[This article is reprinted from Morskoy Sbornik]
In the course of the evolutionary development of our submarines, as well as foreign ones, three basic architectural types of hull construction have been developed:
- Dual hulls, in which the main ballast tanks (TSGB) are positioned in the light outer hull (LK) over the entire length of the pressure hull (PK) and encompass its entire perime-ter, that is, in any place on the submarine the external plating of the light hull appears. An exception is the separate sections of the free flooding space between the inner and outer hulls where the rigid tanks of the auxiliary ballast can be positioned. Auxiliary ballast includes the fast diving or negative tanks, the regulating or variable tanks, and the like.
- One-and-a-half hulls, when they have the external tanks of the main ballast over the entire length of the pressure hull, but the outer hull, which forms these tanks, does not cover the entire perimeter of the pressure hull, that is, the external plating of the submarine in the lower part of its hull is the pressure hull (a variety of one-and-a-half hull types exist, like the type with rolls, where the outer hull and respective-ly, the tanks of the main ballast have been located in the middle part of the pressure hull according to its length and height).
- Single hull, there are no external ballast tanks.
Our domestic, underwater shipbuilding industry has found a use for all these architectural types. We have used the dual hull architecture to achieve the following: the best hydrodynamic contour for surface transit; simplified construction of the pressure hull, which can consist only of cylinders and truncated cones without molded or curved contours, though when necessary it’s possible to use more complex patterns in the cross sections-for instance, a figure eight; an increase in the amount of free flooding space between the inner and outer hulls which gives more reserve buoyancy to ensure surface flotation; the use of external ribs on the pressure hull, which has made more internal space available. Further more, in the free flooding space between the inner and outer hulls, it’s easier to accommodate the rigid tanks of the auxiliary ballast and also part of the fuel or fuel reserves (in the fuel/ballast tanks of the main ballast tanks), and the like.
The hull-and-a-half and the single hull architectures are used (by us) for submarines with relatively small displacement, particularly when the calculated, allowable width of the free flooding space between the inner and outer hull of the double hulled boats was excessive and led to increased displacement and a subsequent decrease in the boat{s) speed and maneuver capabilities.
After the war, all our diesel submarines (including the projects 613 and 641 attack subs, and the project 651 SSGN) were double hulled, except the small projects 615 and A-1615, which were fitted out with the ED-KhPI powerplant.
It’s well known that a submarine will submerge when it is on the surface and blows out all its main ballast tanks, which then fill with sea water. During this activity the elimination of negative buoyancy and the absence of pitch and roll are achieved by observing the following conditions:
- The total volume of the main ballast tanks should be equal to the water-tight volume of the submarine’s hull above the water line {when surfaced), that is, it’s reserve buoyancy in the surfaced position.
- The center of gravity of the above mentioned water tight volume should be located on the same vertical plane with the center of gravity of all the main ballast tanks.
The second of these conditions determines the basic requirement for the distribution of reserve buoyancy (from the main ballast tank) along the length of the submarine. To avoid significant pitch during diving and surfacing, it’s also necessary to ensure balance during flooding while diving and when blowing out the main ballast tanks during surfacing. This is ensured by choosing the proper profile of Kingston valves, flooding ports (when Kingstons are not present), ventilation valves, and also the regulation of the high pressure air equipment, which supplies air for blowing out the individual main ballast tanks.
To effectively fight damage caused by the flooding of the pressure hull, some tanks of the main ballast should be positioned at both ends of the submarine, which would compensate for the pitch (caused by the flooding) when the main ballast is blown out. Theoretically, putting the necessity of maintaining the surfaced submarine’s buoyancy at the top of the list, it can be proven that it would be advisable to concentrate all buoyancy reserves (all tanks of main ballast) at the ends of the submarine. With that, the required buoyancy reserves would be minimal and the submarine’s full underwater displacement would accordingly decrease. However, we traditionally continue to position the main ballast more or less evenly along the length of the submarine and divide them into three groups: bow group and stem group (end groups which include tanks positioned behind the pressure hull, as well as parts of tanks adjacent to them), and the so called middle group positioned approximately in the middle of the submarine.
