Submarine Dive Technology

(c) 2001, Johan J. Heiszwolf

Contents

Live Scale Dive Technology
    Static Diving
    Dynamic Diving
    Aft Hydroplanes
Model Submarine Dive Technology
    Dynamic Diving Technology
    Static Diving Technology
       Vented ballast tank
      Flexible ballast tank
        Pressure ballast tank
      Piston ballast tank
      Membrane ballast tank
      Bellow ballast tank
      Gas operated ballast tank
      Compressed air ballast tank
      Note on gas ballast tanks
    Hybrid Ballast Systems
Conclusions
Literature

Live Scale Dive Technology
    Basically, there are two ways to submerge a boat: dynamic diving and static diving. Many model submarines use the dynamic method while static diving is used by all military submarines. Dynamic diving boats are submarines that inherently float that is, they always have a positive buoyancy. This type of boat is made to dive by using the speed of the boat in combination with the dive planes to force the boat under water. This is very similar to the way airplanes fly. Static diving submarines dive by changing the buoyancy of the boat itself by letting water into ballast tanks. The buoyancy is thereby changed from positive to negative and the boats starts sinking. These boats do not require speed to dive hence this method is called static diving.
Modern military submarines dive use a combination of dynamic and static diving. The boat submerges by filling the main ballast tanks with water. After that, the buoyancy is accurately adjusted with the trim tanks. Once underwater, the depth of the boat is controlled with the hydroplanes.
In the following, the dive methods are treated in detail. We will start with static diving because this is more important for real submarines.

 

Static Diving
    The buoyancy of a submarine can be changed by letting water into the main ballast tanks (MBT). The MBT's can be located in three different ways: (a) inside the pressure hull, (b) outside the pressure hull as additional tanks, and (c) in between the outer hull and the pressure hull. Figure 1 shows the three possible configurations. Drawback of having the MBT inside the pressure hull is obvious: it takes up space that could otherwise be used for equipment, weapons or personnel. This MBT arrangement was used in the WW-I boats and other early submarines. The classical example of a boat with MBT's outside the pressure hull is the German Type VIIC but also American and Dutch submarines in WW-II used this design. Due to the location of the MTB's, they are called saddle tanks. Most modern military submarines use the space in-between the inner pressure hull and the outer hull as MBT.
 

Figure 1: Different locations of the main ballast tank.

There are two different ways the MBT's can be emptied and filled. These methods will be referred to as the the western (USA, UK) method and the Russian method. Please note that the 'Russian' method is not exclusively Russian because it was also used by for example the Dutch triple hull Dolfijn class boats. Figure 3 depicts the both methods, the left hand side of the pictures shows the USA/US method, the right hand side the Russian method. When surfaced, the MBT are entirely filled with air and the main vent valves on top of the MBT are closed. In the USA/UK boats the flood opening at the bottom of the MBT always remains open. Water is prevented to enter the MBT because the air in the MBT is pressurized, at about 10 PSI. In the Russian boats, the bottom flood opening is closed with a valve, a so called Kingston. Because the Kingston prevents water entering, air in the MBT can be at approximately atmospheric pressure. To dive the boat, the vent valves on top of the ballast tanks are opened to let air escape the MBT. Because in the USA/UK boats the air is pressurized, the air roars out of the vents, resulting in a large spray of water, see Figure 2.
 

Figure 2: An Ohio class submarine venting  the forward ballast tanks.

In the Russian technology, the Kingstons at the bottom of the MBT also have additionally to be opened in order to let water enter the MBT. It is claimed that because the air in the USA/UK boats is pressurized (more gas in the MBT and larger friction in the vent valves) the Russian MBT is flooded more quickly.
 

Figure 3: Flooding of the main ballast tanks.

To surface the boat, the water in the MBT's is forced out by pressurized air. When the boat is deeply submerged, the water is forced out using high pressure air to overcome the water pressure. Once the boat is near the surface, the blowing of the MBT's proceeds with low pressure air. Once at the surface, the Russian boats close the Kingston valve and then opens the main vent valve briefly to equalize the air pressure in the MBT with that of the atmosphere. In the USA/UK boats, the main vent valve remains shut to keep the air in the MBT under pressure. The pressure inside the tanks remains equal to that of the low pressure air system.
 

