In the Engine Room (build 144)

[Chromatix]

Of the four major roles players are expected to take on, the Engineer was the most surprising to me in its implementation in the current alpha. Given that the developers are clearly trying for authentic operation in other areas, the number of both basic and subtle problems I found in the engine room were such that I must make a detailed criticism of it.

First, and most obviously, both the helm and the diving controls have been placed aft, along with the engine and motor controls. In every real WW2 (and later) sub I'm aware of, the helm and primary diving controls were actually in the control room, along with telegraphs to relay engine orders aft. There would only be an emergency helm control aft, in case of failure of the remote control system; if remote control of buoyancy systems failed, there would be local hand controls for every vent and valve throughout the length of the boat.

Also fairly obviously, no diveplane controls are provided whatsoever, with depth control being entirely by flooding and draining buoyancy tanks. In reality, the diveplanes were the primary means of depth control (both directly and via angling the whole boat), with the buoyancy tanks being set up for neutral trim and then left well alone under normal circumstances. British and American subs did have a "Q" or "negative" tank to assist rapid diving, but that was for use in urgent situations rather than as the routine method.

The above is excusable as a simplification, given that the game is in an early state of development and no AI crew is available yet. It should also be noted that modern subs use a "one man control" with both helm and diveplane controls operated by one crewman, though the engine orders are still telegraphed.

It seems reasonable to me to provide a helm station in the control room with engine telegraphs, rudder, fore-plane and aft-plane wheels, and controls for the main ballast, Q, safety and trim tanks all in one place, even though it was normal for a total of four or five crewmen plus a Dive Officer to operate these controls. The engine room itself can then be optionally manned, as it is relatively easy for an AI to interpret engine telegraph orders.

But now we come to the propulsion machinery, and I feel obligated to invoke Video Game Developers Have No Sense Of Scale. I've previously led a (partly successful) crusade on this subject for railway-based games - well, *one* railway-based game - and the principles are sufficiently similar to require only slight modification for naval technology.

First, the maximum speeds of the sub (both surfaced and submerged) are ridiculously high for a conventional WW2 design. The well-known American "fleet boat" design was among the first capable of a sustained 20 knots on the surface, and that after several failed attempts at achieving precisely that throughout the 30s. To do it required a long waterline (by sub standards) and the full power of *four* diesel engines. Marulken's top speed of 30 knots with two normal-looking diesels is not credible, even for a "black project".

Meanwhile submerged speeds of even 10 knots were found only in smaller boats (with less surface area) until full streamlining came into vogue with the Type XXI and the post-war GUPPY conversions. Marulken's underwater performance however ranks with the best of the GUPPYs, even though it has the tower design obviously lifted from an early-war German U-boat.

Putting that aside, one would expect the batteries to last a great deal longer when creeping around at low speeds than when dashing around at full speed. This is because, as a rule of thumb, the power requirements scale with the *cube* of the ship's speed, while the distance covered scales only linearly. Yet I found my batteries to be almost drained after an hour or two of creeping around at 3 or 5 knots, forcing me to surface and recharge before I had sufficiently left the bay. That's bad by *First* World War standards of underwater endurance.

If we assume that each battery is nominally 300V, the quoted 4000 Ah capacity corresponds to 1.2 megawatt-hours per battery. So Marulken was somehow using about a megawatt to trundle peacefully about at 5 knots under my command. To put that into context, a megawatt is roughly the full rated output of a Class 33 locomotive's main genset, consisting of a large 8-cylinder diesel engine which wouldn't be *entirely* out of place on a fleet-type submarine - but it would certainly achieve more than 5 knots if so fitted!

Meanwhile the Marulken's miraculous diesels fully recharged both batteries in about a minute flat (so, producing about 60MW or 80,000 horsepower each) - which was fortunate as by then I had an enemy destroyer bearing down on me. So much for remaining undetected.

There's a lot to be said for modelling systems with physical laws and energy equations in mind. I'll expand on that in subsequent posts in this thread.

[Chromatix]

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.

[Chromatix]

First, some electrical laws:

Power = volts * amps (as above)
Volts = amps * impedance
Conductance = 1 ÷ impedance

Hence:
Power = volts * volts ÷ impedance
Power = amps * amps * impedance
Impedance = volts ÷ amps
Conductance = amps ÷ volts

Impedances in series sum, as do voltages.
Conductances in parallel also sum, as do currents.
The voltage sum around any loop in a circuit is always zero.
The current sum into any point on a circuit is always zero (currents in always balanced by currents out).

