Submarines are incredible machines that allow humans to explore the deep ocean. Unlike regular boats that stay on the surface, submarines can dive beneath the water, stay at a certain depth, and then return to the surface when needed. But how do they do it? It all comes down to buoyancy, ballast tanks, and careful control of weight and density.
To help explain how submarines work, let’s take a look at a real submarine from history: the USS Cavalla (SS-244). This submarine was used during World War II and now is preserved at the Galveston Naval Museum in Texas. The USS Cavalla is a Gato-class submarine, meaning it was designed for long missions, stealth, and powerful attacks against enemy ships. War patrols were 60 to 70 days long, usually submerging 18 to 20 hours at a time. Cavalla had a range of more than 11,000 nautical miles on the surface. It used the same principles of buoyancy and ballast tanks that modern submarines use today.
As you learn about buoyancy and ballast tanks on submarines, keep in mind the size of the USS Cavalla, 311-feet-9-inches long, with a 27-foot-3-inch beam, which displaces 1,550 tons while surfaced. The USS Cavalla is so large that it housed a crew of six officers and 54 enlisted.
How a Submarine Floats on the Surface
When a submarine is floating on top of the water, it behaves like any other boat. This happens because of a force called buoyancy.
Buoyancy is the upward force that water applies to objects. If the force pushing up is greater than the force of gravity pulling down, an object will float. If the downward force is stronger, the object will sink.
On the surface, submarines like the USS Cavalla need to stay afloat, so they have large ballast tanks filled with air. These tanks work like empty containers, making the submarine less dense than the water around it. Since air is much lighter than water, the submarine is able to float just like a boat.
USS Cavalla Example: Staying Afloat Before a Mission
During World War II, when the USS Cavalla was patrolling the ocean looking for enemy ships, it would stay on the surface most of the time. This allowed the crew to use their diesel engines for power, since those engines needed air to work. The submarine would only dive when necessary, such as when attacking a target or avoiding detection.
The USS Cavalla was driven by four 1600-horsepower General Electric Motors V-16 diesel engines, two 126-cell Sargo battery banks, and four General Electric propulsion motors to drive her two screws. Cavalla could reach 21 knots surfaced or 10 knots submerged.
How a Submarine Dives Underwater
To sink, the submarine must become heavier. This is where the ballast tanks come into play again.
When the crew wants to submerge, they open special valves that let water flow into the ballast tanks. As the tanks fill with water, the submarine becomes heavier and its overall density increases. When the submarine is denser than the water around it, it begins to sink.
The submarine doesn’t just drop straight down, though. It has hydroplanes—small, wing-like fins—that help control its descent. By tilting the hydroplanes downward, the submarine can glide smoothly into the depths rather than just falling.
USS Cavalla Example: Preparing for an Attack
On June 19, 1944, the USS Cavalla used this exact process to set up a famous attack. During the Battle of the Philippine Sea, the Cavalla spotted the Japanese aircraft carrier Shōkaku. To avoid being seen, the crew quickly flooded the ballast tanks, allowing the submarine to dive beneath the waves. Moving silently underwater, the Cavalla got into position and launched six torpedoes, sinking the enemy ship. This was one of the biggest victories during the war and some say the turning point to U.S. victory in the Pacific Theater!
How a Submarine Stays at a Certain Depth
Once underwater, submarines don’t just sink to the bottom. They need to stay at a controlled depth, sometimes for long periods. This is done through a process called neutral buoyancy.
A submarine achieves neutral buoyancy when its weight is perfectly balanced with the buoyant force of the water. This means it neither sinks nor rises, but stays at the same depth.
To adjust depth, the crew can make small changes to the amount of water in the trim tanks (another type of ballast tank). If they add more water, the submarine becomes slightly heavier and sinks a little more. If they pump water out and replace it with air, the submarine becomes lighter and rises slightly.
The submarine’s hydroplanes also help adjust depth by controlling the angle of movement. By angling them slightly up or down, the submarine can gently glide to different depths without major changes to the ballast tanks.
USS Cavalla Example: Avoiding Detection
During missions, the USS Cavalla often had to stay at periscope depth, which is about 60 feet below the surface. This allowed the submarine to raise its periscope—a long tube with lenses—to see what was happening above the water while staying hidden. The crew carefully adjusted the ballast tanks to keep the submarine at this depth, avoiding detection while gathering information on enemy ships.
How a Submarine Rises to the Surface
When it’s time to surface, the process is reversed. The submarine needs to become lighter, so the crew forces high-pressure air into the ballast tanks. This pushes the water out, making the submarine’s overall density lower than the water around it.
As the submarine gets lighter, the buoyant force of the water pushes it upward. The hydroplanes can also be adjusted to help control the angle of ascent, making sure the submarine comes up smoothly instead of too quickly.
USS Cavalla Example: Returning Home
After completing a successful mission, the USS Cavalla would surface to recharge its batteries and use its diesel engines. Submarines of that time period relied on electric batteries when underwater, but these batteries would eventually run out of power. By surfacing, the crew could start the diesel engines, which powered the submarine and recharged the batteries at the same time.
Why Don’t Submarines Sink All the Way to the Bottom?
You might be wondering: if submarines sink by taking in water, why don’t they just keep sinking forever?
The answer is control. Submarines don’t just rely on ballast tanks; they also use their hydroplanes and precise weight management to stop sinking at a specific depth. The goal is to make the submarine neutral in the water, so it doesn’t keep falling.
If something goes wrong and a submarine takes in too much water, it can become too heavy and sink uncontrollably. That’s why modern submarines are built with emergency ballast systems that can quickly push out water and bring the submarine back up if needed.
The Amazing Science Behind Submarines
Submarines like the USS Cavalla have been using these same scientific principles for over a hundred years. By carefully controlling their buoyancy, ballast tanks, and hydroplanes, submarines can float, dive, stay at specific depths, and rise back to the surface whenever needed.
The next time you're looking for things to do in Galveston, visit the USS Cavalla, the only submarine museum in Texas. Even if you just watch a movie with submarines, think about all the science that goes into making these incredible machines work. From sneaking underwater for secret missions to exploring the deep ocean, submarines are a perfect example of how we use physics to conquer the depths of the sea!
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