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USS Dolphin Research Sub
Official US Navy Photograph
Declassified

Humans are designed for living above the water.  We breathe air.  We have a limited tolerance for cold.  We can't see through murky water.  We cannot handle extreme pressure on our bodies. We cannot swim so swiftly as do aquatic creatures.

A submarine is a vessel designed to help us overcome those limitations and allow us to descend beneath the waves.  To do so submarines must be precisely designed.

Each feature of a submarine is an adaptation to allow humans to survive and carry out our missions of research, pleasure or combat.

This webpage identifies each of the challenges of underwater travel and the steps submarines commonly take to meet that challenge.
 

Challenge 1  Temperature
 

A significant survival consideration for any mariner is staying warm.  The ocean can be a very cold place.  Exposure to its chill will quickly sap the heat away from a sailor and create a risk of fatal hypothermia.  Insulation and temperature regulators are essential on any serious ocean going vessel.

If the craft is a small submersible where the pilot is close to the engine, the temperature may actually get to be too hot.  On larger crafts the problem is usually one of combatting cold.  An efficient craft takes heat from the hot spots and vents that heat to the colder reaches of the sub, while using the cooler air from other regions to keep the hotter areas under control.

Challenge 2  Air

  Breathing

While an individual can survive cold temperatures for a while, they cannot survive more than a few minutes without air.    Specifically, they need the right chemicals in the air to survive.  Humans breathe a mixture of chemicals that consists of about 80% nitrogen and about 20% oxygen.  The oxygen is most critical for our survival, but we cannot safely breathe pure oxygen.

The task of the sub designers is to create a system on board the sub that provides the crew with the right balance of oxygen and other chemicals.  Several methods are used to accomplish this task.

    Storage

The earilest subs simply trapped and held air in the vessel.  This worked for a while, but soon the air became depleted of the usable oxygen and the crew had to surface.  Many personal subs still use this method, but they now store the air in tanks that compress a lot of air into a very small space.  None the less, this method cannot provide for very long term descents.

    Snorkeling

Large naval subs have the ability to use a snorkel to collect air from the surface.  Even though the sub is below the water, it can extend its periscope and snorkel many feet above the waves to reach clear air.  This may be pumped into the ship for breathing and reloading air tanks. The periscope, used for vision, and the snorkel may extend over 25 feet high.

    Scrubbing

Scrubbers are machines that use filters and other chemicals to replace the chemicals in the air that the crew has used up.  The scrubbers commonly remove a chemical we breathe out called Carbon Dioxide and refresh the oxygen content in the air.

    Chemistry

The newer high powered submarines, typically the nuclear powered submarines, have enough power to use electricity to break down sea water into its basic chemistry: hydrogen, oxygen, and salt.  This chemical method can allow for unlimited ability to breathe underwater.

Challenge 3  Movement

    Propulsion

Propulsion is the powering of the vessel through the water.  It can be provided by anything from oars, to air jets, to moing fins, but the most common method is by propeller.  The earliest subs had a screw-like propeller (see image of the Turtle).  This could be powered by a hand crank or by a motor.

The problem with most motors is that the strongest ones are typically powered by gas and generate toxic exhaust. This exhaust would need to be ejected from the submarine creating a trail of bubbles.  This is not a propblem for a research sub ( except it may disrupt the fish), but it is a big problem for a navy sub that needs to leave no trace.

Most naval submarines use either nuclear power or electric power to propel the vessel without exhaust propblems.
 

Efficiency

A successful undersea craft must be able to go up and down at will, but must also be able to go forward and backward and turn easily.   Unlike typical boats, where only a small part of the hull is underwater, submarines have much more exposed surface.  This creates a lot of drag or friction that slows the ship down.  Powerful motors and a smooth, rounded hull-design allows for the best efficiency.

Early subs had large bows and excessive deck equipment (like guns).  These were removed in later designs and the ships have become increasingly streamlined to insure maximum efficiency.  Interestingly, many subs can go faster under water than when cruising at the surface.

 Link to a chart demonstrating increased design efficiency

Depth Control and Buoyancy

What sets a submarine apart from traditional vessels is its ability to descend in the water.  In fact, most boats can go down (we call it sinking), the great thing about submarines is they can also come back up intact.  The ability to float is a ship's buoyancy.  Submarines have control over their buoyancy.  By filling tanks they can increase their weight and sink.  Later, they use air or other compressed gas and blast the water out of their tanks allowing their buoyancy to go up, and allowing the ship to surface.  These water tanks are called ballast tanks.

Historically ships could use other substances besides water to increase the weight of the ship.
Rocks or metal pellets have been used.  Their drawback is that once you have dumped your rocks to go up, you don't have any more ballast to sink again.  An example can be seen below in the drawing of the Turtle submarine from 1776.  It had a detachable weight that allowed it to dive and surface only once per mission.

When there is equal pressure pushing the ship down as there is pressure causing the ship to rise, the craft is at neutral buoyancy.  The ship may then choose to use its fins (elevators and planes) to control the depth of a moving submarine.
 
 

Challenge 4  Undersea Vision

Periscopes

Periscopes are viewing tubes that use mirrors and lenses to extend up out of the water and view what is going on outside of the water.  These periscopes may be more than thirty feet high and have to be exceptionally strong.  They may also be attached to a device called a snorkel that allows the submarine to bring in fresh air.

Sonar

Sonar is a technology that uses sound to bounce off objects and based on the time it takes the sound to  return, a sailor can identify the distance to the object.  Sonar is an abbreviation of Sonic Navigation and Ranging.  Skilled sonar operators can tell the difference between different types of vessels and even different types of sea life.

Radiotelemetry

Radiotelemetry is a process of sending and receiving radiowaves to calculate a submarines location and bearing.  This may come from calculating a distance from a known radiosource, or it may come from communication with satellites as in using global positioning data.  Radio telemetry can be exceptionally accurate, but may be harder to utilize at greater depths.

Challenge 5  Pressure

    Pressure hulls

At sea level the weight of the atmosphere is nearly 15 pounds per square inch.  When descending beneath the waves, every 33 feet adds another 15 pounds of pressure (called another atmosphere of pressure).  At 100 feet deep, the pressure is 4 times the pressure at sea level, or approximately 60 pounds per square inch.  Few vessels could survive this pressure unless they were designed to be very strong.

Submarines must be able to stand up to the crushing pressures of the depths.  Submarines are commonly made of strong metals and very thick glass or plexiglass for viewing portals.  It takes clever engineers to design a craft that has the best combination of strength without becoming too heavy or bulky to move efficiently through the water.

A round cross-section tends provide the best strength with the least extra bulkiness.  Thus most submarines are fairly round when viewed from the front.   They may be long but they taper to a point at the tail to allow the water to slip over and release the tail easily, this creates a very efficient and fast design.

The Early Submarine, The Turtle
(drawing by the author, R. Cureton)

The turtle was completed in 1776 and designed by David Bushnell.
The one-person craft ascended and descended by using a floodable ballast tank, but it also had a rigid ballast weight that it could detach to surface.  It used the screw propellors shown for its slow speed travel.  In combat it used a narrow screw to bore up and into the hull of enemy ships.

The creator/author of these pages welcomes your ideas for
improving this website.
Email the Webmaster and Author, Robert Cureton, Ph.D.

Design and content copyright 2001 by Robert Cureton, Ph.D. All rights reserved.