The middle group of ballast tanks is isolated so that when the ballast from these tanks is blown out while the submarine is surfacing, there is no severe pitch, and the submarine is in broached position with a definite lateral and longitudinal stability. A minimal part of the deck is above the water to allow the crew to come out and perform repairs on the superstructure. The volume of the middle group of main ballast tanks is usually up to 30 percent of the submarine’s total buoyancy reserve, or 8 to 10 percent of its normal water displacement. Also, the middle group is used for:
- Surfacing and submerging the submarine in two steps. While surfacing, only the middle group is blown out; after surfacing the end groups are blown out by exhaust gases of the diesel engine, which saves high pressure air. While submerging, the end groups of tanks of the main ballast are flooded first, and then the middle group. That helps to decrease possible pitch and roll when the upper stringers enter the water and lateral stability becomes minimal-a normal occurrence for submarines of two hull and hull-and-a-half architectural types.
- Surfacing of the submarine with minimal pitch in instances when an accident is not connected with the flooding of the submarine’s end sections with seawater. The normal procedure in this case is maximum acceleration with a simultaneous blowing out of the middle group of main ballast tanks. Then, depending on the rate of pitch in-crease, the high pressure air is supplied to the end group of main ballast tanks.
Also, in many cases diesel electric submarines charge their batteries and replenish pressurized air reserves while in a broached position, since in the event of a sudden attack by the enemy, diving from this position is much faster then from the surface.
Forming of a group of main ballast tanks in the middle section of the submarine and achieving the above mentioned goals on double hulled and hull-and-a-half types submarine is not complicated technologically. As far as single hull submarines are concerned, the main ballast tanks located in the middle part of the submarine must be strong, which significantly increases the weight and reduces usable volume of the submarine. That becomes very important with the increase of the submarine’s maximum depth.
These reasons were the basis for making the presence of the middle group of main ballast tanks on domestically produced submarines not only a tradition, but the rule.
Comparing the advantages and disadvantages of double hulled submarines with single hulled submarines it is notable that an increase of the buoyancy reserve on the former (up to 25-30 percent as compared to the latter), and therefore increased full underwater displacement, practically does not inhibit their speed and maneuverability. For example, at the fixed power of the engine, the full speed (Vmax) is related to the full underwater displacement (D) by an equation (Vmax- D-2/9), and the radius of established circulation (when the vertical rudder area is fixed) is reversely proportional D 1/3. That means that at the above mentioned conditions, the increase of water displacement in 1.2 times causes a 4 percent reduction in full speed and 6 percent increase in circulation radius. The rate of speed increase is proportional to D, which is not very important.
The influence of an underwater displacement increase on the required engine power (Ne) is more noticeable. Here, the equation Ne- D2/3 is taking place. Therefore, the decrease of D 1.2 times will cause the reduction of Ne by 13 percent. The area of outer hull is also changing proportionate(y to D- 2/3. This particular reason, as well as the low hydrodynamic noise of submarines of this architectural type (which is especially important at higher speeds), resulted in exclusive use of single hulled submarines with a small buoyancy reserve (around 12-15 percent) and placement of light tanks of main ballast in the end sections of the submarine (without the middle group) by the American Navy. The example of this type of submarine is the Los Angeles class. The quantity of sections on this class is reduced to three, which makes it impossible for her to stay on the surface when even an insignificant area of its section is flooded.
However, in this article the attention should be paid to the absence of the middle group of the main ballast tanks, which is probably a normal consequence of the transition to the classic single bull architectural type; there the middle group (as a geometric term) is losing its meaning. We can only talk about some unidentified group of main ballast tanks which includes separate ballast tanJcs positioned at the ends of the submarine, the purging of which brings the submarine to a mid position (between periscope depth and surfaced), which still can be called broached. Therefore, this group can be called not middle group but the broached group.
Such a broached group on single hull submarines with the main ballast tanks located at both ends of the submarine can be formed with the same purposes as the middle group. However, when the submarine is surfacing, its use may cause heavy pitching if for some reason the main ballast tank at one end would not blow out, or the main ballast would not blow out evenly.
There was a case in submarine history, when in the 1920s the main ballast tanks on the stem of an American single hulled submarine blew out, and the ballast on the bow did not. The submarine was practically in a vertical position, with only a small part of its stem above the water. She was able to float in this position because of a small amount of air in its rear main ballast tanks. Fortunately, the submarine did not have any negative buoyancy, otherwise it would have sunk. It was completely helpless, because the group of tanks on the bow could not be blown out; there was no middle group and residual air in the stern-positioned tanks could not be blown out because they were higher than the stern ventilation valves. Only after 40 hours and because of a series of lucky events was the crew of 28 rescued.
