In the Engine Room (build 144)

Chromatix · 06-06-16, 06:52 PM

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 · 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.

Chromatix · 06-06-16, 11:32 PM

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 · 06-07-16, 05:28 AM

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 · 06-07-16, 08:26 AM

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 · 06-08-16, 09:00 AM

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.

Chromatix · 06-08-16, 10:29 AM

I'm not sure I made any comment about the game format in the above. But since it's been brought up:

There is indeed a definite and fundamental distinction between a "mission based" game such as Dangerous Waters and an "open world" one such as Silent Hunter. It's generally a lot easier to tell a story with the mission-based format, since the player is ensured to be in the right place at the right time every time. However, I consider the open-world format to be more immersive and to offer more replay value.

In the spaceflight genre, a similar distinction can be seen between the likes of Freespace and Wing Commander, which are mission-based, versus Elite and X-Universe which are open-world. The X-Universe games make a heroic effort to impose a storyline on the gameplay, which is not entirely successful and often merely interferes with the player's own goals; Elite prefers to provide a universe and let the player do whatever he wants with it.

One factor which makes the mission-based format the obvious choice for HMS Marulken is that it's much harder to use time compression in a multiplayer game, since either all players must experience the same time compression, or they will get out of step (which may offer unfair advantages). This is doubly so when not all the players are in the same ship or even the same squadron/fleet.

With that said, I don't think it's impossible to make a multi-crew subsim in open-world format. Time compression could be handled by simply handing over the conn to the (AI) Officers of the Watches while at cruising stations, whose standing orders would be to alert the senior officers (that is, the players) when something interesting happened, while pre-emptively taking any immediately necessary evasive action. Contacts would then be tracked in from the edge of detectability, and a significant part of the gameplay would be conducting approaches.

HMS Marulken is obviously intended to be a very storyline-driven game. However, perhaps the technology (or at least the experience gained) could be adapted to a future open-world game along these lines.

Beardmoresam · 06-08-16, 02:36 PM

I have to agree with Chromatix in that there is something to be said for building on the real physics right from the beginning. In particular, I would like to talk about buoyancy as I feel Chromatix has got the propulsion line taken care of.

I do not believe it would be too difficult to manage buoyancy simulation even with the tens of tanks built into the submarines of the era. Indeed buoyancy and trim were and remain easily calculated by hand and are therefore well within our computers abilities.

Although no engineer my self, I would be glad to throw out some equations or perhaps even a working algorithm if required.

Chromatix · 06-08-16, 04:26 PM

I suppose it fits the theme of the thread, since the diving controls are currently in the engine room. In fact I was already considering writing something on the subject. Let me have a stab at it:

Fresh water has a density of (almost exactly) 1kg per litre, or 1 metric tonne per cubic metre. In fact that was part of the original basis of the definition of metric units. Sea water density is usually a little higher, which means that a given object immersed in it will be a few percent more buoyant. In a submarine context this is potentially very significant. This is also why cargo ships have several different safe waterlines marked on the side, in case they plan to travel between areas of differing water density.

Baltic Sea water is a lot closer to "fresh" than ocean water, because it has several large rivers flowing into it (the catchment areas of most of Eastern Europe and almost all of Northern Europe) and only the narrow and shallow Øresund, Lillebælt and Storebælt straits as an outflow. This flow actually does reverse about twice a decade on average, when spring tides coincide with strong winds in the right direction, but almost all the time it flows outward, into the North Sea, to some degree. Since HMS Marulken is supposedly set (almost?) entirely in the Baltic, this implies buoyancy figures must be based on Baltic-specific figures rather than the usual oceanic ones.

Marulken's developers could reasonably assume that only one seawater density exists (that of the Baltic), but more ambitious developers might want to consider that water in ballast tanks might have a different density to the surrounding seawater, if the sub has moved into a different density since the tanks were flooded. This can easily occur during a dive, as density layers sometimes exist and were reported to act as sub-surface "support" depths which it was more difficult to dive past. It can also occur if entering a river outflow.

