DIVE!

Take the plunge into the sinking science of submarines.

Last spring, divers plunged into the Atlantic Ocean four miles off the South Carolina coast, hoping to solve a 136-year-old mystery. Thirty feet underwater they chiseled three feet of sediment (eroded rock particles) off one of history's strangest contraptions, forged from iron locomotive boilers--the 28-ton Hunley, the first submarine ever to sink an enemy ship in combat.

On February 17, 1864--near the end of the Civil War--the Confederate Hunley stole into Charleston harbor and rammed a torpedo into the USS Housatonic. The Union warship sank in three minutes, but minutes later the Hunley mysteriously disappeared as well, killing all nine crewmen on board.

On August 8, 2000, through an extraordinary engineering feat, salvagers hoisted the sub from its watery grave to cheering crowds in Charleston (see "Raising the HUNLEY"). Whether or not scientists solve the mysterious sinking of the Hunley, one thing is certain: the sub's strike proved once and for all that no ship could again rule out an attack from below.

AMAZING ADVANCES

Submarines have undergone an amazing sea change since the days of the Hunley. (The world's first attack sub, the American egg-shaped Turtle, was deployed during the Revolutionary War in 1776.) While the Hunley's "engine" consisted of a propeller cranked by crewmen's aching arm and back muscles, today's subs are fueled by nuclear power. Onboard nuclear reactors split atoms of the element uranium to release energy in a process called fission, which in turn churns out heat and steam to spin an electrical generator. The latest subs stock enough nuclear fuel to power the ships for their lifespan--approximately 33 years.

While the Confederate sub crammed in nine crewmen side by side, modern subs can house more than 135 crewmen, stretch up to 171 meters (560 feet), and weigh up to 19,000 tons. Whereas the Hunley could trudge only a few miles from port, nuclear-powered subs can dive deeper than 800 feet (actual depth is classified information), travel at about 25 knots (46 kilometers per hour), and stay underwater for about three months at a time!

SILENT SERVICE

Officially adopted by the U.S. Navy in 1900, subs played a key role in winning both World War II (1939-1945) and the Cold War (1945-1989). The Navy proudly calls its submarine corps the "silent service." "Stealth is our thing," says Lt. Commander Tom Monroe. Today the Navy boasts two stealthy sub types: the bigger ones, called "boomers," carry nuclear missiles; smaller "fast attack" subs are designed to destroy enemy subs and ships. Both types spy on marine vessels and plant enemy sea mines.

Older subs deployed in World War II featured square ship hulls and lacked the engine power to stay underwater for more than a day. In the late 1940s, designers developed teardrop-shape subs inspired by whales and porpoises--the new shape slashed water friction (rubbing force). Launched in 1954, the celebrated USS Nautilus--the world's first nuclear-powered sub--was among the first to sport the teardrop or hydrodynamic shape.

Today's subs navigate and spy by "hearing" their way through water. They use sonar, which stands for "sound navigation and ranging." But subs tend to avoid active sonar, which sends out pulsing sound waves that ping when they rebound off marine vessels. "It's like blowing your horn," says Mickey Garverick, former USS Andrew Jackson sub captain. Instead, subs eavesdrop on other underwater craft using devices called passive sonar, which only picks up sound waves rather than transmit them. What do subs hear? "Believe it or not, even a mass of shrimp makes a lot of noise," says Lt. Jason Crain. "It's a clicking sound."

Differences in seawater temperature also help subs hide out. By lurking right below the thermocline, the layer between the surface and deeper waters, subs can avoid some sonar pings from ships or aircraft trying to detect them. That's because sound waves distort when passing through varying water temperatures.

SUB SCIENCE

How do massive subs float, sink, and resurface? A sub floats on the ocean surface when the weight of the water it displaces, or pushes aside, is equal to its own weight. This creates an upward force, or buoyant force, which acts opposite to Earth's downward pull of gravity. (Buoyant force is one part of what's called Archimedes' principle after the Greek mathematician.)

All subs feature inner and outer hulls. Between the two hulls is a space shaped like an inner tube called the ballast tanks. They hold air and/or seawater (see diagram). When a sub is ready to dive underwater, vents on top of the ballast tanks release air and let in heavier seawater--this impacts the sub's density, or how solid it is. When a submarine's density is greater than the surrounding water, it starts to sink--creating negative buoyancy.

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Once submerged, crewmen constantly supervise seawater levels in the ballast tanks so that the sub maintains neutral buoyancy--allowing the ship to cruise underwater at a constant depth. Operators steer the sub up and down using tail flaps called hydroplanes or "planes."

To resurface, operators release compressed air into ballast tanks; the air forces out seawater until the sub's density is less than that of the water around it--positive buoyancy. Sound simple? Last February, the USS Greeneville made headlines when it resurfaced too quickly during a training maneuver near Hawaii. Compressed air flushed water out of its ballast tanks, causing the 6,000-ton sub to suddenly become much lighter than surrounding water. The lethal result: The Greeneville accidentally rammed a Japanese fishing trawler, killing nine civilians on board. "You're coming up with tremendous force," says Garverick. "It'll move the whole first half of the submarine right out of the water."