***

The electric motors in both types of propulsion machinery are similar: dual-armature and separately-wound, designed for DC power supply. Their basic characteristics are remarkably similar to those of a Bo-Bo locomotive, so I can base this description closely on practical work I've previously done on these.

A separately-wound motor is typically used in four configurations:

1: With the field connected in parallel with the armature (emulating a "shunt wound" machine), it acts either as a constant-speed motor (with torque varying with voltage & current) or as a generator (when it is turned in the opposite direction to its "motoring" tendency). The latter is normally more useful - this is how a motor can be used to recharge the batteries, using their residual charge to "bootstrap" the field.

2: With the field connected in series with the armature (emulating a "series wound" machine), it acts as a powerful and flexible motor whose torque varies with the square of the armature current, and impedance linearly with the motor speed. The torque direction does not depend on the polarity of the applied power.

3: As 2, but with the field reversed in polarity relative to the armature, the motor acts in the opposite direction with the same characteristics. This would be used to go astern.

4: The field can also be driven and controlled directly and independently of the armature. This is the normal configuration of a dedicated generator (as the armature voltage varies with the product of speed and field strength) or a traction motor being used as a rheostatic brake, but some relatively modern motor applications also make use of this feature for more precise control.

The two armatures on each motor may be connected in series or in parallel, and the two motors may also be connected separately (effectively in parallel) or in series. The slowest but most economical configuration is with armatures and motors in series. The fastest is with armatures and motors in parallel. With motors in parallel and armatures in series, an intermediate speed range is available.

Submarines usually have two main batteries of equal capacity, one mounted forward of the control room and the other aft. These may also be connected in series ("group up" for more voltage) or in parallel ("group down" for longer endurance). They may also instead be connected separately to each motor, or used individually with both motors. Ideally the batteries should be kept at the same state of charge as each other, but this is not always possible; care must be taken to avoid reverse-charging any cells, as this can damage them. The batteries, being almost universally of the basic wet-cell lead-acid type, require significant amounts of distilled water for topping up, and product hydrogen gas when charging which must immediately be vented overboard to prevent an explosion.

The combination for slowest speed and longest endurance is thus "group down, motors and armatures in series"; this is what the engine room selects when "dead slow" or "silent speed" is rung up on the telegraph. Conversely, for flank speed submerged, the configuration would be "group up, motors and armatures in parallel".

However, it is dangerous to start a heavy-duty motor by simply connecting it to power, especially in a high-power configuration; the impedance of a motor at rest is nearly zero, ie. a short-circuit, and can even be negative if the motor is being hot-reversed (as in a "crash stop"). There is thus a set of starting resistances incorporated into the armature series/parallel control switch, limiting the starting current until the motor comes up to speed. The starting resistances are built only for intermittent duty - they cannot safely be used for continuous running at reduced speed.

The engineer must select "Series 1" (armatures in series, all starting resistances in) and wait for the ammeter needle to drop to a safe current, then advance to "Series 2" (cutting out one resistance bank) and wait again, then "Series 3" and finally "Full Series". If in fact parallel operation is required, he then continues in the same form to "Parallel 1", "Parallel 2" and "Full Parallel". On American boats at least, the Parallel positions are on the opposite side of the "neutral" position to the Series positions, so he must go back through the Series starter positions and the neutral while the motor is still spinning.

Once "Full Series" or "Full Parallel" are selected, motor power and speed can be fine-tuned if required by means of the field-weakening control. This shorts out part of the field winding in the motor, resulting in a lower overall impedance and thus higher armature current for a given voltage. Selecting a degree of "weak field" is likely when flank speed is called for, or when trying to match the motors to the engines on a diesel-electric boat at high power settings.

(NB: all of the above is automatically controlled on a railway locomotive. Marine applications, especially naval, tend to do it the hard way instead.)

While when running on batteries, the "group up" and "group down" configurations are the only two supply voltages available, when running on engines the diesel-electric boat has a continuous series of voltages available by varying the field strength of the generators and the speed of the engines. In practice the engine speed is normally set for the maximum continuous rating, and the field strength is then adjusted until the engine torque (measured by the fuel injectors' rack position, which is controlled by the speed governor) required to maintain that speed also corresponds to the maximum continuous rating. If the maximum field strength is reached without sufficiently loading the engine, the field weakening control is used on the motors.