Once you know the density of the water and the location and volume of each ballast tank, the weight and balance contribution of the water in them is easy to calculate. Likewise the buoyancy contribution of the outer hull. Beware the "free surface effect" in partly-filled tanks; fortunately MBTs are almost always completely full or completely empty, so there is no free-surface effect in them. Flooded compartments however will very much be subject to it.

Ideally a dived submarine should have zero net buoyancy. Most of the difference between the strong positive buoyancy required to stay on the surface and this dived condition is achieved by completely flooding the main ballast tanks. This effectively reduces the buoyant volume to that of the pressure hull (assuming a double-hulled or saddle-tank boat, with the MBTs outside the pressure hull). This is still significantly more than required the balance the weight of the boat though, so the safety tank (if fitted) is also flooded and the trim tanks are partly flooded to fine-tune the buoyancy.

The trim tanks are located at the extreme ends of the boat and are used not only to obtain zero net buoyancy, but to achieve correct fore-and-aft balance as well. They are usually kept in their dive-adjusted condition even while surfaced, so that they don't need to be set up again for every dive. Sea water could be pumped into, out of, or between the trim tanks at will, but the pump used for this was often rather noisy and could potentially give away the sub's position. Using compressed air to "blow" the tank made less noise, but this was in limited supply.

It was normal practice to carry out a "trim dive" immediately after leaving port to perform this adjustment, since taking on fuel, ammunition, supplies and exchanging some of the men would have substantially changed the sub's overall weight and balance. This "trim dive" would be repeated at regular intervals thereafter if no other reason to dive was found, so that the sub would always be ready to dive safely if required. Such dives were also useful for training purposes.

Fuel was usually carried in large external tanks very like the MBTs, and as it was used, it was displaced by water from below. This mostly compensated for the change in submerged weight due to consumed fuel. When a fuel tank was completely drained, some of them could be converted into normal MBTs, improving surfaced buoyancy.

The centre of buoyancy must be somewhat above the centre of mass in order to keep the boat upright. This condition must be maintained both when surfaced and when dived, and for preference even having taken damage. The term "metacentric height" is key here, and there is much literature on the subject.

Blowing the MBTs using compressed air while submerged would only be done to effect a very rapid surfacing, either in an emergency or as part of a "battle surface" for a gun action. The consumption of compressed air would be very high, and it would be prudent to be sure of recharging this supply before needing to dive again.

For normal surfacing, the dive planes would be put on full rise and the safety tank would be blown, putting enough of the sub above water to run the engines and low-pressure compressors; this low-pressure air would be used to empty the MBTs.

To dive safely, it is absolutely vital that all openings in the pressure hull are sealed shut. As well as manual checks and automatic indicators, it was usual to let a little compressed air into the boat and observe on the barometer (and the crew's eardrums) whether it was held in. This indication was robust even in the face of failure of the other two checks, and gave much confidence when making a rapid dive under combat conditions.

To make a "crash dive", British and American subs had a "negative buoyancy" tank which was normally kept empty, but could be flooded and blown independently of the trim system. There was a particular "fill mark" corresponding to the empty state; some water was kept in it so that blowing it "to the mark" wouldn't leak too many bubbles which might be spotted. German U-boats didn't have such a tank, but instead had most of the men run through the boat into the forward torpedo room in order to quickly get a down-angle on the boat. Obviously they had to run back again when normal trim was again required.

So much for static buoyancy. As mentioned earlier, depth control was primarily by means of the dive planes, which on a WW2 sub were almost always provided both forward and aft. On the surface, the fore planes were usually folded. Obviously they had no effect when stopped, so the buoyancy tanks had to be used instead.

Japanese submarines often had an automatic trimming system for depth control while stopped, but it tended to "hunt" badly (ie. did not settle on a stable state) and, because it relied on the trim pumps, made a lot of noise. Manual control of the trim made much more economical use of the pumps, so was both quieter and more effective.

Dive planes were normally sized to be capable of providing several tons of up or down force at silent-running speeds. This was sufficient to correct a considerable degree of error in the trim, which was an important safety consideration when performing a trim dive or performance damage control. Obviously, greater dive plane forces were available at higher speeds, so if a large trim error was present, the dive officer could request (and usually got) a higher speed for purposes of depth control.