Figure 4: Blowing of the main ballast tanks.

Figure 5 shows the location of the MBT's in a modern diesel electric submarine. The bulk of the MBT's are located at the bow and aft sections of the boat and a small MBT surrounds the pressure hull in the center of the boat. A large portion of the space between the pressure hull and the outer hull is occupied by the fuel tanks. It is important to note that the MBT is only used to change the buoyancy of the boat from very positive (the boat is surfaced) to slightly positive (the boat is just still on the surface, decks awash this is called). The optimal rig for a submerged boat is neutral buoyancy: the boat neither floats nor sinks. This situation is accomplished by the use of the main trim tanks (MTT) located in the center of the boat. Once the MBT is full of water, the MTT is carefully filled with water until a neutral buoyancy is obtained.  For a submarine with a given weight,  the amount of water required inside the MTT depends on for example the salt content and the temperature of the surrounding water. Maintaining neutral buoyancy in a submarine is a continuous procedure. For example the diesel engines consume fuel and the personnel eats food so that the total weight of the boat steadily decreases during a mission. This means that while progressing with the mission, the amount of water in the MTT has to be increased to maintain neutral buoyancy.
Also the density of the surrounding water plays an important role. A well known example is the downstream area of a river where fresh and salt water mix leading to a different density than in the open sea. If a submarine enters such a region, the trim has to be adjusted. For military submarines an obvious action that changes the buoyancy of the boat is the launch of a torpedo. For this purpose, military submarines have a special ballast tank located in the vicinity of the torpedo room to compensate for the weight loss of the torpedo. Usually the water level in the MTT is adjusted using high pressure pumps rather than high pressure air because the latter makes much more noise. Some of the MTT tanks can however be emptied using pressurized air to get a quick blow in case of an emergency. Once a neutral buoyancy is obtained with the MTT's, the depth of the boat can be changed using the speed of the boat and the angle of the dive planes. This is thus dynamic diving, see below.
 

Figure 5: Location of the different tanks in a modern diesel electric boat. Picture adapted from Gabler (1987).

At neutral buoyancy conditions, it is also important that the submarine maintains a horizontal angle. For this purpose the submarine is equipped with two sets of trim tanks located in the bow and aft section of the boat. Both fore and aft trim tanks are connected with a line so that water can be pumped back and forth to obtain the required horizontal angle of the boat. Further note that in the military submarine of Figure 5 a large section of the boat is free flooded. With the use of the free flooding sections, the overall size of the ballast tanks can be kept to a minimum.

Dynamic Diving
    Once the boat is trimmed to more or less neutral buoyancy, the depth of the boat is controlled with the hydroplanes. To use the hydroplanes the boat requires speed to create a force on the tilted planes. At slow speeds, the fore hydroplanes are exclusively used to keep the boat at the required depth. The fore planes can be located on the hull near the bow or on the sail of the boat. Because bow mounted hydroplanes are located further from the center of gravity, the depth control is more accurate with these types. Arguments for locating the fore planes on the finn of the boat are (a) improved performance of the spherical sonar array in the bow because the fore hydroplanes generate noise and (b) bow mounted hydroplanes can be damaged during docking of the submarine. Penalties for placing the fore planes on the fin are (a) the operating gear takes up space in the fin where room badly is needed for the masts, (b) the ice breaking performance is decreased, (c) at periscope depth the planes are close to the surface so their performance is adversely affected by the surface turbulence and finally (d) the hydroplanes are closer to the center of gravity and are thus less effective. Note that while improving the Los Angeles class submarine (688I) the US Navy relocated the fore planes from the sail to the bow. At sufficiently high submerged speed (more than 12 knots), the fore planes are no longer needed to control the depth of the submarine. At these speeds, they are rotated in a neutral or slightly dive position. Because the fore planes generate noise, many submarines are capable of retracting the forward bow planes at high speeds. All this considering, we may conclude that (retractable) bow planes are more favorable. It may be added that the author is not aware of boats having both dive planes on the bow and on the sail.
 

Figure 6: Location of the dive planes and their angles during the dive of Figure 7.

