A further observation about the 6B/346 Mk 11 “Navigator’s” watch

Quote: The balance isn’t shock-protected!?

Ah! An astute observation.

Nope, none of the JLC-manufactured 6B/346 units were outfitted with shock protection. At least not from the factory (more on this later)

Likewise, none of the first-generation IWC-made 6B/346 watches had a shock-protected balance. However, later IWC units did add shock protection.

Quote: Yikes.

Indeed, there were some Field Complaints related to broken balance staffs. In the end, it became enough of an issue that most of the non-shock-protected examples were retired first (prior to 1970, apparently), and the units which remained in use into the 70’s and 80’s were all shock-protected models. And here’s where it gets murky.

Conventional wisdom says that only the IWC units were ever shock-protected. However, a few parachoc equipped JLC models do exist! Whether this surgery was done by HMRS Watchmakers as part of an Official Field Retrofit, or by some misguided (though well-meaning, one assumes) civilian watchmaker after the fact is a bone of contention. Personally I think it’s the latter, as their low numbers don’t seem to jive with the Official Field Retrofit theory. Alas, the official records don’t specify, and the men who could answer the question from personal experience have all passed. So probably no way to ever know for sure.

Suffice to say that I wouldn’t touch a shock-protected 6B/346 with a ten-foot pole. OTOH, if it was an 6B/346 I lusted for ( It’s not, but if ) then I would settle only for a parachoc model.

Quote: Doesn’t sound like a typical mil-spec product to me.

Sounds exactly like a typical mil-spec product.. only one of WW2 vintage.

Bear in mind that no military-issued chronometers in WW2 used a shock-protect balance. This is true for both Allied and Axis forces. Your vaunted B-Uhr? Not shock-protected, not as built by any of the Big Five Manufactures. The humble A-11? Nope, no shock-protection there either, not in any of it’s myriad iterations.

Now, Allied forces did recognize the need, and the solution was the Elgin Master Navigational Aviation Chronometer:

These were basically a marine chronometer mounted in the Rube Goldberg spring-loaded isolation cage. Produced only in relatively low numbers, it’s my understanding that these would be issued only to the lead plane in a bomber formation, with everyone else making do with an av-certified A-11 wristwatch.

Quote: Any idea why they left that feature out of that movement?

You bet! I’ve seen a dozen different theories as to why. Eleven of which are total bunk.

The reason is that shock-protection was in it’s infancy at that time – Incabloc® was only invented in 1934, right? Watchmakers tend towards a certain staid conservatism in Engineering, and so it didn’t exactly take the horological world by storm. Some old-school watchmakers detested it, at first, and were convinced that it compromised the precision of the balance. And there’s solid evidence that this was the case, early on. Some of the primitive versions of parachoc, when activated, did not return the balance to precise alignment.

These problems were largely solved by the start of WW2 hostilities, of course, and completely solved by the time the 6B/346 specification was issued. But watchmakers being watchmakers, still not all were 100% on-board with the new technology. And the government hacks writing the spec didn’t have a clue, soooooo…

In the end, it was the practical real-world testing of the testosterone-poisoned bomber crews which provided irrefutable evidence that parachoc worked. Since 6B/346 timepieces existed both with and without, it made the point crystal clear, and served to hasten the mainstream adoption of Incabloc, Etachoc, Kif, Diashock, Parashock, etc. So, looked at a certain way, you can thank the 6B/346 chonos for the fact that all your modern mechanical timepieces feature a balance wheel shock-protection system. Yet one more reason why these watches are treasures of horological history!

Thanks for asking!

The UK Ministry of Defence 6B/346 Mk 11 “Navigator’s” Wristwatch

First let me say that so many fine articles have been written about this iconic wristwatch over the years that, frankly, I’m a bit embarrassed to set my hand to the task. Lest I be accused of hubris, I will freely admit that I’m breaking no new ground here. Indeed, I’ve learned ‘most all I know of the Mark Eleven from those writings, and I will hyperlink a selection of them before I close. Also, I must say that the common perception of modern watch enthusiasts that “Pilot’s Watch” is synonymous with the Beobachtungsuhren (or “B-Uhr”) designs of the German Luftwaffe of WW2 strikes me as a grave disservice. So today we will focus on the aviation timepieces used by the Allied forces and which were, arguably, superior to their Axis counterparts.

OK. Where to start? I suppose that to truly tell the story of the Mk 11, it’s necessary to go all the way back to John Harrison and the marine chronometer.

While these instruments are beyond the scope of this article, their impact on the modern world should not be under-estimated. Here at last was a means of accurately determining longitude, something of a Holy Grail for all sea-faring nations of the time.

Fast forward to the Aviation Age, and celestial navigation was still the only practical method of determining longitude. A precision timepiece is an integral part of celestial navigation, of course, and one supposes that actual ship’s chronometers were used early on. And I suspect there may have been some railways crossover, with Railroad Certified pocket watches being pressed into aviation service.