IIRC, the plane forces scale with the square of the speed, except that they also reverse sense (like the rudder) when moving astern. This latter effect (and others) can be accounted for by remembering that the angle of attack of the plane on the water flow is what matters, not the absolute angle setting of the planes. This also applies to the rudder and the propeller blades!

The stern planes primarily have the effect of changing the *angle* of the boat, in much the same way as the rudder does on course. Obviously when at an angle, the hull itself acts as a giant (if somewhat inefficient) dive plane, and the propellers' thrust angle also changes. Angle is normally shown on an inclinometer consisting of a curved, fluid-filled tube with a bubble in it; hence the term "bubble" used for angle in USN phraseology.

The fore planes obviously act primarily on the front half of the boat, but if the stern planesman is instructed to maintain a level angle (as he normally would when near periscope depth, to avoid broaching), he will naturally end up following the fore plane setting, making the fore planesman primarily in control of *depth* rather than angle.

There are normally three different types of depth gauge in the boat, duplicated wherever required. One gives a very precise reading of shallow depths (covering optimum periscope and listening depths), another is calibrated for the full test depth of the boat (and then some), and there are also direct readings of sea pressure, not calibrated for depth, which are provided in other compartments than the control room.

Compensation and impulse tanks were provided forward to assist with firing torpedoes, and to counter the sudden change in trim occasioned by the torpedo leaving the tube. The first submarines capable of deploying weapons discovered the need for this the hard way, as they tended to stand on their tails immediately after detaching them! The compensation tank was generally drained into the forward trim tank so that another torpedo could be fired using it. If aft tubes were fitted, the compensation and impulse tanks would be duplicated there for obvious reasons.

Onkel Neal · 06-08-16, 04:48 PM

Quote:

Originally Posted by Beardmoresam

I have to agree with Chromatix in that there is something to be said for building on the real physics right from the beginning. In particular, I would like to talk about buoyancy as I feel Chromatix has got the propulsion line taken care of.

I do not believe it would be too difficult to manage buoyancy simulation even with the tens of tanks built into the submarines of the era. Indeed buoyancy and trim were and remain easily calculated by hand and are therefore well within our computers abilities.

Although no engineer my self, I would be glad to throw out some equations or perhaps even a working algorithm if required.

No doubt the physics should approximate reality. This is a demo, right? The final game would have lifelike submarine characteristics.

Beardmoresam · 06-09-16, 05:28 AM

Quote:

Originally Posted by Onkel Neal

No doubt the physics should approximate reality. This is a demo, right? The final game would have lifelike submarine characteristics.

I would certainly be glad if it did.

As you quite rightly point out, it's early days and there is only so much you can do at once. I do still think it would be best to get these fundamentals in from the beginning to avoid generating loads of work redesigning the entire way the submarine works once it's all refined and all the tasks that would imply.

As well demonstrated by Chromatix, there is a significant wealth of knowledge throughout the community which can be readily drawn upon.

If the developers are interested, the community may find it more beneficial to gather the information they have in a more organised and structured manner, perhaps using google docs or a similar tool that everyone can contribute to and that facilitates its use by the developers.

If there is an appetite for a SUBSIM library as it were, I'd be happy to throw together a prototype when permitted by my work commitments.

rentacow · 06-09-16, 02:04 PM

For anyone interested, this video is one of the best i have found which explains the mechanical workings of a sub from the 40s.

Chromatix · 06-09-16, 03:43 PM

I suppose the GUPPY conversions (post-war) were technically begun in the 40s. Fleet boats didn't have snorkels or streamlined superstructure until then. Most of the other details remained the same though.

It should also be remembered that the American fleet boats were probably the most sophisticated submarines in service at the time, and did not reflect the general state of submarine technology in other countries, which is what a Swedish "black project" would have drawn on. The German U-boats (aside from the Type XXI) largely reflected First World War design principles, but with updated materials and construction techniques.

Chromatix · 06-11-16, 12:09 AM

So, seawater density depends on salinity, temperature, and to a lesser extent depth (due to pressure; water is not *quite* incompressible, though close).