Figure 7 shows how the fore and aft dive planes are used during a dive. At the start of the dive the aft plane is rotated upwards so that the stern of the boat is forced upwards. The fore hydroplanes are rotated downwards thus forcing the bow of the boat down. During the dive the aft hydro planes are moved to the neutral position and the dive angle is controlled with the fore hydroplanes only. Close to the the required depth, the aft planes are rotated down and the fore planes up to level off the boat. At slow speeds the depth of the boat is maintained by the fore planes only.  During the first dive, the water level in the main trim tanks is adjusted to obtain a neutral buoyancy so that the required depth can be maintained with a nearly horizontal position of the hydroplanes.

 

Figure 7: Angle of the dive planes during a drive.

Aft Hydroplanes
    Figure 8 shows the positioning of the aft hydroplanes as used by military submarines. Type A is the configuration applied by many modern military submarines. The hydroplanes are located in front of the screw. Note that the rudder blades are of different size. The bottom plane is smaller than the top one so that the boat can be put on the bottom of the sea (bottoming). Types B and C have the hydroplanes behind the screw. This is a configuration used by older submarines. The hydroplanes behind the screw are still used by the double screw Russian Tango and India class boats. The arrangement of D has the rudder behind the screw but has the dive planes in front of it. This type of arrangement was used for the German 205 and 206 class boats. Type E has the hydroplanes tilted 45 degrees, the so called X-tail configuration. No distinction between the rudder and the dive planes can be made. To steer and dive the boat, all of the four hydroplanes are used. In old submarines each set of hydroplanes, fore dive, aft dive and rudder, were operated by a separate person that manually turned a control wheel to the desired angle. It is obvious that an X-tail can only be operated by electronics or computer control. Because all four planes are used for both horizontal and vertical movement, the control of the boat is more subtle. Due to the 45 degrees tilting of the hydroplanes, bottoming is made possible without having to decrease the size of the lower dive planes. The X-tail configuration is used by the Dutch Walrus (Figure 9), the Swedish Vatergotland and the Australian Type 471.

 

Figure 8: Positioning of the aft hydroplanes for single screw boats. (side, aft and top view).

 
Figure 9: X-tail configuration of the Dutch Walrus Class, picture from Miller (1990).

Model Submarine Dive Technology
    In the previous sections, the dive technology of real submarines was explained. It was shown that the bulk buoyancy of the boat is changed with the MBT followed by fine tuning with the MTT and finally the correct depth is maintained using the hydroplanes. Of course the ultimate model submarine should operate in exactly manner. Due to the small scale however, application of the real submarine technology is not always possible. In the following, some of the available model diving technologies will be treated.

Dynamic Diving Technology
    The fully dynamic diving boats are the most simple model submarines available. These boats have an inherent positive buoyancy which means that they will float back to the surface if control is lost. This is a major advantage for model submarines. Two German model manufacturers sell dynamic diving submarines: Robbe The Seawolf (not the real one) and the U-47 (a Type VII-C boat) and Graupner sells the Shark. On the internet Charles Darley  has an excellent web site showing the building of a fully dynamic submarine. To get a positive buoyant boat under water, the force on the hydroplanes has to overcome the upward force of the floating boat. This requires a combination of sufficient speed or sufficiently large hydroplanes. Of course the closer the boat is rigged towards neutral buoyancy the smaller the required downward force of the hydroplanes. Figure 10 shows the angle of the dive planes to keep a positive buoyant sub under water. At low speed both planes have a downward angle. The aft hydroplanes are needed to prevent the stern of the bow rising above the surface. Just like for the real boats, at sufficiently high speeds the aft hydro plane can be moved to a neutral position and depth control can be maintained with the fore plane only.
 

Figure 10: Angle of the dive planes, left low speed, right high speed..

In general, the force on a hydroplane can approximately be calculated from the following equation:
 

F = C A sin(a) 0.5 r v2 F : Force on hydroplane (kg)
C : Friction coefficient (C = 0.1)
A : Area of hydroplane (m2)
v : Velocity of boat (m/s)
r : density of water ( r = 1000 kg/m3)
a : angle of hydroplane (deg)
Equation 1: Force acting on a hydroplane.

