On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

Thread: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

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  1. #1
    Moderator Emeritus Crusader's Avatar
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    On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    We - like many a forum - have had several discussions on the origin and the purpose of the elongated three-minute-markers on chronograph 30-minute subdials otherise marked in 5-minute intervals (confusingly, JohnF has come up with a vintage elongated-4-minute-marker variant).

    A number of explanations have been advanced for the elongated three-minute markers, with the need of users (even military ones) to time American long-distance calls measured in three-minute intervals looming quite large.

    Looking anew at a Longines A-7 (a U.S. Army Aircorps issued aviation chronograph) from the mid 1930s on MWR (http://www.mwrforum.net/forums/showthread.php?t=22713) and seeing that its 30-minute chronograph subdial is marked ONLY in 3-minute intervals, I am increasingly convinced that the purpose of the elongated three-minute marks is rather basic - its purpose is to make reading the elapsed time easier ... looking quickly at a 30-minute subdial marked with 5-minute intervals only, one may easily be mistaken between, say, 3 and 4 minutes past the interval. Not so with the additional elongated 3-minute markers.

    Another way to address the same problem is the 15-minute register as used in the German aircraft clocks of WWII (they switched to 15-minute subdials from 30-minute subdials), and by Breguet for the French Type XX/21 chronographs after the war.

    The thesis is open for discussion.
    Last edited by Crusader; March 5th, 2009 at 21:49.
    Cheers,

    Martin ("Crusader")

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    Re: On The Three-Minute Markers on Chronograph Subdials

    Quote Originally Posted by Crusader View Post
    We - like many a forum - have had several discussions on the origin and the purpose of the elongated three-minute-markers on chronograph 30-minute subdials otherise marked in 5-minute intervals (confusingly, JohnF has come up with a vintage elongated-4-minute-marker variant).

    A number of explanations have been advanced for the elongated three-minute markers, with the need of users (even military ones) to time American long-distance calls measured in three-minute intervals looming quite large.

    Looking anew at a Longines A-7 (a U.S. Army Aircorps issued aviation chronograph) from the mid 1930s on MWR (http://www.mwrforum.net/forums/showthread.php?t=22713) and seeing that its 30-minute chronograph subdial is marked only in 3-minute intervals, I am increasingly convinced that the purpose of the elongated three-minute marks is rather basic - its purpose is to make reading the elapsed time easier ... looking quickly at a 30-minute subdial marked with 5-minute intervals only, one may easily be mistaken between, say, 3 and 4 minutes past the interval. Not so with the additional elongated 3-minute markers.

    Another way to address the same problem is the 15-minute register as used in the German aircraft clocks of WWII (they switched to 15-minute subdials from 30-minute subdials), and by Breguet for the French Type XX/21 chronographs after the war.

    The thesis is open for discussion.
    That longine is an interesting watch with its rotated dial! Nice for telling time with your arm in a very neutral position!

    I totally agree with you re: the purpose of elongated 3-minute markers, btw.

  3. #3
    Member lysanderxiii's Avatar
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    That has been my hypothesis all along.

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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    I used to be a Royal Navy Navigator, and we always used to mark our charts for projected future times along our track by DR (dead reckoning) or EP (estimated positions) calculation, in time intervals that were simple multiples or fractions of six minutes. The reason was very simple - a 6 minutes is 1/10th of an hour. So if you ship is doing 15 knots (15 nautical miles per hour), in 6 minutes it will do 1.5 nautical miles. Depending on the scale of the chart in use, you might want to mark the chart every 3 minutes, or every 12. In the same way, we'd aim to fix our position at 6 minute (or 3 or 12 minute) intervals since we could then very quickly calculate the speed made good - as being 10 times (or 20 times or 5 times) the distance covered between adjacent fixes.

    So I suspect that the 3 minute interval markings on the chrono may be to assist with navigation calculations...

  6. #5
    Member lysanderxiii's Avatar
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    Quote Originally Posted by HappyJack View Post
    I used to be a Royal Navy Navigator, and we always used to mark our charts for projected future times along our track by DR (dead reckoning) or EP (estimated positions) calculation, in time intervals that were simple multiples or fractions of six minutes. The reason was very simple - a 6 minutes is 1/10th of an hour. So if you ship is doing 15 knots (15 nautical miles per hour), in 6 minutes it will do 1.5 nautical miles. Depending on the scale of the chart in use, you might want to mark the chart every 3 minutes, or every 12. In the same way, we'd aim to fix our position at 6 minute (or 3 or 12 minute) intervals since we could then very quickly calculate the speed made good - as being 10 times (or 20 times or 5 times) the distance covered between adjacent fixes.