It's fairly well-known that fresh water has a maximum density at +4°C, which is why rivers and lakes usually don't freeze all the way to the bottom. This density maximum decreases more rapidly with increasing salinity than the freezing point does, until at about 23 promille, it theoretically lies below the freezing point.

Normal ocean salinity varies from 33 promille (arctic regions) to 36 promille (tropics), the average figure being 34.7 promille. However the Mediterranean Sea (38 promille) and Red Sea (40 promille) are noticeably more saline, while the Black Sea (18 promille) and Baltic Sea (8 promille) are so much less saline that they are officially classed as "brackish", in the same category as river estuaries. This also puts the Baltic Sea at a density profile closer to that of fresh water, including having a density peak somewhere above freezing point (which is why the Baltic freezes over relatively easily, but not deeply).

For a game restricted to the Baltic Sea, the above matters little; I mention it for completeness sake. It's also perhaps worth mentioning that the Baltic has very little tidal activity, though it does sometimes experience local level changes due to storm surges. Tides and currents don't appear in Silent Hunter either, but that's not because they shouldn't be there!

Another significant effect for submarines is that the pressure hull physically contracts under pressure at depth, which is why it creaks when changing depth. This contraction, which could quite easily be measured by the crew, has a significant effect on the sub's submerged buoyancy, and may imply that trim changes must be made if a significant amount of time is to be spent at deep submergence. It was usual to carry a small degree of positive buoyancy for safety reasons, except when actively "hovering" at zero speed, and this will at least partly compensate for the loss of buoyancy due to hull contraction.

As for the maximum diving depth, this was usually quite a lot deeper than the "test depth" officially published. Many designs incorporated a large margin in the pressure hull design strength, so that the sub would have a good chance of surviving a depth charge attack while at test depth. For example, the test depth of British subs was often 300 feet while the actual design depth was 500 feet. Commanders often exploited this difference, preferring to run deeper in order to *avoid* the depth charge attack, rather than counting on the strength of the hull to survive it.

In most of the American "fleet boat" designs, there was a common weakness in that the vent risers for the foremost and aftmost MBTs (located below rather than around the pressure hull) passed through the pressure hull, appearing as pipes (at sea pressure!) within the torpedo rooms. That's probably why you see pipes often being the first point of failure in movies involving a WW2 era sub, requiring frantic damage control activity to stop the resultant flooding. This weakness was corrected in the last of the fleet-boat classes, which however entered service only in the last months of the war, and it seems unlikely that many other navies' designs would have incorporated the same weakness.

06-06-16, 06:52 PM	#1
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	In the Engine Room (build 144) 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.

06-08-16, 09:00 AM	#6
Onkel Neal Born to Run Silent Join Date: Jan 1997 Location: Cougar Trap, Texas Posts: 21,363 Downloads: 541 Uploads: 224	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. __________________ SUBSIM - 26 Years on the Web

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.

06-06-16, 11:32 PM	#3
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	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.

06-07-16, 05:28 AM	#4
rentacow Machinist's Mate Join Date: Sep 2014 Location: Onakushcloud Posts: 126 Downloads: 58 Uploads: 0	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.

06-07-16, 08:26 AM	#5
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	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.

06-08-16, 10:29 AM	#7
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	I'm not sure I made any comment about the game format in the above. But since it's been brought up: There is indeed a definite and fundamental distinction between a "mission based" game such as Dangerous Waters and an "open world" one such as Silent Hunter. It's generally a lot easier to tell a story with the mission-based format, since the player is ensured to be in the right place at the right time every time. However, I consider the open-world format to be more immersive and to offer more replay value. In the spaceflight genre, a similar distinction can be seen between the likes of Freespace and Wing Commander, which are mission-based, versus Elite and X-Universe which are open-world. The X-Universe games make a heroic effort to impose a storyline on the gameplay, which is not entirely successful and often merely interferes with the player's own goals; Elite prefers to provide a universe and let the player do whatever he wants with it. One factor which makes the mission-based format the obvious choice for HMS Marulken is that it's much harder to use time compression in a multiplayer game, since either all players must experience the same time compression, or they will get out of step (which may offer unfair advantages). This is doubly so when not all the players are in the same ship or even the same squadron/fleet. With that said, I don't think it's impossible to make a multi-crew subsim in open-world format. Time compression could be handled by simply handing over the conn to the (AI) Officers of the Watches while at cruising stations, whose standing orders would be to alert the senior officers (that is, the players) when something interesting happened, while pre-emptively taking any immediately necessary evasive action. Contacts would then be tracked in from the edge of detectability, and a significant part of the gameplay would be conducting approaches. HMS Marulken is obviously intended to be a very storyline-driven game. However, perhaps the technology (or at least the experience gained) could be adapted to a future open-world game along these lines.