For example, if we would have a model submarine with square hydroplanes of dimension 5 x 5 cm, this leads to an area of the individual hydroplane of A = 2.5 10-3 m2. With a boat traveling at a speed of 10 km/h (v = 2.78 m/s) and a hydroplane down angle of a = 30 degrees, the down force on the hydroplane is about half a kilogram. Since the boat has two forward dive planes the nett down force would be close to a kilogram. This means that the positive buoyancy of the boat can also be allowed to be a kilogram. However, at lower speeds than 10 km/h or at hydroplane angles less than 30 degrees, we will not able to submerge such a boat! Equation 1 can be used to design the size of the hydroplanes or the allowable positive buoyancy for a given hydroplane size. Please note that the value of the friction coefficient C is dependent on the design of the hydroplane, C=01 is the maximum value. In updates of this page more accurate values of the friction coefficient C will be given. Generally, for fully dynamic diving models one would want the boat rigged close to neutral buoyancy. That is, the decks awash situation. In that case, the boat can be submerged at low speeds with a realistic size of the dive planes. However if the boat is already close to neutral buoyancy, the surface running is not very realistic. To obtain realistic surface and submerged operation ballast tanks are needed.

Static Diving Technology
    In real submarines, MBT's are filled by venting the air inside the tanks and are emptied by blowing compressed air in to them. For model submarines a number of alternative methods are available.

Vented ballast tank
    The vented tank (Figure 11) can be used to decrease the buoyancy of the boat from positive to slightly positive (decks awash). If the flood valve is opened, the air can escape through the vent and water fills the tank. The tank can be emptied by pumping water out of the tank while air is sucked back into the tank through the vent. Note that in order for this system to work, the top of the vent line must be above the water level. That is why the vented tank cannot be used to give the boat neutral or negative buoyancy. With a filled tank the boat can dive using the hydroplanes. Note that if a bi-directional pump is used, the flood valve is not needed. To prevent water getting in to the ballast tank when running submerged, the diameter of the vent line should be kept small. Please note that the vented ballast tank is not very convenient as a ballast system.

 

Figure 11: Vented ballast tank.

Flexible ballast tank
    The flexible tank (Figure 12) consists of a rubber balloon placed inside a rigid tank. To flood the tank, the valve is opened and water is pumped into the tank. The valve is closed to prevent water getting out once the tank is flooded. The air originally present in the rigid tank is vented into the pressure hull of the boat. This will lead to an increase of the pressure inside the hull. If the volume of the ballast tank is not to large compared to the air volume inside the pressure hull this is not a problem. Note that the inside of the submarine is usually packed with equipment so the air volume is certainly not equal to the hull volume.
 

Figure 12: Flexible ballast tank.

Wilhelm Sepp, builder of a model of the Nautilus, used an inflatable toy ball as flexible tank, see Figure 13. The ball is filled and emptied by a Robbe gear pump. The tilted section of the wooden panel is connected to a micro switch and rests on the balloon. Once the balloon is completely filled the micro switch closes so that the power to the pump is cut off and the balloon cannot be filled beyond its capacity.

 

Figure 13: Flexible tank, empty and full, by Wilhelm Sepp.

Pressure ballast tank
    The pressure ballast tank (Figure 14) consists of a sealed ballast tank capable of with standing a significant pressure increase (5 bar or so). To flood the tank water is pumped into the tank with a high pressure water pump. Because the air in the ballast tank cannot escape the air is compressed. To empty the tank, the water pump pumps the water out of the tanks again. Note that because the pressure build-up inside the ballast tank it can never be completely filled. Assuming a maximum pressure of 5 bar inside the tank, about 80 percent of the volume of the ballast tank can be used.
 

Figure 14: Pressure ballast tank.

On the internet, the company SubTech sells a special T-valve that can be fitted on the ballast tank. This T-valve releases air into the boat when the tanks is filled and lets air into the tank if the tank is emptied. In that way, the ballast tank does not have to withstand high pressure. In my opinion a drawback of this system is that the ballast tank is connected to the interior of the boat. When the tank overflows, the water ends up inside the pressure hull containing the electronic RC equipment. A maximum water level detector that cuts the power to the pump can prevent this.

