Problem is trying to hit what you can't see. Trying to hit a tank other than by sheer luck using such an artillery system would have been nearly impossible. The observer would have to phone the FDC command post for the fort who would then plot the fire order and transmit the solution to the guns after calculating it.
Just the lag alone would ensure that a relatively fast moving target would not come under fire. So, while I agree completely that a heavy wall HE or even a common HE round from a 75mm gun would demolish the 1940 German tanks hitting one would be very rare other than by pure luck.

Sure, but how long would it take for a turret commander to start firing independantly over "iron sights" rather than wasting the time, phoning the FDC Command Post? They were French, but they weren't stupid.

There are no sights! The guns have none, iron or otherwise. There is no way to see out of the turrets or casemates in which the guns are mounted. There is no appature in the turrets or casemates for a sight. Nor are there any slits, periscopes, or other vision devices in them.
The guns themselves are trunnoned at the muzzle. This allows for better protection of the gun barrel as nothing is exposed to fire. There is an armored plate on the casemates that protects the gun and crew. Not having any other openings in the plate or turret makes it easier to make it gas tight and also gives less openings for an attacker to use against the fort.
In fact, in the 75mm turrets the upper level (firing platform) only has room for the two gun loaders. In front of their position are the two chain hoists that feed ammunition up to the turret. The loaders literally cannot even touch the face of the turret or position themselves between the guns. There is no room for this. The elevation and train mechanisims along with the fire direction pointers are on the level below the guns. So, those training and elevating the guns are doing so from one story below the actual gun postions and can see nothing.
These guns were not intended for engaging tanks particularly. Instead, they were intended to act purely as artillery to defend the intervals between the forts and for firing on the occasional enemy target to their front (the turrets not the casemates).

Kemps book has a photo of a 135mm casement howitzer. That example has traverse & elevating handwheels on the gun mount. Like the 75mm example it has no 'sight' so the gun crew cant do much more than open the breech and peer down that narrow opening. Which from experince is not the best way to aim a cannon.

Now I'm trying to imagine how the crew aligned the cannon tubes with the traverse & elevation indicators in the turrets and casements. The traditional bore sighting method used on field artillery carriages dont seem suitable. I suspose the original installation crew or ordinance techs had some suitable measuring equipment, but would the regular crew have the same for checking alignment after firing?

Actually massed artillery fires worked fairly well at Crehen, Thisenes, around Merdorp, and with the First Army at the Gembloux Gap area. I dont have numbers at hand for tanks destroyed by artillery fires vs French tank fires. There are several complaints in the German after action reports about the French artillery strikes accurately & repeatedly hammering their tanks.

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Wow, you guys really know a lot about these guns. I would have had no idea which guns were where, let alone which had sights or not.

Still, I remember something I heard once about tanks not being too effective on their own, unless their supporting vehicles and infantry come along with them. What effect would massed French heavy artillery fire have on these formations? How long could groups of German tanks survive behind the Maginot Line before they were cut off and destroyed? (Surely the French had some kind of anti-tank weapon saved for this problem.)

Or, how does the German infantry and horse-drawn artillery ever get through the Line? Does a few hundred German tanks sneaking through constitute a total breakthrough?

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For a good part of the Maginot Line you could expect fire from only 2 to 4 75mm guns in fortifications until you closed to under about 4000 meters where some additional weapons could come into play. At that range and under an attacker might come under fire by 2 135mm howitzers, 2 to 4 75mm "Mortars" (short range high trajectory versions of the 75mm gun), and/or 2 to 4 81mm mortars.
At ranges under 1000 meters the various machinegun turrets might be able to start engaging targets. At around 500 meters automatic rifles and small grenade launchers and 60mm mortars (lance-grenade) could begin to engage targets as well. Of course by this time the fort was mostly trying to prevent attacks on itself rather than engage the enemy offensively.
For the most part only the turret and cupola mounted weapons could fire to the front of the forts. Casemated guns almost always were faced to fire along the line of fortifications in defensive support only. This means that against a tank attack only a mere handful, say one to three, antitank guns mostly of 25mm could be brought to bear on a tank formation until it was within the line itself.
Now, one has to note that the French thought that just two 75mm guns in a turret were the equivalent of two field batteries of 75mm guns (a dozen). The turret guns were supposedly capable of 12 rounds per minute sustained and 24 for short periods. The block the turret was located in held 600 rounds per gun and more was available from the supply and admin block to the rear of the combat block by tram.
While artillery could, and often was, effective against tanks it mostly was just capable of forcing them to move elsewhere rather than outright mass destruction.