    So I suspect that the 3 minute interval markings on the chrono may be to assist with navigation calculations...
    When these making first start to show up, the average aircraft when anywhere from 2 to 5 miles in one minute, three minute cues on the watchface would be rather coarse.

    However, it does show that the readability of the watch must be something that can be performed as rapidly as possible, as pilot work-load had alway been rather high, because as technology allows more things are automated, technology creates morethings for the pilot to monitor/control.

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    Member HappyJack's Avatar
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    Quote Originally Posted by lysanderxiii View Post
    When these making first start to show up, the average aircraft when anywhere from 2 to 5 miles in one minute, three minute cues on the watchface would be rather coarse.
    Hmm...not sure what you mean by "rather coarse". When you practice visual navigation you choose the scale appropriate to your speed, so at 300 knots (600 km/h), you'd cover 30km in 3 minutes. That's 12cm on a 1:250000 scale navigation chart, which is a hand's width - knowing you do a hand's width every 3 minutes will help a pilot work out, very quickly, where he is.

  8. #7
    Moderator Emeritus Crusader's Avatar
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    Hmm, I think it has to do with the fact that an airplane, even in the early days when the three-minute markers were introduced, would move at a multiple of the speed of a ship, thereby increasing navigational errors substantially.

    Unless the three-minute interval was in use in shipborne navigation prior to its use in aviation, I would find it difficult to see navigational requirements as the principal cause of the three-minute marks.
    Cheers,

    Martin ("Crusader")

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    Member whifferdill's Avatar
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    With regard to the A7 in the link above, anyway, I tend to agree with HappyJack - in aviation as well as nautical navigation - the pilot / navigator is always interested in distance per hour or distance per minute or multiples thereof - everything hinges on fractions or multiples of 60 - including position over the earth ( minutes and seconds of arc ).

    A useful aviation time piece, especially one from the days when practical dead reckoning was employed, might reasonably be expected to highlight fractions of the 60 as a whole - hence the 30 minute sub-dial and highlights of tenths of that ( 3, 6, 9 etc ).

    Knowing groundspeed and direction, one might plot position at 1/10 of an hour intervals on a chart. Easier to do when this time segment is highlighted on whatever you are using to time the leg.

    I think the above is probably separate to the elongated 3 minute interval marks found on other chronographs such as the Benrus Skychief etc, but for those involved in aviation and navigation, working in multiples or fractions of 60 rather than 100, is just natural and normal, and will therefore be at the fore of the mind when looking at features like this on any watch designed for aviation / navigational use.

    So in a way - it is almost certainly done for legibility, but I agree with happyJack that what lies behind this might well have its routes in dead reckoning navigation.
    Last edited by whifferdill; March 8th, 2009 at 00:33.

    Minutes are the Milestones of Aviation - Fred, in 'Fools and Birds Fly'

  10. #9
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    Quote Originally Posted by whifferdill View Post
    So in a way - it is almost certainly done for legibility, but I agree with happyJack that what lies behind this might well have its roots in dead reckoning navigation.
    If that were the case, one would need an explanation for the four-minute markers on the vintage pieces which JohnF has come up with ... of course, it could be so easy as one dialmaker who was misinformed, or clueless, about the purpose of the elongated three-minute marks ...
    Cheers,

    Martin ("Crusader")

  11. #10
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    Re: On The Three-Minute Markers on Vintage Pilot's Chronograph Subdials

    Hi -

    Contemplate this: while the 3-6-9 markings are good for initial timing for a 20x division (60/3=20), the 4-8-12 markings are good for initial timing for a 15x division (60/4=15). That makes as much sense as the 12x division (60/5=12) that we're all used to...

    But I've checked and checked and checked and checked. There really seems to be no meaningful, "why, of course!" explanation that I've been able to find. I am convinced that it has to be a military/aviation thing, given that we simply do not see this on any other kinds of watches: I just don't know why. I still like my true headings navigation trick theory, but have no explanation of why this doesn't also work using three-minute legs.

    I'm also close to acquiring a Benrus SkyChief with that elusive dial, but the logistics are not looking good right now.

    JohnF
    Last edited by JohnF; March 8th, 2009 at 21:15. Reason: SkyChief, not Skymaster
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