The First Military Dive Watch: The 3646 Dive Watch

                   

For the elite underwater combat divers of World War II, a reliable timepiece wasn't a luxury - it was a lifeline. With covert missions in murky harbors, planting demolition charges on enemy ships, or navigating treacherous underwater environments. Precision timing was paramount, and a standard watch simply wouldn't suffice. This urgent need for robust, legible, and highly water-resistant timepieces spurred the creation of the earliest purpose-built military dive watches, forever changing the landscape of horology.

Why WWII Changed Everything

Unlike previous conflicts, World War II saw an unprecedented surge in specialized underwater warfare, distinct from earlier naval actions. So, you might be thinking, "Hold on, naval warfare was intense during World War I, right? Why didn't they need these dive watches back then?" Great question! It reaches the core of how naval warfare shifted between World War I and World War II.

In WWI, if a WWI submarine needed to "dive," it was mostly to hide or attack, not to deploy human combatants for complex, timed missions. Divers back then were more like underwater mechanics or recovery specialists. They are clearing fouled propellers, inspecting hulls, salvaging sunken goods. There simply wasn't a widespread tactical need for a watch designed for individual combat divers operating independently, deep underwater, for extended periods.

Image credit: BBC Bitesize – “World War One: The war at sea”  https://www.bbc.co.uk/bitesize/articles/zw4c8p3

Fast forward to World War II, and suddenly, the underwater world became a whole new battlefield. It wasn't just about torpedoes anymore. It was about frogmen (divers involved in underwater combat). These were highly trained, brave combat divers who could approach enemy vessels and port defenses unseen, placing demolition charges with pinpoint accuracy. This wasn't salvage work; this was sabotage. This wasn't an inspection; this was infiltration. And for these silent warriors, every second counted. They needed a way to time their approach, synchronize their charges, and know when to get out of dodge before things went boom. A regular wristwatch would be waterlogged, unreadable in the dark, and utterly useless. This massive shift in naval strategy and the emergence of these elite underwater units created an urgent, undeniable demand for specialized timepieces.

Source credit: Museum Crush – “Buster Crabb and the dawn of underwater frogman warfare in World War Two”  https://museumcrush.org/buster-crabb-and-the-dawn-of-underwater-frogman-warfare-in-world-war-two/

How Rebreathers Unleashed the Frogmen

So, what allowed these frogmen to become the silent terrors of the deep, opening the door for dive watches? Beyond the evolving military doctrines, it was a crucial piece of technology: the closed-circuit rebreather. And understanding how these ingenious devices work is key to understanding why dive watches became indispensable.

Forget those clunky open-circuit SCUBA tanks where every exhale sends a stream of bubbles to the surface, practically shouting "Here I am!" to anyone above. Rebreathers are far more subtle. Think of it like a personal, closed-loop life support system for your breath.

Here's the simple magic:

• 1. Exhale into a Bag (Your "Atmosphere" Recycler):

  The body uses roughly 5% of the oxygen in each breath. Inhaled air is about 21% oxygen; exhaled air is still around 16%. When a diver exhales into a rebreather, that breath (a mix of oxygen and carbon dioxide) doesn’t just bubble out into the ocean — it goes into a special breathing bag within the unit.

• 2. Counter Lung:

  The counterlung acts as a buffer between the diver’s lungs and the rest of the system. It stores the exhaled gas before it undergoes carbon dioxide removal and oxygen replenishment.

• 3. Scrubbing the Bad Stuff (The CO2 Filter):

  That breathing bag connects to a scrubber — a canister filled with chemical absorbent, usually soda lime. This is where the magic happens. As your exhaled breath passes through, the scrubber chemically removes carbon dioxide (the waste product that makes you feel breathless). The carbon dioxide reacts with the calcium hydroxide in the soda lime, forming calcium carbonate, which is trapped within the scrubber material. This process effectively removes the carbon dioxide from the breathing gas. The reaction also regenerates the hydroxide ions, allowing them to continue absorbing more carbon dioxide.