The first purpose-designed wristwatch for astro-navigation was the Longines Seconds-Setting aka “Longines Weems” of 1929, designed by Professor Philip Weems of the United States Naval Academy. Aside from central-seconds, itself a very unusual feature at the time, the seconds hand stopped (or “hacked”) during time-setting, allowing the watch to be precisely synchronized to a reference time source. These were quickly followed in 1932 by an improved version designed by the famous aviator Charles Lindbergh, the “Longines Hour Angle”. Mind you, some consider these not so much an improvement, as simply being tailored to Lindbergh’s personal taste.

As far as I can tell, these Longines models were never officially issued to the Armed Forces of any country. However, there can be no doubt that they were informally adopted by some military aviators in both the USA and Britain. When WW2 hostilities began, Britain had a sudden need for astro-navigation timepieces in far greater numbers than could be filled by the aviator’s personal watches. Luckily there was an off-the-shelf solution in the A-11 wristwatch which had been adopted by the USA in 1941.

Britain formally adopted the US-made A-11 (under MoD Specification 6B/234) in 1942. The A-11 was manufactured by Bulova, Elgin, and Waltham, and the specification was rather broad, accommodating a number of slight variations between the three main models. They all share the characteristic of having central-seconds, seconds-setter (“hacking”), with black dial and full arabics.

The A-11 was a rugged, reliable timepiece, and did yeoman service during WW2. Indeed, some have gone so far as to call it “The Watch That Won The War”. However, it was something of a compromise in that true chronometer-grade performance was not a given with any random example. The hacking feature mitigated this somewhat, of course, and the watches destined for actual astro-navigation use were hand-picked from the most accurate examples.

After the war the UK MoD set out to remove this compromise, and issued the 6B/346 specification in late 1946 or early 1947. The specs called for a watch which was, on the surface, very similar to the A-11. The main difference is that chronometer-grade performance was specified, along with anti-magnetic properties.

The resulting watch, the 6B/346 Mk 11 “Navigator’s”, was produced by two companies, Jaeger-LeCoultre (JLC) and International Watch Company (IWC), and was issued to RAF and RAAF forces from 1948 to 1953.

Click the photos for larger version

 

While both the IWC and the JLC examples are fine watches, my favorite by far is the Jaeger-LeCoultre, and so this is the model I will focus upon.

The main external difference between the two is visible in the side profile:

The IWC watch has lugs with a less-pronounced downturn than the JLC shown and, while the lugs are long, they are not quite as long as the JLC lugs. Some surviving examples have signed crowns, and these appear to have been added during field refits. The watches were returned to stores for overhaul every 2-3 years, and several running changes were retrofitted as a matter of course.

One thing that didn’t change was the anti-magnetic protection.

The dial is fashioned of soft iron, with the movement capped by an iron “dust cover”, effectively shielding the movement with a full faraday cage. I’ve heard it said that the Mk 11 was the first wristwatch designed from the outset to offer this level of anti-magnetic shielding.

But the real star of the show, and IMHO the one thing that gives the JLC a leg up over the IWC, is the movement.

The JLC caliber 488SBr movement is a 12.5 ligne, 16-jewel work of art featuring Glucydur balance, Breguet-type Nivarox hairspring, non-magnetic curb pins, and impeccable finishing. This movement was specific to the 6B/346 watches, whereas the caliber 89 used by IWC was used in many other watches. Yet one more reason why, despite conventional wisdom, the JLC is for me a far more desirable timepiece compared to it’s IWC sibling.

The case is full stainless steel (or “StayBrite” as JLC called it), the caseback threaded (note the four spanner notches vs. the more common 6), with the physical dimensions being 35.3 mm w/o crown, 43.5 mm across the lugs, 12.6 mm thick, with 18mm lugs and a Perspex (acrylic) crystal. Water-resistance was stated to be 20 feet, but this a bit misleading as this also applied at the low pressures encountered at operational altitudes.

The caseback markings on this example place it as S/N 2249, issued in 1948. The “B/97” marking is a bit of a mystery. It appears on some, but not all RAF-issued models, and never on RAAF-issued models. Some consider it a decommissioning mark, but I personally think it’s more likely an indicator of a field retrofit.

The exact retirement date of the Mk 11 timepieces is also a bit of a mystery. Their most high-profile use was arguably with the aircrew of the English Electric Canberra  bomber. This aircraft remained in service until 2006, but the watches were retired in favor of GPS long before that time. It would seem that the phase-out began in the mid 1970’s, and was complete by the end of the decade. So a ~ 20-year service life.. Quite a remarkable feat!

And here I’ll close. Again, I realize that this article is nowhere as comprehensive as it could be. For that, I encourage you to peruse the links I will provide. Suffice to say that, for this writer, the  Mk. 11 remains the pinnacle of aviation wristwatch evolution, the closest thing to a haute horlogerie wristwatch any Armed Forces has ever issued their personnel. Not coincidentally, it is also the closest thing to a “Bucket List Watch” for me – The one wristwatch I feel I need to own before I leave this mortal coil.

Thanks for reading!

-Ricky Mac

Further Reading:

The Navigator: Icons of Wristwatch Design 1 by Carlos Perez

SteveG’s excellent photo essay

The Jaeger LeCoultre Mark XI by Francis Chang

Jaeger-LeCoultre Navigator’s Mark 11 Wristwatch Tracking Page

“Man Is Not Lost” January 2004 article from Horological Journal