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The definitions contained here are exact meanings of the terms commonly used in reference to World War II submarines and their operation. These terms and explanations represent accepted interpretations and provide an understanding of the functions of the equipment. 

Surface condition - A submarine is in surface condition when she has sufficient positive buoyancy to permit running on her main engines. 

Diving trim - The term diving trim designates that condition of a submarine when it is so compensated that completing the flooding of the main ballast, safety, and bow buoyancy tanks will cause the vessel to submerge with neutral buoyancy and zero fore-and-aft trim. 

Rigged for dive - A submarine is rigged for dive by so compensating the vessel and preparing the hull openings and machinery that the vessel can be quickly and safely submerged and controlled by flooding the main ballast tanks, using the diving planes, and operating on battery-powered main motors. 

Running dive - A running dive consists of submerging a submarine while running on battery power. 

Stationary dive - A stationary dive consists of submerging a submarine without headway or sternway. 

Quick dive- A quick dive consists of rapidly submerging a submarine while running on main engines. 

Submerged condition - This term designates a condition of a submarine in which all fixed portions of the vessel are completely submerged and the variable ballast is so adjusted that the submarine has approximately neutral buoyancy and zero fore-and-aft trim. 

Final trim - Final trim is the running trim obtained after submerging in which the fore-and-aft and over-all weights have been so adjusted that the boat maintains the desired depth on an even keel at slow speed and with minimum use of the diving planes. 

Compensation - Compensation is the process of transferring ballast, in the form of water, between the variable tanks, and between the variable tanks and sea, to effect the desired trim. 

Main ballast tanks - Tanks that are provided primarily to furnish buoyancy when the vessel is in surface condition and that are habitually carried completely filled when the vessel is submerged. 

Variable ballast tanks - Ballast tanks that are not habitually carried completely filled when submerged and whose contents may be varied to provide weight compensation are known as variable ballast tanks. These ballast tanks are constructed to withstand full sea pressure. 

Negative tank - The Negative Tank is a variable ballast tank providing negative buoyancy and initial down-angle. Submarines normally will operate submerged in neutral buoyancy and without trim when the negative tank is nearly empty. It is used to reduce the time required in submerging from surface condition, to reduce the time required to increase depth while operating submerged, and to prevent broaching when decreasing depth. It may be blown or pumped. 

Safety tank - The Safety Tank is a heavily reinforced main ballast tank arranged to permit pumping as well as quick blowing to regain positive buoyancy. Under normal submerged conditions, the blowing or pumping of this tank will bring the conning tower above the surface. 

Bow buoyancy tank - The Bow Buoyancy Tank is a free-flooding vent-controlled tank with its main volume above the normal surface waterline. It is located in the extreme bow of the vessel and is formed of the plating of the superstructure. Its function is to provide reserve surface buoyancy, emergency positive buoyancy in the submerged condition, and to aid in surfacing. 

Auxiliary tanks - The auxiliary tanks are variable ballast tanks located at or near the submerged center of buoyancy, and are used to vary the over-all trim of the boat. 

Trim tanks - The trim tanks are the variable ballast tanks nearest the bow and stern of the boat and are used to provide fore-and-aft compensation. 

Normal fuel oil tanks - Tanks designed solely for containing the engine fuel oil are known as normal fuel oil tanks. Note that because fuel tanks are located outside the pressure hull and cannot withstand the pressure of the sea during a dive they must always be completely filled with fluid. As fuel is withdrawn from these tanks it is replaced with seawater. The remaining fuel oil floats on this compensating water so fuel is withdrawn for use from the top of the tank. Remaining fuel quantity is determined by locating the oil/water interface (via tubing installed in the tanks) and then calculating fuel volume from the tank geometry. 

Fuel ballast tanks - The fuel ballast tanks are designed to be utilized as fuel oil tanks for increased operating range. When empty, they may be converted to main ballast tanks, providing additional freeboard and thereby increasing surface speed through decreased drag. 

Expansion tank - The Expansion Tank, connected between the head box and the compensating water main, admits sea pressure to the fuel oil tanks. It receives any overflow from the fuel tanks resulting either from overfilling the fuel system or from temperature expansion. The bilges are pumped into this tank to prevent leaving an oil slick or polluting a harbor. 

Collecting tank - The Collecting Tank, connected to the fuel oil tanks through the fuel transfer line, serves as a water and sediment trap for the fuel oil being transferred to the fuel pump. 

Clean fuel oil tanks - The clean fuel oil tanks are storage tanks located within the pressure hull. They receive clean fuel oil fromthe purifiers and are the supply tanks from which the engines receive their clean fuel. 

Poppet valve drain tank - The two poppet valve drain tanks are located under the platform deck of the fore and aft torpedo rooms immediately below the breech of the torpedo tubes. The air and water from the poppet valves, incident to the firing of torpedoes, is discharged into these tanks. 

Fresh water tanks - The fresh water tanks contain potable water for drinking, cooking, and certain sanitary facilities. 

Battery fresh water tanks - The battery fresh water tanks are storage tanks for the distilled water used in watering the main storage batteries. 

Sanitary tanks - The sanitary tanks receive and store the ship's sanitary drainage until conditions permit overboard discharge. 

WRT tanks - The WRT (Water 'Round Torpedo) tanks are variable ballast tanks located in the forward and after torpedo rooms for flooding or draining the torpedo tubes. When a torpedo is fired an amount of water equal to the weight of the torpedo is added to the WRT tank to compensate for the loss of weight an thus maintain trim. 

Main vents - The main vents are valves operated hydraulically, or by hand, for venting the main ballast tanks when flooding. They are located in the top of the vent risers (vent pipes) of the main ballast tanks. 

Emergency vents - The emergency vents are stop valves in the vent risers near the tank tops and are used in case of damage to the main vents. They permit sealing the tank to prevent accidental flooding and also permit blowing the tank if desired. 

Venting - Venting consists of permitting a flow of air into or out of a tank. 

Riding the vents - Riding the vents is a surface condition in which the main ballast tanks are prevented from completely flooding by the closed main vents which prevent the escape of air. 

Flood valves - Flood valves are hinged covers at the bottom of certain ballast tanks, which may be opened to admit or expel sea water. 

Flooding - Filling a tank through flood ports, open flood valves, or other sea connections, is known as flooding. 

Blowing - Blowing a tank consists of expelling its contents by admitting compressed air into the tank to force out the water. 

Pumping - Pumping a tank consists of using a pump to transfer liquid from the tank to sea, from sea to tank, or from one tank to another. The tanks must be vented during this operation. (In the case of pumping between tanks the venting can be between the tanks rather than to atmosphere.) 

Bow planes - The bow planes are horizontal rudders, or diving planes, extending from each side of the submarine near the bow.