• 4. Adding Fresh Air (The Oxygen Booster):

  Once the carbon dioxide is removed, the remaining gas is mostly oxygen — but slightly depleted. A small, precisely measured amount of fresh oxygen is automatically or manually added from a high-pressure tank back into the loop.

• 5. Inhale:

  Now, the diver inhales this refreshed, CO₂-free, oxygen-rich air back from the breathing bag. It’s a continuous closed loop, allowing maximum use of every liter of oxygen in the tank.

Image credit: Research Gate – “The use of mixture model theory in CFD for the chemical reaction between CO2 and soda lime in closed circuit rebreather scrubbers” https://www.researchgate.net/figure/Closed-Circuit-Rebreather-schematic-A-complete-automated-system-is-shown-for-gas_fig5_272188342

This brilliant system solves several critical problems that plagued earlier diving methods and paved the way for tactical underwater operations:

• • No Bubbles:

  This is the biggest one. No exhaust bubbles mean the diver leaves virtually no trace, making them nearly invisible to surface patrols. For combat divers, silence and invisibility are essential.

• • Oxygen Efficiency (Longer Dive Times):

  Because the rebreather recycles precious oxygen, it dramatically reduces oxygen consumption compared to open-circuit systems. A small oxygen tank can last for hours, not minutes. This directly extends mission times, allowing divers to travel greater distances, spend more time at the target, and perform complex tasks, making a reliable, legible watch indispensable for planning and executing those longer operations.

• • Beating Nitrogen Narcosis & Decompression Sickness:

  Traditional air (which is about 78% nitrogen) can lead to nitrogen narcosis (a disorienting "rapture of the deep" at shallower depths) and decompression sickness (the dreaded "bends" if ascending too quickly). Rebreathers, especially those designed for combat divers, often use precise oxygen-rich mixes. By eliminating or drastically reducing nitrogen in the breathing gas, these rebreathers almost entirely prevented nitrogen narcosis and significantly mitigated the risk of decompression sickness on shallower, tactical dives. This meant divers could operate more clearly, effectively, and safely, allowing them to focus on the mission, where timing (and thus, a reliable watch) was critical.

  Close-circuit rebreathers also have diluents that dilute the oxygen to prevent oxygen toxicity and ensure that your volume of breathable air doesn't decrease  as you descend in the water.

  
  

  So, in essence, the rebreather unlocks the full potential of the human combat diver, turning them into silent, effective underwater agents. And now that the rebreather has pushed the edge for underwater warfare, it has become essential to have a dive watch capable of enduring this new medium of naval tactics. The Navy now consistently goes under water for an extended period, prompting the timing of such dives to be critical. Although the rebreather allows the diver to dive longer than ever before, it's not something that can indefinitely let you survive underwater. It's important to time your dives and have a watch that serves that purpose without breaking. You're counting on every second to live; a watch stops being a mere accessory and becomes mission-critical.

Panerai's Precision Legacy: The Italian Connection

Now, let's talk about the company that became synonymous with these early military dive watches: Officine Panerai. They weren't just some upstart watchmaker jumping on a trend. Panerai had a history with the Royal Italian Navy, a deep-rooted relationship stretching back to the early 1900s.

Panerai, founded in Florence in 1860, was a watch shop, watch workshop, and watch-making school. Over time, the company developed a close relationship with the Royal Italian Navy, providing specialized instruments-such as compasses, depth gauges, and luminous devices-designed to meet the demands of maritime operations. Because of this long-standing reputation for quality and innovation in specialized naval instruments, when the Royal Italian Navy needed a highly specific, cutting-edge wrist-mounted instrument for their newly formed Decima Flottiglia MAS frogmen units, Panerai was the obvious, trusted choice. They understood the unique demands of naval operations better than almost anyone.

Image Source: Doug Kincaid User from Pinterest - https://in.pinterest.com/pin/1149332767384780072/

So, why did the iconic Panerai Ref. 3646, often called the "Radiomir," emerge in 1938, even before the official declaration of WWII for Italy?