06-08-16, 02:36 PM	#8
Beardmoresam A-ganger Join Date: Oct 2011 Posts: 74 Downloads: 437 Uploads: 0	I have to agree with Chromatix in that there is something to be said for building on the real physics right from the beginning. In particular, I would like to talk about buoyancy as I feel Chromatix has got the propulsion line taken care of. I do not believe it would be too difficult to manage buoyancy simulation even with the tens of tanks built into the submarines of the era. Indeed buoyancy and trim were and remain easily calculated by hand and are therefore well within our computers abilities. Although no engineer my self, I would be glad to throw out some equations or perhaps even a working algorithm if required.

06-08-16, 04:26 PM	#9
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	I suppose it fits the theme of the thread, since the diving controls are currently in the engine room. In fact I was already considering writing something on the subject. Let me have a stab at it: Fresh water has a density of (almost exactly) 1kg per litre, or 1 metric tonne per cubic metre. In fact that was part of the original basis of the definition of metric units. Sea water density is usually a little higher, which means that a given object immersed in it will be a few percent more buoyant. In a submarine context this is potentially very significant. This is also why cargo ships have several different safe waterlines marked on the side, in case they plan to travel between areas of differing water density. Baltic Sea water is a lot closer to "fresh" than ocean water, because it has several large rivers flowing into it (the catchment areas of most of Eastern Europe and almost all of Northern Europe) and only the narrow and shallow Øresund, Lillebælt and Storebælt straits as an outflow. This flow actually does reverse about twice a decade on average, when spring tides coincide with strong winds in the right direction, but almost all the time it flows outward, into the North Sea, to some degree. Since HMS Marulken is supposedly set (almost?) entirely in the Baltic, this implies buoyancy figures must be based on Baltic-specific figures rather than the usual oceanic ones. Marulken's developers could reasonably assume that only one seawater density exists (that of the Baltic), but more ambitious developers might want to consider that water in ballast tanks might have a different density to the surrounding seawater, if the sub has moved into a different density since the tanks were flooded. This can easily occur during a dive, as density layers sometimes exist and were reported to act as sub-surface "support" depths which it was more difficult to dive past. It can also occur if entering a river outflow. Once you know the density of the water and the location and volume of each ballast tank, the weight and balance contribution of the water in them is easy to calculate. Likewise the buoyancy contribution of the outer hull. Beware the "free surface effect" in partly-filled tanks; fortunately MBTs are almost always completely full or completely empty, so there is no free-surface effect in them. Flooded compartments however will very much be subject to it. Ideally a dived submarine should have zero net buoyancy. Most of the difference between the strong positive buoyancy required to stay on the surface and this dived condition is achieved by completely flooding the main ballast tanks. This effectively reduces the buoyant volume to that of the pressure hull (assuming a double-hulled or saddle-tank boat, with the MBTs outside the pressure hull). This is still significantly more than required the balance the weight of the boat though, so the safety tank (if fitted) is also flooded and the trim tanks are partly flooded to fine-tune the buoyancy. The trim tanks are located at the extreme ends of the boat and are used not only to obtain zero net buoyancy, but to achieve correct fore-and-aft balance as well. They are usually kept in their dive-adjusted condition even while surfaced, so that they don't need to be set up again for every dive. Sea water could be pumped into, out of, or between the trim tanks at will, but the pump used for this was often rather noisy and could potentially give away the sub's position. Using compressed air to "blow" the tank made less noise, but this was in limited supply. It was normal practice to carry out a "trim dive" immediately after leaving port to perform this adjustment, since taking on fuel, ammunition, supplies and exchanging some of the men would have substantially changed the sub's overall weight and balance. This "trim dive" would be repeated at regular intervals thereafter if no other reason to dive was found, so that the sub would always be ready to dive safely if required. Such dives were also useful for training purposes. Fuel was usually carried in large external tanks very like the MBTs, and as it was used, it was displaced by water from below. This mostly compensated for the change in submerged weight due to consumed fuel. When a fuel tank was completely drained, some of them could be converted into normal MBTs, improving surfaced buoyancy. The centre of buoyancy must