Piston ballast tank
    The piston ballast tank (Figure 15) consists cylinder and a movable piston, just like a giant syringe. The piston can be moved with a thread, a cogwheel and a small motor. The outer end of the cylinder is directly connected to the surrounding water. In the piston ballast tank no air is present. Just like the flexible tank the pressure inside the boat increased if the piston tank is filled with water. If the position of the cylinder is measured, for example with a linear potentiometer connected to the thread, the buoyancy of the boat can very accurately be adjusted. Due to the large stroke of the piston, these types of ballast tanks are mostly fitted horizontally, like in Figure 12. This means that during filling of the tank with water the axial center of gravity of the boat is affected. For example if the boat is balanced to run horizontally with a full ballast tank, the angle of the boat is no longer zero with an empty tank. This drawback can be overcome by using two piston tanks in the aft and bow section of the boat.
 

Figure 15: Piston ballast tank.

The piston tank can be purchased from Norbert Bruggen, see Figure 16. In this system, a small electric motor drives the piston in the plastic cylinder. Two micro switches interrupt the current to the motor if the piston is in either fully retracted or fully deployed.

 

Figure 16: Piston ballast tank by Norbert Bruggen.

Membrane ballast tank
    The membrane ballast tank (Figure 17) is a simplified version of the piston tank. It consists of a rigid disk that can be moved up and down with a thread connected to a motor, just like the piston tank. The disk is connected to the cylinder via a flexible rubber membrane. When the disk is retraced, water is allowed into the boat. A nice aspect of the membrane ballast tank is that the water tight sealing is very easy. As long as the rubber membrane is properly attached to both disk and tank, leaking is not possible. In a piston tanks the sealing between the piston and the cylinder is quite critical. Drawback of the membrane tank is that the stroke of the piston is not very large so the change in buoyancy of the submarine is not very large. To make optimal use of the membrane tank, the diameter of the cylinder should be rather large compared to its height. The system is however ideal for small, or micro, model submarines. Thorson Feuchter has a nice collection of boats based on this principle.

 

Figure 17: Membrane ballast tank.

Bellow ballast tank
    The bellow ballast tank (Figure 18) is a variation on the membrane ballast tank. Instead of a flat membrane a rubber bellow is used. This has the advantage that the stroke of the disk is increased so that more water can be taken into the boat. Rubber bellows of sufficient diameter, 5 to 10 cm or so, can be found in car parts shops. In cars they are for example used to seal off moving parts of the steering equipment. Under pressure, the zig-zag wall of the membrane may pop out, resulting in a sudden increase of the ballast volume (and sinking of the sub). To prevent this, it is recommended to fit the bellow inside a cylinder.

 

Figure 18: Bellow ballast tank.

Gas operated ballast tank
    The liquid gas system (Figure 19) consists of a storage cylinder with pressurized gas, a ballast tank and two valves. This system resembles the ballast system of a real submarine very closely. To flood the tank, the valve in the vent line is opened and water is allowed into the tank via the opening in the bottom. If the required volume of water is taken in, the vent valve is closed. The tank can be emptied by forcing pressurized gas into the tank by opening the blow valve. If we want the model boat to be able to blow the ballast tank a number of times, the stored amount of gas should be sufficient. Carbon dioxide (CO2) is an option because cylinders with this gas are relatively cheap and readily available from Paint ball shops. In paint ball, cylinders of 50 to 500 gram are commonly used. If CO2 cylinders are used a reduction valve to bring back the pressure to about 2-3 bar is necessary. CO2 cylinders are also used in model Warships, an excellent web site giving information on CO2 cylinders is R/C Warship.
 

Figure 19: Gas operated ballast tank.
Figure 20: A CO2 cylinder (14 oz).