During emergency surfacing, heavy pitching is very dangerous for a submarine without Kingston valves on its ballast since it leads to a reduction of the ballast tanks’ air pressure through the flooding ports, and it is also dangerous for submarines with Kingston valves on the main ballast tanks because the air pressure is lost through the valves when they’re open.
During this heavy pitching, the amount blown out of the main ballast tanks is significantly reduced. As in the Black Sea in 1957, when during an emergency dive the hull-and-a-half submarine, number M-351 of the project A-615 class, sank and laid on the bottom at a depth of 84 meters with a pitch of about 60° at the stern. The cause was the intake of sea water into the diesel engines through the air feed shaft. The sixth compartment was two-thirds flooded and the water seeped through the transverse ulkhead and penetrated to the end of the seventh compartment. The submarine could not independently surface because the severe angle of pitch had reduced the volume of main ballast tank blow out. Only after three days was the submarine rescued by the Navy’s emergency rescue service.
For submarines with their main ballast tanks positioned only in the extreme bow or stem, controlling pitch while surfacing (with the help of blowing out the main ballast tanks) becomes complicat-ed. To some degree this is possible to illustrate by holding a barbell in a horizontal position with both hands in the center, then disturbing the weights at the ends, Of course, more complications will arise during surfacing while not underway, or at a slow speed.
It becomes difficult to design single hull submarines with main ballast tanks positioned only in the extreme bow or stern. First, all the diving and surfacing calculations should be conducted with the utmost precision, in particular the flooding and purging the main ballast tanks. Secondly, additional measures should be taken to synchronize the supplying of air, the opening and closing of the Kingston valves and the ventilation vents of the main ballast tanks. Third, the design should provide a balanced, stable, surfaced position if one of the main ballast tanks in one of the ends of the submarine is damaged (this type of damage can occur at any time, for example, during a collision). And with this, the general reserve buoyancy decreases, the danger of heavy pitching recedes, and the difference between the trim and broached positions disappears.
In conclusion, and considering the foregoing, I would like to put forward three questions for working submariners to discuss:
- Is it possible to do without the middle group of main ballast tanks, understanding that this group is centered along the length of the submarine?
- Is it necessary to design single hull submarines with their main ballast tanks located in the extreme ends of the hull, and having only a relatively small buoyancy reserve (up to 20 percent)?
- Finally, in connection with the creation and operation of the submarine with a relatively small buoyancy reserve (this is a characteristic of single hull submarines), it’s worth considering increasing the effectiveness of the pressurized air system when combating incoming water in the pressure hull when submerged.
At the current time, during emergency surfacing, supplying high pressure air into a damaged compartment, before the submarine has begun to surface is not recommended, and in fact is forbidden. During this, the pressurized air should only be used for blowing out the main ballast tanks, since only in this way (with the current buoyancy reserves) can the positive buoyancy needed for emergency surfacing be created. Feeding the high pressure air into the damaged compartment can only reduce the negative buoyancy. In this case, blowing the main ballast tanks is more effective than feeding high pressure air into the compartment. Thus the submarine will have the property of single compartment surface flotation, and consequently by effectively flowing out all of the main ballast tanks, it’s theoretically possible to always have a positive buoyancy in a damaged submarine.
If the full buoyancy reserve is small and specific, and single compartment surface flotation bas been effected only during the partial flooding of the damaged compartment, blowing out only the main ballast tanks can lead to loss of air (pressure) from the already purged main ballast and, accordingly, to the useless expenditure of pressurized air. After the complete purge of the main ballast tanks, it will be necessary to direct pressurized air into the damaged compartment (if, of course, the rupture is situated lower than the compartment’s water line). If the subma-rine continues sinking and seawater again beings flowing into the main ballast tanks, then it’s necessary to again tum on the pressurized air and feed it into the main ballast tanks, etc. However, this principle can only be realized with water level sensors in the main ballast tanks and compartments, with the sensors preferably automated. I won’t stop here and detail the significance of accidents/casualties involving seawater penetrating the pressure hull of a submerged submarine, and the possibility of increasing the submarine’s speed to utilize the hydrodynamic drying capabilities of the hull.
We will gratefully accept and consider opinions an suggestions on the topics discussed. Material can be sent to the following address: 197061, g. Saint Petersburg, P-61, TsND MO RF, or to the Editor Morskoy Sbomik, 103175, Moskva, Cbaplygina, 15 .
