Wet submarines and dry submarines are two very different ways to travel underwater. A wet sub, often called a swimmer delivery vehicle, is flooded with water, so the crew rides in diving gear and breathes from scuba or supplied gas. A dry sub keeps its crew inside a sealed cabin at about one atmosphere of pressure, similar to being inside a small underwater spacecraft.
Comparing them helps students connect buoyancy, pressure, life support, and vehicle design to real marine engineering.
Key Facts
- Hydrostatic pressure increases with depth: P = P0 + rho g h.
- At 10 m depth in seawater, pressure is about 2 atm total, including atmospheric pressure.
- Buoyant force equals the weight of displaced water: Fb = rho water g V displaced.
- A wet sub is flooded, so the crew experiences the surrounding water pressure directly.
- A dry sub has a pressure hull that keeps the cabin near 1 atm while resisting outside pressure.
- For neutral buoyancy, weight equals buoyant force: W = Fb.
Vocabulary
- Wet sub
- A wet sub is an underwater vehicle whose crew compartment is flooded, so riders must wear diving equipment.
- Dry sub
- A dry sub is an underwater vehicle with a sealed pressure hull that keeps people in a dry air-filled cabin.
- Pressure hull
- A pressure hull is a strong sealed structure designed to withstand the force of surrounding water at depth.
- Buoyancy
- Buoyancy is the upward force a fluid exerts on an object because the object displaces some of the fluid.
- Life support
- Life support is the equipment that provides breathable gas, removes carbon dioxide, and helps maintain safe conditions for people.
Common Mistakes to Avoid
- Treating a wet sub like a dry cabin is wrong because a wet sub is intentionally flooded and the crew must handle depth pressure like divers.
- Ignoring atmospheric pressure in depth calculations is wrong because total pressure underwater includes both surface air pressure and water pressure.
- Assuming all submarines dive by becoming heavier than water is wrong because many adjust ballast to become nearly neutrally buoyant for controlled motion.
- Forgetting that dry subs need strong pressure hulls is wrong because the outside water pressure can crush a sealed cabin if the structure is not designed for the depth.
Practice Questions
- 1 A wet sub operates at a depth of 20 m in seawater with density 1025 kg/m3. Using P = P0 + rho g h, with P0 = 101000 Pa and g = 9.8 m/s2, find the total pressure in pascals and in atmospheres.
- 2 A small dry sub displaces 3.0 m3 of seawater with density 1025 kg/m3. What buoyant force acts on it? Use g = 9.8 m/s2.
- 3 A team must travel underwater for several hours while staying warm, dry, and protected from high pressure. Explain why a dry sub is more suitable than a wet sub for this mission.