be somewhat above the centre of mass in order to keep the boat upright. This condition must be maintained both when surfaced and when dived, and for preference even having taken damage. The term "metacentric height" is key here, and there is much literature on the subject. Blowing the MBTs using compressed air while submerged would only be done to effect a very rapid surfacing, either in an emergency or as part of a "battle surface" for a gun action. The consumption of compressed air would be very high, and it would be prudent to be sure of recharging this supply before needing to dive again. For normal surfacing, the dive planes would be put on full rise and the safety tank would be blown, putting enough of the sub above water to run the engines and low-pressure compressors; this low-pressure air would be used to empty the MBTs. To dive safely, it is absolutely vital that all openings in the pressure hull are sealed shut. As well as manual checks and automatic indicators, it was usual to let a little compressed air into the boat and observe on the barometer (and the crew's eardrums) whether it was held in. This indication was robust even in the face of failure of the other two checks, and gave much confidence when making a rapid dive under combat conditions. To make a "crash dive", British and American subs had a "negative buoyancy" tank which was normally kept empty, but could be flooded and blown independently of the trim system. There was a particular "fill mark" corresponding to the empty state; some water was kept in it so that blowing it "to the mark" wouldn't leak too many bubbles which might be spotted. German U-boats didn't have such a tank, but instead had most of the men run through the boat into the forward torpedo room in order to quickly get a down-angle on the boat. Obviously they had to run back again when normal trim was again required. So much for static buoyancy. As mentioned earlier, depth control was primarily by means of the dive planes, which on a WW2 sub were almost always provided both forward and aft. On the surface, the fore planes were usually folded. Obviously they had no effect when stopped, so the buoyancy tanks had to be used instead. Japanese submarines often had an automatic trimming system for depth control while stopped, but it tended to "hunt" badly (ie. did not settle on a stable state) and, because it relied on the trim pumps, made a lot of noise. Manual control of the trim made much more economical use of the pumps, so was both quieter and more effective. Dive planes were normally sized to be capable of providing several tons of up or down force at silent-running speeds. This was sufficient to correct a considerable degree of error in the trim, which was an important safety consideration when performing a trim dive or performance damage control. Obviously, greater dive plane forces were available at higher speeds, so if a large trim error was present, the dive officer could request (and usually got) a higher speed for purposes of depth control. IIRC, the plane forces scale with the square of the speed, except that they also reverse sense (like the rudder) when moving astern. This latter effect (and others) can be accounted for by remembering that the angle of attack of the plane on the water flow is what matters, not the absolute angle setting of the planes. This also applies to the rudder and the propeller blades! The stern planes primarily have the effect of changing the angle of the boat, in much the same way as the rudder does on course. Obviously when at an angle, the hull itself acts as a giant (if somewhat inefficient) dive plane, and the propellers' thrust angle also changes. Angle is normally shown on an inclinometer consisting of a curved, fluid-filled tube with a bubble in it; hence the term "bubble" used for angle in USN phraseology. The fore planes obviously act primarily on the front half of the boat, but if the stern planesman is instructed to maintain a level angle (as he normally would when near periscope depth, to avoid broaching), he will naturally end up following the fore plane setting, making the fore planesman primarily in control of depth rather than angle. There are normally three different types of depth gauge in the boat, duplicated wherever required. One gives a very precise reading of shallow depths (covering optimum periscope and listening depths), another is calibrated for the full test depth of the boat (and then some), and there are also direct readings of sea pressure, not calibrated for depth, which are provided in other compartments than the control room. Compensation and impulse tanks were provided forward to assist with firing torpedoes, and to counter the sudden change in trim occasioned by the torpedo leaving the tube. The first submarines capable of deploying weapons discovered the need for this the hard way, as they tended to stand on their tails immediately after detaching them! The compensation tank was generally drained into the forward trim tank so that another torpedo could be fired using it. If aft tubes were fitted, the compensation and impulse tanks would be duplicated there for obvious reasons.