Simple: militaries don't wait for war to start before developing the tools they anticipate needing. The political climate in the late 1930s was already tense. Nations were preparing, innovating, and quietly building up their specialized forces. The Italian Navy, recognizing the strategic potential of underwater combat divers, put out the call to Panerai for a watch that could meet these extreme new requirements. The 1938 date reflects this proactive preparation, a testament to foresight in military planning.

The Luminescent Visionary: Panerai's Glow-Up

Panerai truly shone (again, pun intended!) when it came to making these watches readable in the pitch black of the ocean depths. Were they the absolute first to put luminescence on a watch? Not quite. Luminous paints, often radium-based, had been used on watch dials for decades, particularly for military field watches, to allow visibility in low light.

However, Panerai's genius wasn't just applying luminescence; it was optimizing it for extreme underwater conditions. Their patented "Radiomir" compound, a radium-bromide-based substance, glowed with incredible intensity. But that wasn't enough. They understood that even the brightest paint could be obscured by reflections or simply not be punchy enough when you're deep beneath the surface with zero ambient light.

This led them to their most brilliant innovation: the "sandwich style dial."

Here's how it works and why it was a game-changer for legibility:

• • The Bottom Layer (The Luminous Heart):
• Panerai created a solid disc as the base layer. This entire disc was generously coated with their powerful Radiomir luminous compound. Think of it as a solid, glowing foundation.

• • The Top Layer (The Stencil):
• On top of this luminous base, they placed another, thinner dial disc. This top disc wasn't solid; it had cut-outs - stenciled openings - precisely where the numerals (3, 6, 9, 12) and hour markers would be.

• • The Glowing Effect:
• When these two layers were put together, the brilliant Radiomir paste from the bottom layer would literally shine up through the cut-out numerals and markers on the top layer.

Why was this so effective?

• • Maximized Luminosity:
• Unlike simply painting thin lines of luminous material onto a solid dial, the sandwich construction allowed for a much thicker, more uniform, and therefore brighter application of the luminous compound. More glowing material meant more light output.

• • Sharper Definition:
• The crisp, defined edges of the cut-outs created incredibly sharp and clear glowing numerals, even in total darkness. There was no smudging or fading of painted lines.

• • Durability:
• The luminous material was somewhat protected within the sandwich, making it less prone to flaking or degradation over time compared to a surface application.

This simple yet revolutionary design ensured that a frogman, even in the murkiest depths or the dead of night, could glance at his wrist and instantly read the time, a crucial factor for mission success and personal survival.

Why Rolex Said "Yes!" to Panerai

So, Panerai had the vision for the ultimate dive watch and the groundbreaking luminous technology, but they weren't watchmakers in the sense of producing movements and cases on a large scale. For that, they turned to a burgeoning giant in the watch world: Rolex.

Why Rolex? At the time, Rolex was already making waves with its innovations in water resistance. Their big claim to fame was the "Oyster" case, patented back in 1926.

What made the Oyster Case unique and perfect for Panerai's needs?

• • Screwed-Down Crown:

  This was the game-changer. Instead of a simple push-in crown that could leak, Rolex developed a crown that screwed down onto the case tube, creating a watertight seal much like a submarine hatch. This was crucial for dive watches.

• • Screwed-Down Caseback:

  Similar to the crown, the caseback was also threaded and screwed tightly onto the middle case, providing another robust barrier against water ingress.

• • Sealed Bezel:

  The crystal was also sealed securely into the middle case.

Image Source: Analog Shift – “Rolex Oyster” https://www.analogshift.com/products/rolex-cushion-case?srsltid=AfmBOopFAu1nHNcOlil77miKiyVYTHmZ2b8D9NSk_xAqXwTY0bm5x3Mn

These innovations combined to create an impenetrable fortress for the delicate movement inside. When Panerai approached Rolex, it was a logical partnership.

Panerai was primarily an instrument maker and didn't have the large-scale wristwatch manufacturing capabilities of a company like Rolex, particularly for intricate movements and the revolutionary Oyster cases. Sourcing these components from a specialized expert like Rolex was far more efficient and effective than trying to build them from scratch or source them from less proven suppliers.

