Ohio Class, Ballistic Missile Nuclear Powered Submarine, USS Kentucky SSBN737

(By Johan J. Heiszwolf)

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.

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. 

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. 

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 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.  

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 fin 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 favourable. It may be added that the author is not aware of boats having both dive planes on the bow and on the sail. 

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 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. 

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. 

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