06-09-16, 02:04 PM	#12
rentacow Machinist's Mate Join Date: Sep 2014 Location: Onakushcloud Posts: 126 Downloads: 58 Uploads: 0	For anyone interested, this video is one of the best i have found which explains the mechanical workings of a sub from the 40s.

06-09-16, 03:43 PM	#13
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	I suppose the GUPPY conversions (post-war) were technically begun in the 40s. Fleet boats didn't have snorkels or streamlined superstructure until then. Most of the other details remained the same though. It should also be remembered that the American fleet boats were probably the most sophisticated submarines in service at the time, and did not reflect the general state of submarine technology in other countries, which is what a Swedish "black project" would have drawn on. The German U-boats (aside from the Type XXI) largely reflected First World War design principles, but with updated materials and construction techniques.

06-11-16, 12:09 AM	#14
Chromatix Watch Join Date: May 2011 Posts: 26 Downloads: 11 Uploads: 0	So, seawater density depends on salinity, temperature, and to a lesser extent depth (due to pressure; water is not quite incompressible, though close). It's fairly well-known that fresh water has a maximum density at +4°C, which is why rivers and lakes usually don't freeze all the way to the bottom. This density maximum decreases more rapidly with increasing salinity than the freezing point does, until at about 23 promille, it theoretically lies below the freezing point. Normal ocean salinity varies from 33 promille (arctic regions) to 36 promille (tropics), the average figure being 34.7 promille. However the Mediterranean Sea (38 promille) and Red Sea (40 promille) are noticeably more saline, while the Black Sea (18 promille) and Baltic Sea (8 promille) are so much less saline that they are officially classed as "brackish", in the same category as river estuaries. This also puts the Baltic Sea at a density profile closer to that of fresh water, including having a density peak somewhere above freezing point (which is why the Baltic freezes over relatively easily, but not deeply). For a game restricted to the Baltic Sea, the above matters little; I mention it for completeness sake. It's also perhaps worth mentioning that the Baltic has very little tidal activity, though it does sometimes experience local level changes due to storm surges. Tides and currents don't appear in Silent Hunter either, but that's not because they shouldn't be there! Another significant effect for submarines is that the pressure hull physically contracts under pressure at depth, which is why it creaks when changing depth. This contraction, which could quite easily be measured by the crew, has a significant effect on the sub's submerged buoyancy, and may imply that trim changes must be made if a significant amount of time is to be spent at deep submergence. It was usual to carry a small degree of positive buoyancy for safety reasons, except when actively "hovering" at zero speed, and this will at least partly compensate for the loss of buoyancy due to hull contraction. As for the maximum diving depth, this was usually quite a lot deeper than the "test depth" officially published. Many designs incorporated a large margin in the pressure hull design strength, so that the sub would have a good chance of surviving a depth charge attack while at test depth. For example, the test depth of British subs was often 300 feet while the actual design depth was 500 feet. Commanders often exploited this difference, preferring to run deeper in order to avoid the depth charge attack, rather than counting on the strength of the hull to survive it. In most of the American "fleet boat" designs, there was a common weakness in that the vent risers for the foremost and aftmost MBTs (located below rather than around the pressure hull) passed through the pressure hull, appearing as pipes (at sea pressure!) within the torpedo rooms. That's probably why you see pipes often being the first point of failure in movies involving a WW2 era sub, requiring frantic damage control activity to stop the resultant flooding. This weakness was corrected in the last of the fleet-boat classes, which however entered service only in the last months of the war, and it seems unlikely that many other navies' designs would have incorporated the same weakness.