An interesting alternative to CO2 is the use liquefied gas, for example canisters used for air brushing (propane), canisters used to clean photo equipment or electronics called 'dust-off' (dimethylether/tetrafluorethane) or propell (tetrafluorethane). Because these gases are stored as a liquid,  the amount of gas that can be stored is quite large. It is also very easy to refill the gas tank in the submarine form a larger stock cylinder. With CO2 do it your self refilling is not that easy so that spare cylinders have to be taken to the lake. An additional advantage of liquid gas is that the pressure inside the storage vessel is about 3 to 4 bar so there is no need for a pressure reduction vale. A very serious draw back of these gases is its flammability. If the storage vessel leaks an explosive mixture may form inside the pressure hull of the boat. The sparks of the electric motor are sufficient to detonate it! The only real safe liquid gas is tetrafluorethane. The dust-off product contains about 20 percent of the flammable dimethylether and is potentially hazardous.

In the gas ballast system the electric valves used in the gas line (the blow valves) can be standard solenoid valves used in laboratory equipment. To prevent draining of the batteries, valves that are normally closed should be used. Using CO2 with a pressure reduction valve or liquid gas, the pressure at which they remain closed should be about 5 bars. Miniature solenoid valves can be obtained from Clippard.

The vent valves that let air out of the ballast tank to submerged the boat are different. The pressure difference between the air in the tank and that of the ambient air is only a couple of cm water. Therefore the opening of the vent valve should be quite large to let our air at a sufficient flow rate to get a realistic dive. Because the pressure difference is also quite small when the vent valve is closed, and thus the boat is submerged, we can make these valves ourselves. Note that many of the above-mentioned solenoid valves have an opening of less than 1 mm and do usually not like water getting in to it, these types of valves are not very suited.

Compressed air ballast tank
    The ballast tank system that uses compressed air is identical the one used in real submarines. This system is similar to the gas operated ballast tank but in this case the gas bottle is replaced by a cylinder that is filled with a compressor. Small compressors can be found in car accessories shops. They sell small 12 volt compressors that are intended to inflate car tires. These compressors are capable of compressing air to about 6 to 8 bar. These pressures are high enough to be careful with the storage cylinder. It is smarter to buy a commercial cylinder or use a empty CO2 cylinder than to make one by your self. The pressure is however relatively low pressure if you consider the amount of gas that can be stored. If we would assume that the compressed air cylinder is half the size of the MBT, we can only blow the MBT two to three times. This is not much compared to CO2 or liquid gas systems. In general, boats with on board air compressors refill the air supply each time they run on the surface after a dive. Special care should be taken to prevent water being sucked into the compressor. To prevent this, the air intake should be fitted with a valve that closes if the boat is submerged.
 

Figure 21: Gas operated ballast tank.

The use of a gas compressor is applied in a boat made by Harry Grapperhaus. Two cylinders of 0.75 liter and  0.1 liter are filled with the compressor to about 6 bars. The main ballast tanks is about 3 liters so that with a full air cylinder only one blow of the MBT is possible. Harry uses the MBT also to control the trim of the boat. For this purpose the MBT is fitted will a total of 6 solenoid valves. Two valves are used for the controlled blowing of the MBT. These valves are connected to the air 0.75 liter cylinder via a pressure reduction valve (2.5 bar). By using two blow valves, the air flow rate can be more or less be regulated. One solenoid valve is directly connected to the 6 bar small cylinder of 0.1 liter. This valve is used for quick blowing of the MBT.

The venting of the MBT is controlled by three solenoid valves so that the air flow rate can be adjusted in three steps. To get a realistic dive of the model all three valves are opened simultaneously. Once close to neutral buoyancy, only one valve is used to regulate the depth of the model. In the model of Harry Grapperhaus, the MBT always remains partly filled.

 

Figure 22: Arrangement of the valves in the boat of Harry Grapperhaus.

