This page attempts to explain what happens to a submarine when it dives.

To start click on the below link for a general explanation of what happens on a nuclear submarine.

The diving basics for a nuclear submarine. Click here.

This next sequence attempts to show the diving operation for a fleet diesel boat of WWII design. The operation is broken into steps for sake of understanding and is not represented to be the typical sequence which would more or less happen simultaneous. A fleet boat could do 3 types of dives. A Quick (Crash) dive where underway on diesel propulsion on the surface, a Running dive where underway on electric motor propulsion on the surface or a Stationary Dive where totally stopped on the surface. This sequence approximates what happens on a Stationary Dive. It relates the weight added, in which tanks it is added and the general draft of the boat at various stages. In an as built condition the boat weighs 1500 tons and draws 14 feet. On the surface, loaded and crewed it weighs 1750 tons.

The Main Ballast Tanks are flooded with 360 tons of water. These tanks are shown in blue on the diagram. The tanks have an open grated flood port on the bottom and a vent valve on top. In this condition the boat displaces a total of 2110 tons, draws 22 feet of water and the main deck is still not awash. There is still a good bit of positive bouyancy.

Filling the Bow Bouyancy tank with 32 tons of seawater. This makes the overall boat weigh 2142 tons and helps make the overall trim bow heavy. The Bow Bouyancy tank is shown in light blue on the diagram.

We add 23 tons of seawater to the Safety Tank. This makes the boat weigh 2175 tons and draws 24 feet of water. At this point the decks are just awash. The Safety Tank is sized to just about offset what would be gained if the Conning Tower were to flood, hence the name.

We add 55 tons to the Trim system and distribute it to FWD, AFT and Auxiliary trim tanks. These tanks are shown in light aqua blue. This puts the boat in just about a neutral bouyancy condition. This is the amount that is normally required under most typical loading conditions. This amount is normally maintained once established but it can be varied if required, say for very shallow waters on the surface.

We flood about 7 tons of seawater into the Negative Tank. This is the tank that makes the overall boat into a negative bouyancy condition and allows it to break the surface tension and actually dive. The diving depth is anticipated and the negative tank is blown to level off at the desired depth and do a final trim. The Negative Tank is part of the Variable Ballast system and has valving on both the top and bottom for venting / flooding. The Negative Tank is shown on the diagram in red.

To surface, Blow the Bow Bouyancy Tank and Safety tank. This surfaces the Conning Tower and makes the boat overall bow light. Complete the surfacing by blowing the Main Ballast Tanks with low pressure air / Blower to raise the entire hull with the desired freeboard. It should be noted a fleet boat is most fuel efficient on the surface with the least wetted surface. So if long distances or speed is important the boat wants to be as high as possible, weather permitting. This creates a system of trade offs. The higher the hull is raised, the longer the Quick (Crash) diving times. In general once a dive is made and good trim conditions are acquired for the loading of the boat, that trim condition is generally maintained between surface and dived operations.