Overcoming the Dangers of Underwater Travel

For centuries, the ambition to navigate beneath the ocean surface was frustrated by physical realities. Water is roughly eight hundred times denser than air, exerting rapid increases in hydrostatic pressure with depth, while resisting movement and depriving occupants of light and breathable air. Early pioneers recognised that any underwater vessel had to resolve three interconnected dilemmas: structural resistance to implosion, vertical movement through buoyancy adjustment, and horizontal propulsion. The earliest documented working submersible, demonstrated by Cornelis Drebbel in the River Thames during the early seventeenth century, relied on a greased leather outer covering stretched over a wooden framework. While Drebbel's craft managed brief submerged journeys at shallow depths, its performance was strictly curtailed by the structural limitations of flexible hide.

By the late eighteenth century, attention turned towards rigid wooden hulls and mechanical controls. During the American Revolutionary War, David Bushnell designed the Turtle, an egg-shaped wooden craft intended to attach explosive charges to enemy hulls. Bushnell made notable progress in managing buoyancy. To submerge, the operator admitted water into a bilge tank via a foot-operated valve; to surface, the water was expelled using brass hand pumps. Lead ballast attached to the base provided stability, and a portion could be detached in an emergency to ensure rapid ascent. However, the craft relied entirely on single-person manual effort. The operator had to steer, pump water, and crank screw propellers simultaneously, severely constraining the vessel's range and effectiveness.

At the beginning of the nineteenth century, American engineer Robert Fulton expanded upon these principles with his copper-sheathed craft, the Nautilus. Fulton recognised that relying solely on static buoyancy—filling and emptying tanks—made precise depth-keeping difficult. In response, he incorporated horizontal diving rudders, or hydroplanes, which used forward motion to generate vertical hydrodynamic forces. Furthermore, Fulton introduced a dual-propulsion system: a collapsible mast and sail for surface cruising, paired with a hand-turned propeller when submerged. Although French trials demonstrated that the Nautilus could remain underwater for several hours using compressed air stored in an iron cylinder, military authorities declined to adopt the technology, partly out of ethical objections to underwater warfare.

As iron and steel replaced timber in naval architecture during the mid-nineteenth century, inventors constructed stronger hulls, though not without setbacks. In 1850, German engineer Wilhelm Bauer constructed the Brandtaucher, an iron submersible designed to break naval blockades. Instead of conventional water tanks, Bauer employed a heavy sliding weight that shifted along a track inside the hull to pitch the vessel up or down. During a demonstration in Kiel Harbour, the vessel pitched too steeply, causing the weight to break loose and crash forward. Water ballast rushed to the bow, and the hull partially caved in under the seabed pressure. Bauer and his crew survived only by allowing internal air pressure to equalise with the water outside before forcing the hatch open.

Beyond structural strength and buoyancy, the internal environment posed severe physiological risks. In an enclosed hull, an occupant consumes roughly twenty-five litres of oxygen hourly while generating toxic carbon dioxide. Early experimenters frequently misjudged the rate of atmospheric deterioration, mistaking the effects of carbon dioxide poisoning for simple fatigue. Moreover, the open flame lamps used for illumination accelerated oxygen depletion and created serious fire hazards. Although some craft attempted to cleanse the air using trays of slaked lime to absorb carbon dioxide, prolonged submersions remained dangerous until reliable electrical lighting and chemical life-support systems appeared towards the end of the century.

Propulsion remained the ultimate impediment to practical submersibles. Human muscle power, even when geared, was insufficient for navigating against ocean currents. In the 1860s, French designers developed the Plongeur, which utilised an engine driven by compressed air stored in massive tanks. While this eliminated the smoke of combustion, the heavy reservoirs drained rapidly, leaving the vessel with an extremely short range. Other inventors tested steam engines that were sealed before submerging, relying on latent heat in insulated boilers to supply underwater power. However, this approach raised interior temperatures to unbearable levels, often exceeding fifty degrees Celsius, causing severe heat exhaustion among the crew.

The resolution to these compounding engineering dilemmas finally emerged in the 1890s through the synthesis of internal combustion and electric power. By pairing an efficient petrol or diesel engine for surface travel with heavy rechargeable storage batteries for underwater propulsion, designers achieved a viable balance between endurance, speed, and safety. When running on the surface, the combustion engine propelled the craft while recharging the battery banks, which then provided silent, fume-free power beneath the waves. This dual-propulsion layout, combined with refined hydroplanes and compartmentalised pressure hulls, transformed the submersible into a dependable instrument of naval strategy.