Air is trapped in the ballast tanks which surround the boat. This makes it positively bouyant (why it floats). To submerge the boat the air is vented out of the tanks via the main ballast tank vents which are at the highest point in the tank. As the air escapes seawater comes into the tank through the flood grates located at the bottom of the main ballast tanks. This makes the boat heavier or negatively bouyant and causes it to sink. Once submerged then the crew adjusts the weight of the boat (called trimming the boat) so that it is as close to neutrally bouyant as possible.
To control its buoyancy, the submarine has ballast tanksand auxiliary, or trim tanks, that can be alternately filled with water or air. When you fill it with water, it sinks, When you first let the water out then you let the air in (from compressed air in the submarine) it floats.
Normal ships are designed to move on the surface of the water. Submarines are designed to be able to submerge and travel under the water's surface, as well as on the surface. When normal ships go down, they stay down. Submarines are able to come back up after they go down. All American submarines are nuclear powered. With the exception of Aircraft carriers, surface ships are powered conventially.
ability of an object to float or submerge depends upon the density of the object and on the density of the liquid in which it is submerged if the density of an object is greater than the density of the liquid then the object submerges in the liquid and when its density is less than the density of liquid then it floats on the liquid as in submarines when it takes water in its hollow tank its density increases and it submerges and when it pushes out water from its hollow tanks its density decreases and ot floats on water
By their pumps and ballast-tanks: allowing water into the tanks to raise the vessel's density so can submerge, or pumping air into the tanks to displace the water and allow the submarine to surface.
Submarines must be able to float to load and offload crew and cargo and to have repairs and maintenance. They are designed to submerge either for underwater exploration or to pass undetected by ships at the surface and planes and satellites above.
The density will rise and submerge the submarine.
to sink below the surface of water
It is a matter of buoyancy. There are large tanks that can be filled with sea water or with air. To submerge they pump the air into compressed tanks and the water fills it up sinking the sub. When they want to go back up, they blow the water out using the compressed air.
PLANKTON
a green liquid
Displacement - the ship displaces an amount of water equal to its weight. Provided that this dowes not submerge the vessel, it will float.
A submarine can submerge by filling ballast and trimmer tanks with sea water. The additional weight will take the submarine down. To resurface, the submarine will blow the sea water out of the tanks using compressed air. The air in the tanks make the submarine buoyant, so it rises and floats on the surface. Though a stationary submarine can submerge in this way, it usually submerges going forward and using diving plane (fins) to steepen the dive, so getting under the surface faster.