LIFE UNDER PRESSURE

Submariners go through months of special training before they ever take the plunge. A typical schedule on a nuclear sub lasts 18--not 24--hours. Crewmen work six-hour shifts; then spend remaining hours training, studying, exercising, and sleeping. On a fast attack sub, more than 130 men share a space the size of a three-bedroom house. "You can go to the john and shut the door or take a shower, but that's about it for privacy," says Garverick. "A submarine is built for speed, not comfort."

How do submariners breathe, drink, and eat? A special oxygen generator strips oxygen molecules from distilled seawater, then pumps oxygen into crewmen's air supply. Computerized sensors monitor fresh air and moisture levels on board. Special systems transform seawater into fresh water through the process of destination, in which boiling seawater evaporates to separate salt and impurities from the liquid.

All food is kept in cold storage--enough to prepare up to 50,000 meals and snacks! "The biggest problem with a nuclear submarine is that you can only carry about 90 days' worth of food," says Garverick. "It's not the crew that. wears out. We just flat out eat everything that's on board." Biodegradable trash gets compressed into air-tight cylinders, which are shot out of the bottom of the ship. But before that happens, sonar operators listen to make sure no "enemy" can detect the can rattling down the garbage tube!

ANATOMY of a NUCLEAR SUBMARINE

The U.S. Navy officially incorporated submarines into their arsenal of marine vessels 100 years ago. The first subs, the Holland class (like cars, subs come in different models) ran on gasoline and electric engines.

Currently in the works: the Virginia-class attack submarines. This high-tech model runs on nuclear power, speeds at more than 25 knots (46 km/h), and will be the first sub to navigate without a periscope--replaced by hightech cameras and electronic fiber-optic imagery systems. Cutting-edge sonar arrays will accurately map the ocean floor, detect mine fields, and rapidly pinpoint enemy targets. Scheduled for delivery: 2004.

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HANDS-ON SCIENCE SINK OR SWIM

What makes sub sink or rise? Build a Cartesian Diver--the earliest version of this experiment was devised by French scientist Rene Descartes (1596-1650).

YOU NEED:

2-liter plastic soda bottle with screw-on cap * (*)pen cap (without hole on top) * (*)modeling clay * room-temperature water * tall drinking glass

TO DO:

1. To make the diver, roll the clay into a marble-size ball and stick it on the tail of the pen cap.

2. Fill the drinking glass with water.

3. Gently drop the cap into the glass. Adjust the amount of clay until the cap floats upright, with just the tip above the water.

4. Fill the soda bottle with water and carefully drop the pen cap into the bottle. Tightly cap the soda bottle.

5. Squeeze the bottle. Observe. (If your diver doesn't sink, you may need to add more clay to the pen cap's tail.)

6. Release the bottle. Observe.

(*) An eyedropper works as well. Fill dropper with water. Adjust the amount to follow Step 3.

CONCLUSIONS:

What happens to the pen cap when you squeeze or release the bottle? Why? How does this relate to subs?

DON'T STOP NOW:

Experiment with different amounts of clay. What happens to the diver's surfacing or diving speed? Why?

Did You Know?

* The USS Triton became the first submarine to circle the globe in 1960. The voyage followed 16th-century explorer Ferdinand Magellan's route and took 84 days.

* Subs aren't only used for military purposes. They have been used to map the ocean floor, to study deep-sea fish, and to study the topography and biodiversity of deep-sea thermal vents.

Cross-Curricular Connection

Research: Write a short report on a technological advance due to submarine design and development.

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National Science Education Standards

Grades 5-8: motions and forces * abilities of technological design * understanding about science and technology

Grades 9-12: understanding about science and technology * abilities of technological design * science as a human endeavor

Resources

"Dive! Dive! Dive!," How Stuff Works January/February 2001

"Probing a Sea Puzzle," Time, June 19, 2000

Directions: Read "Dive!" Match the word (s) on the left column with the correct phrase at the right.

-- 1. negative buoyancy   a. eroded rock particles

-- 2. sediment            b. sound navigation and ranging

-- 3. density             c. the layer between the surface
                             and deeper waters

-- 4. boomers             d. smaller subs designed to destroy
                             enemy subs

-- 5. fast-attack subs    e. an upward force which is opposite
                             to that of gravity

-- 6. sonar               f. in water, when an object's density is
                             greater than the surrounding water

-- 7. buoyant force       g. mass or solidity of an object

-- 8. thermocline         h. bigger subs that carry nuclear missiles

1. f 2. a 3. g 4. h 5. d 6. b 7. e 8. c

RELATED ARTICLE: Raising the HUNLEY

Finding the wreck of the Hunley in 1995 left scientists elated--dismayed. After more than a century of burial in 28 feet of sea water, its iron hull had become overly brittle and could have easily disintegrated if the Hunley were even slightly budged. Hoisting the sub intact seemed impossible.

After five years of planning, engineers lowered a specially designed metal truss to the seafloor from a 600-ton barge and crane. Divers slid nylon slings under the hull and attached them to the truss. Vinyl sacks full of liquid foam were wedged between the truss and the Hunley's hull; the foam hardened into a concrete-like shell to cushion the hull as the crane lifted the sub to the surface.

On August 8, 2000, the Hunley saw daylight for the first time in 136 years. Silt within the hull had preserved sailors' skeletons, and artifacts like buttons and cloth. Last March, archaeologists uncovered the first human bones. Eventually, the Hunley will become a prize exhibit at South Carolina's Charleston Museum. But what sank it still remains a mystery.