Ask A Scientist

How does a submarine sink and move at the same time?

Asked by: Stephanie Black
School: Glenwood Elementary, Vestal School District
Grade: 5
Teacher: Mrs. Russell
Hobbies/Interests: Reading and writing. 
Career Interest: Reading and writing

Answer from Theodore Brewster

Programmer Analyst, Binghamton University

Department: Information Technology Services
Research area: Databases, networks, programming languages, and software. 
Interests/hobbies: International affairs and history, particularly American and Russian

Unlike a surface ship, a submarine can move in 3 dimensions. A modern surface ship uses one or more propellers to move forward (and, less efficiently, backwards); it turns by manipulating a rudder behind the propeller which acts as a lever against the water flowing past it to pivot the vessel in one direction or the other. 

A submarine also has one or more propellers, and a rudder, to control movement forward (and backward) and to turn. (On a modern submarine, the rudder is ofen in front of -- "forward of" -- the propeller(s), but the principal is the same.) On the surface, a sumarine is controlled just like any other ship, by propeller and rudder. A submarine, however, can also move up and down in the water. 

To submerge (to go under water) a submarine takes on water -- it floods "ballast tanks" to make it heavier -- causing the "boat" to sink (submarines are always referred to as boats). To "surface," a submarine forces excess water from its ballast tanks with compressed air, making it lighter, causing it to rise. 

When submerged, most modern submarines try to maintain "neutral buoyancy" by shifting air and water between the various air ballast tanks. With neutral buoyancy, a submarine becomes more like a fish, and can remain at a particular depth under water with little effort. Once successfully submerged with neutral buoyancy, the submarine can turn right or left with the rudder as it would on the surface, like any other ship. A submarine also has "diving planes" that act like the rudder, but vertically, moving the boat up and down. Modern submarines have diving planes on the "sail" (the part of the boat that used to be called the "conning tower" but is now much larger and higher) and at the rear of the boat (the stern). (Older submarines had forward diving planes attached to the hull that would fold outward when the boat needed to go down.)

The diving planes allow the submarine much greater control over vertical movement than using the air and ballast alone. Together, the diving planes and the shifting of water and air in the boat permit control over not only the direction, but also the speed of vertical movement. So the propeller drives the boat through the water, up, down, right and left as controlled by the rudder, diving planes, water ballast and air. 

Last Updated: 9/18/13