slip under load. The engine clutch is a "plate clutch" and can slip, which allows it to be engaged while the engine is running.

This drivetrain can be set in one of four states:

1: Engine driving propeller - both clutches engaged, motor dead.
2: Motor driving propeller - engine clutch disengaged, motor connected to battery.
3: Engine recharging battery - tail clutch disengaged, motor becomes generator, connected to battery.
4: Starting engine - engine clutch disengaged, supply high-pressure starting air directly to cylinders.

Because the motor must be run at full speed to match the battery voltage when generating, it is impractical to use an engine for both charging and propulsion simultaneously. Hence it is normal to use one engine to charge the battery (with its propeller dead in the water) and the other for propulsion until the batteries are fully charged. However if a rapid charge rate is required, both engines can be used for charging, at the expense of having no propulsion at all.

A disadvantage of this system is that to drive both propellers on the surface, both engines must be running. When trying to accelerate, the available power is limited by the maximum torque of the engines, not their maximum power, since the engines run at shaft speed which (except at dead-slow speeds) is closely related to speed through the water. At low speeds it is sometimes more efficient to run only one engine, even though that leaves the opposite shaft trailing. A more serious drawback is that, within the confines of a submarine hull, it is difficult to connect more than one engine to each shaft.

The diesel-electric system solves that last drawback, by mechanically isolating the engines and propeller shafts from each other. Here is the layout of any random American fleet boat:

[ENGINE]---[GEN] [ENGINE]---[GEN] [MOTOR]---§propeller§
[ENGINE]---[GEN] [ENGINE]---[GEN] [MOTOR]---§propeller§

Each motor and generator may be independently attached to one of several electrical buses, including the ones attached permanently to the batteries. At least one of these buses allows the output of one or more engines to be used directly for propulsion, bypassing the batteries. There is also no need (and no provision) for mechanically disconnecting the engine in order to start it.

The system is very flexible. Any combination of 1-4 engines can be used for propulsion or battery charging, provided only that each individual engine is dedicated to at most one of those tasks. Fleet boat patrol reports frequently refer to "two engine speed", etc rather than specific speeds in knots or the conventional series of engine orders. (An occasional reference to "five engine speed" implies that a smaller auxiliary engine supplemented the propulsion power.)

However, because these boats predate by at least 20 years the introduction of high-power silicon rectifiers, it is absolutely critical that the output voltage of the generator equals or slightly exceeds that of the bus it is attached to *before* it is so attached, and that it is disconnected from a bus it is driving in concert with other generators *before* its voltage is reduced (eg. by shutting down the engine). Failing to do so will reverse the current flow through the generator, causing it to try to act as a motor, for which it is emphatically not built.

Hence the diesel-electric system requires more skill and discipline from the engineering department of the boat than the direct-drive system does.

The controls in Marulken's engine room appear to broadly match those required in diesel-electric machinery, rather than direct-drive machinery. However, only the bus switches and a crude armature rheostat are provided, which is a gross understatement of the full electrical controls actually required.

06-06-16, 08:15 PM	#2
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	First, a note about units of speed, distance and power: 1 nautical mile (nm) = 1.15 English miles = 1.852 km. 1 knot (1 nm/hr) = 1.15 mph = 1.852 km/h = 0.5144 m/s. 1 fathom = 6 feet = 2 yards = 1.8288 m. 1 horsepower (hp) = 745.7 W. Power = force * distance Power = torque * rotation speed Power = volts * amps Power = energy ÷ time Now, ignoring nuclear subs and rare oddities, there are two main arrangements for submarine propulsion machinery: "direct drive" and "diesel-electric". The two are very different in practical operations. The direct-drive layout is the older type, and was used on all the early-war U-boats and the smaller British subs during WW2. There are normally two propeller shafts, with one diesel engine and one electric motor attached to each shaft as follows: [ENGINE]---{clutch}---[MOTOR]---E{clutch}E---§propeller§ [ENGINE]---{clutch}---[MOTOR]---E{clutch}E---§propeller§ The tail clutch is marked with Es to emphasise that it is a "dog clutch", and cannot be engaged while in motion (if you try, it'll be like a failed gear change with a car's manual transmission). A dog clutch is however very reliable and does not slip under load. The engine clutch is a "plate clutch" and can slip, which allows it to be engaged while the engine is running. This drivetrain can be set in one of four states: 1: Engine driving propeller - both clutches engaged, motor dead. 2: Motor driving propeller - engine clutch disengaged, motor connected to battery. 3: Engine recharging battery - tail clutch disengaged, motor becomes generator, connected to battery. 4: Starting engine - engine clutch disengaged, supply high-pressure starting air directly to cylinders. Because the motor must be run at full speed to match the battery voltage when generating, it is impractical to use an engine for both charging and propulsion simultaneously. Hence it is normal to use one engine to charge the battery (with its propeller dead in the water) and the other for propulsion until the batteries are fully charged. However if a rapid charge rate is required, both engines can be used for charging, at the expense of having no propulsion at all. A disadvantage of this system is that to drive both propellers on the surface, both engines must be running. When trying to accelerate, the available power is limited by the maximum torque of the engines, not their maximum power, since the engines run at shaft speed which (except at dead-slow speeds) is closely related to speed through the water. At low speeds it is sometimes more efficient to run only one engine, even though that leaves the opposite shaft trailing. A more serious drawback is that, within the confines of a submarine hull, it is difficult to connect more than one engine to each shaft. The diesel-electric system solves that last drawback, by mechanically isolating the engines and propeller shafts from each other. Here is the layout of any random American fleet boat: [ENGINE]---[GEN] [ENGINE]---[GEN] [MOTOR]---§propeller§ [ENGINE]---[GEN] [ENGINE]---[GEN] [MOTOR]---§propeller§ Each motor and generator may be independently attached to one of several electrical buses, including the ones attached permanently to the batteries. At least one of these buses allows the output of one or more engines to be used directly for propulsion, bypassing the batteries. There is also no need (and no provision) for mechanically disconnecting the engine in order to start it. The system is very flexible. Any combination of 1-4 engines can be used for propulsion or battery charging, provided only that each individual engine is dedicated to at most one of those tasks. Fleet boat patrol reports frequently refer to "two engine speed", etc rather than specific speeds in knots or the conventional series of engine orders. (An occasional reference to "five engine speed" implies that a smaller auxiliary engine supplemented the propulsion power.) However, because these boats predate by at least 20 years the introduction of high-power silicon rectifiers, it is absolutely critical that the output voltage of the generator equals or slightly exceeds that of the bus it is attached to before it is so attached, and that it is disconnected from a bus it is driving in concert with other generators before its voltage is reduced (eg. by shutting down the engine). Failing to do so will reverse the current flow through the generator, causing it to try to act as a motor, for which it is emphatically not built. Hence the diesel-electric system requires more skill and discipline from the engineering department of the boat than the direct-drive system does. The controls in Marulken's engine room appear to broadly match those required in diesel-electric machinery, rather than direct-drive machinery. However, only the bus switches and a crude armature rheostat are provided, which is a gross understatement of the full electrical controls actually required.