An increase of speed is then normally effected by bringing another engine "on line". To do this, it must first be secured from battery charging (if that is in progress) or started (otherwise). It is then brought up to speed and the field strength adjusted so that the voltage is slightly higher than the bus it is attaching to. Only then is it tied to the bus. At this point it is still taking very little of the load, so the field strength is increased further. The other engines will now be unloaded somewhat and must have *their* field strengths increased to compensate, or the motor field weakened, or even the motors reconfigured to a higher-power combination.

Obviously the reverse process must occur to bring an engine "off line" for any reason.

By contrast, a boat with direct-drive machinery is quite easy to run on diesels. Disengage the engine clutch, start the engine, engage the tail clutch, carefully let in the engine clutch, then adjust the engine speed or power setting until the desired propeller RPM is achieved.

Incidentally, when charging batteries, they are always connected either individually or in "group down", never in "group up". The latter would require twice the generator voltage to effect a charge.

[rentacow]

Good stuff for the devs to think about. At this point I think they really should be leaning toward the higher complexity, more realistic end of the gameplay spectrum given the core audience.

For now, being very early in development I expect things to be refined using input like this.

I know Oscar and Einer will read this so here is my number 1 suggestion:
Let the game be modded! It will take a huge workload off of your shoulders and allow creative players to implement whatever level of complexity they want while you guys can focus on expanding and optimizing the core framework. Mods have arguably been a huge contributing factor to the success of silent hunter and many other open world, simulation or niche genere games.

[Chromatix]

I do believe tis sort of thing has to be incorporated in the core game code to be really effective; we're talking about subtle qualitative effects as well as gross quantitative ones. If it's left to modders, then only the ships that a particular modder works on will get that modder's preferred implementation of the machinery.

However, if done well in core, it actually makes adding new ships and subs easier to add in future, since their performance characteristics tend to naturally "fall out of" a physically-based model, instead of having to be laboriously calculated and tweaked in a process that is inevitably poorly documented.

While we're on the subject, here's something you might not know about steam-powered ships: they have multiple boilers, only some of those boilers are alight and producing steam under normal cruising/patrolling conditions, and it takes a great deal of time to "light off" additional boilers when a need for increased performance is realised.

A diesel engine can be started and put on line in a matter of seconds with a well-practiced crew, but an oil-fired boiler takes several minutes at best - and as much as an hour if the crew are trying to prolong the life of the machinery by avoiding excessive heat stress.

This has implications for how enemy destroyers react when they notice your presence. If they're pottering along at cruising stations, they might have just one (of three) boilers alight for reasons of economy, giving them roughly one-third of rated engine power, corresponding to about 69% of maximum speed (again, the cube law for power against speed is in effect). A typical destroyer nominally capable of 35 knots will thus have an initial reaction speed limited to about 24 knots, and won't accelerate as rapidly to that speed as it might have done on trials. This is potentially enough of a difference to allow a submarine to dive and escape.

As for a coal-fired boiler, as many merchant ships still had in WW2... several hours can be expected to raise steam in a fresh boiler, even under favourable conditions, except for the very smallest marine engines where one hour may suffice. Most coal-fired ships used the standard "Scotch boiler", and simply fitted more of them for increased power output.

[Onkel Neal]

Thank for the data, Chromatix, very good resource.

As you pointed out, there will be some compromises for playability. No one wants to spend 6 hours patrolling without contacts or engagements.

In previous subsims, the player either a. was on patrol in a zone where contact was likely or b. sailed across the ocean ("go anywhere I want") for hours waiting for contact, and employing 2056x time compression, which effectively had him using option a.

The main thing most players want is a dynamic campaign, where the ships and planes one encounters appear in a non-scripted, random fashion. With HMS Marulken and its focus on team play, I would prefer patrol zones with random ship placement, and random waypoints. I would not be interested in getting three guys in a game and then spending 2 hours trying to make contact. 30 minutes, ok, but the longer the missions, the more likely a member of the crew will need to leave for real life affairs.

I think the devs know what they are doing and so far, so good.