For Rolex, this was an unparalleled opportunity. Having their Oyster cases and movements tested in the most demanding, real-world, combat-diving scenarios imaginable was invaluable. Think of it as the ultimate stress test. Success in such conditions would provide irrefutable proof of the Oyster's superior robustness and water resistance. This kind of "field testing" by elite military units offered a level of credibility and prestige that no civilian advertising campaign could ever buy. It implicitly reinforced Rolex's reputation for making "tool watches" that could truly withstand anything.

Image Source: Rolex Magazine – “The Royal Italian Navy Frogmen - The First Professional Dive Watch In Action” https://www.rolexmagazine.com/2007/07/chapter-3-complete-history-of-rolex_27.html?m=1#gsc.tab=0

While Rolex was happy to supply parts, they weren't directly dealing with the Italian military's procurement, modifications, or specific tactical requirements. Panerai handled that, acting as the primary contractor and taking on the customization and direct liaison. Rolex provided the core, cutting-edge technology, benefiting from the association without being bogged down by the full military production process.

Rolex supplied their large "cushion" shaped Oyster cases (often 47mm, huge for the time but necessary for underwater legibility) and their hand-wound movements, most notably the Rolex Cal. 618. Fun fact: This movement wasn't originally designed for wristwatches. It was actually a robust pocket watch movement from Cortebert (a Swiss movement manufacturer) that Rolex adapted for their use. Its large size and sturdy construction made it ideal for the demanding conditions of a military dive watch.

This collaboration was a win-win: Panerai got the vital components needed to build their specialized instruments, and Rolex gained invaluable real-world testing and a pioneering role in extreme-use watches, further cementing their reputation for robust timepieces.

Missions of the Frogmen: Where the Radiomir Shone

These Panerai Radiomir Ref. 3646 watches weren't just pretty faces; they were integral tools in some of WWII's most audacious and impactful underwater operations. They were primarily issued to the elite Italian Decima Flottiglia MAS, a truly fearsome Naval unit. Later, they also found their way into the hands of the German Kampfschwimmer (combat swimmers), who conducted similar daring missions.

Image Source: Panerai Magazine – “Human Torpedo Documentary” https://www.paneraimagazine.com/2012/09/human-torpedo-documentary.html?m=1

So, what kind of operations were they pulling off, and how did the Radiomir help?

The Alexandria Raid (1941): This is perhaps the most famous mission. Six Italian frogmen, riding human torpedoes (nicknamed "pigs"), infiltrated the heavily defended British naval base in Alexandria, Egypt. Under the cover of darkness, they attached limpet mines to the hulls of two British battleships, HMS Queen Elizabeth and HMS Valiant, and a tanker. The mines detonated hours later, crippling the ships and significantly impacting British naval power in the Mediterranean. How did the Radiomir help?

• • Precise Bomb Setting:

  The frogmen used the watch to accurately set the fuses on their limpet mines. Even a few minutes off could mean mission failure or premature detonation, putting their lives at risk.

• • Synchronized Infiltration:

  The large, glowing dial allowed them to coordinate their movements with their partners and the human torpedoes, ensuring they approached the targets together and under the cover of optimal darkness.

• • Extended Mission Duration:

  With rebreathers allowing for hours underwater, the watch was their constant companion, keeping track of their dwindling air supply and the time remaining for their operation.

These missions were incredibly risky, often involving swimming through anti-submarine nets, avoiding patrol boats, and dealing with strong currents. The Panerai Radiomir Ref. 3646 wasn't just a stylish accessory; it was a fundamental piece of their combat gear. Its unparalleled legibility, water resistance, and robust construction directly contributed to the success of these daring operations, allowing the frogmen to time their attacks, synchronize their movements, and return safely from their dangerous underwater endeavors.

The Legacy Continues...

The partnership between Panerai and Rolex, forged in the crucible of WWII, laid the groundwork for the modern dive watch. It established the core principles of what a professional dive watch should be: supremely water-resistant, incredibly legible in darkness, and robust enough to withstand the most extreme environments. From those shadowy wartime missions, the lineage of dive watches we admire today truly began, a testament to innovation born from necessity, and the brave individuals who trusted these silent sentinels with their lives.