Note on gas ballast tanks
    Remember the distinction made between the Russian and US/UK boats in section Static Diving? The Russian boats use a valve, the Kingston, to seal the bottom opening of the ballast tank to prevent water entering. The US/UK boats keep the ballast tank under pressure to prevent water entering. The designs of Figure 19 and 21 do not have a Kingston valve. In one only uses a gas ballast tank to adjust the buoyancy of the boat, one can run in to trouble. Let us assume that the ballast tank is halfway filled with water to get the boat at  neutral buoyancy and the boat is at a depth of 1 meter. At 1 meter below the surface the pressure of the surrounding water is 0.1 bar as a result the pressure of the gas inside the ballast tanks is also at 0.1 bar. If we would move this boat upwards, the water pressure will decrease resulting in an expansion of the gas in the ballast tank. The expanding gas will force water out of the ballast tank so that the boat gets lighter and will rise even more. On the other hand, if we would move the neutral buoyancy boat downward, the gas in the tank is compressed and more water gets in to the ballast tank. This will sink the boat. We may conclude that boats with a partially filled gas ballast tank are inherently unstable. For model boats this may not be a problem as long as the depth of the boat is controlled by the hydroplanes. Stable depth control at zero velocity is however not possible. Of course if the boat is fitted with Kingston valves water cannot enter the ballast tank and the problem is solved. The author is not aware of any model boats equipped with Kingstons. A different way to get a stable depth control is to use the MBT either completely full or completely empty. The trim of the boat is obtained with separate trim tanks. This is the hybrid ballast system, see below.

Hybrid Ballast Systems
    Just like real submarines use main ballast tanks (MBT) to submerge and main trim tanks (MTT) to rig the boat for neutral buoyancy, the ideal submarine should operated likewise. A nice example of such a boat is the one made by Ralf Diederich. This boat has a compressed air system as MBT. The air cylinder has a volume of 1.2 liters and is filled by two compressors to 6 bar. The tank is filled by two compressors to reduce the filling time.
 

Figure 23: Two compressors in Ralf Diederich's boat. 

This results in a stored gas volume of 7.2 liters. The MBT has a volume of 3.7 liters. This means that with a full compressed air cylinder only just two full blows of the MBT are possible. The exact buoyancy of the boat and the horizontal trim is controlled by two piston tanks located in the bow and the stern section of the submarine. The model boat is prepared for operation as follows. First the both piston tanks are emptied. Then the main ballast tank is flooded completely. The boat will sink to some extent but will still float on the surface. Then the both piston trim tanks are carefully flooded. The pistons inside the trim tanks are moved using  two proportional channels of the radio transmitter. The water level in the trim tanks is adjusted in such a way that the boat is kept horizontal. The trim of the boat is completed once only the top of the sail is above the water level and the boat has a horizontal angle. Since in this condition the boat has a nearly neutral buoyancy, the depth of the boat can be adjusted using the dive planes even at a very low speed. When RC operating the submarine, the trim of the piston tanks is left unaffected and the depth of the boat is only controlled by blowing or emptying the MBT's and by using the dive planes once the boat is submerged. This is pretty much the same way real submarines operate!

 

Figure 24: Two compressors and one piston tank in the aft section of Ralf Diederich's boat. 

Do not think hybrid ballast systems are a recent invention of model submariners. Figure 25 shows a submarine desing by George Garrett in 1878. The boat has a main ballast tank (A). Water from the ballast tank can be removed with a piston pump (B) and the tank can be flooded with the vent valve (C). The trim of the boat is made with a piston ballast tank (D).
 

Figure 25: Design by George Garrett, 1878. [Compton-Hall, 1999].

Conclusions
    For model submarines, a number of different ballast tanks systems were identified. These types can roughly be grouped into three different ways of operation: (a) mechanical attenuated systems (piston, membrane, bellow), (b) pump systems (flexible tank, pressure tank) and gas operated (CO2, liquid gas and pressurized air). The mechanical attenuated tanks are ones that control the buoyancy in the most accurate way but they are rather slow. Pump systems are relatively easy to construct because they use few parts. The gas operated tanks are the most complex systems but are very similar to the live scale submarine technology. With gas systems the blowing of the MBT can be carried out very fast, in fact even an emergency blow can be carried out! An accurate and realistic model boat can be constructed using a mix of these ballast systems.

 

Glossary
MBT: Main Ballast Tank
MTT: Main Trim Tanks

Literature
David Miller, 1990, 'Moderne Gevechtswapens: Onderzeeboten', Uitgeverij Helmond, Helmond, The Netherlands.

Ulrich Gabler, 1987, 'Unterseebootbau, Bernard & Graefe Verlag, Koblenz, Germany, ISBN 3-7637-5286-2.

Compton-Hall, Richard, 1999, 'The submarine Pioneers, Sutton Publishing, ISBN 0-7509-2154-4.