The ways one could expect the Germans to try and penetrate the Maginot Line were:

1. Use aircraft to suppress the supporting field formations and artillery, particularly the Ju 87. By doing this the line is isolated from much of its support.

2. Use 88mm and 105mm AA guns firing AP ammunition to destroy the cupolas on a fortification blinding it. The loss of the cupolas, particularly the observation ones, would leave the fort largely useless.

3. Use extremely large artillery to smash the forts. The maximum standard of construction was to make the Maginot forts immune to 420mm HE rounds. The German 54cm, 60cm, and 80cm rounds would likely have smashed even the heaviest fortifications. Targetting the tunnel from the supply, admin, and entry blocks to the combat block would have been very effective. This would cut off the combat block from the rear and leave it to fail when ammunition, power, and food ran out. As the Germans had a very good idea of where these were and their vulnerability (German contractors did alot of the work building the Maginot Line due to shortages of labor in France) this is a good possibility.

4. Attack the supply and administrative blocks by paratroop and glider. These blocks represent the major ammunition, supply, and resupply positions to the Maginot forts. They are also the entry points into the forts themselves. Blinded and isolated the forts become largely useless due to loss of ammuntion supply.

5. Local attackers could expect success using the same hollow charges that were used on Eben Emael in Belgium. These could penetrate the 20 to 30 cm armor of any Maginot Line turret or cupola. The problem was getting close enough to set the charge.

1, 2 and 3 I agree it would be effective especially the air force bit as the line AA defenses weren't as developed as they should. Oh and the big German tracked guns of course would be able to dish out a lot of punishment (like they did in Sevastopol)

4 and 5 I don't agree as much. the Maginot was much Sturdier than thr Eben Emael and with a much bigger Garrison. If you landed by Glider there were numerous other ways for the defenders to make a sortie and the Paratroopers would be in trouble.

The artillerie in the Maginot line was made using overlaping fields of fire so if the Germans aproached they'd be greeted by a more massive defensive than the one you mentioned. In that set, the tanks woul be forced to retreat as a blind but intense artillerie barrage (especially the 135mm shells but the 75mm would do the trick) would shred a tank to pieces.
A 75mm HE round could pierce in a good shot go trough 10~13mm of armor the pzkw IV ausf D had 10mm thick top turret armor and 12mm upper hull so the heaviest German tank would be very vulnerable to a sustained barrage. Even the early Tigers were prone to this problem as the 152mm Russian artillerie could break the 25mm Turret roof during sustained fire.



Cheers...

Miguel, I think you are overstating the abilities of area fire against armour of simple HE fire before DPICM and other exotic ammunition. You can't guarantee direct hits against small and mobile targets as tanks within an impact zone. At most you can hope that your fires will land close enough to cause damage to tracks or wheels or vision equipment. Infantry, soft vehicles, open top vehicles is another matter, but tanks?

If you assume a battallion concentration (exaggerating), you'll have 12 soizante-quinze firing at a sustained rate of 2 rpm, so 24 rounds per minute on a 200x100m impact zone 20,000 m2. It would have to be a lucky hit to land on a 37mm PzIII deck

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That was one of the problems at Crehen/Thisenes. The German infantry had trouble geting close enough to assit the tanks. The French hit any German groups attempting to attack with multi battalion concentrations.

To breakthrough a fortified zone the defenses have to all be nuetralized. That requires a methodical & well planned series of attacks. You can infiltrate through and disrupt a fortified zone but a true breakthrough requires eleminating effective enemy fires across your lines of communication through the zone. The 'stoss truppen' and infiltration tactics of 1918 are a example of this. The leading companys could get into the enemy defense and disrupt it, but complete sucess depended on how well the following battalions could eliminate the by passed strong points.

Your assumptions here need to be validated. The descriptions of French artillery doctrine, training in the 1930s, and eyewitnes accounts from several battles I have here show battalion and multibattalion concentrations to the be norm for the French light and medium artillery. The 'Group Command Post system used by the French created a centralized control for the battalion, and multibattalion 'groupments'.

Second the French recognized the realative efficiency of concentration in time as well as space. That is they understood that fourty eight rounds hitting a target simultaneously cause higher casualties than fourtyeight rounds fired sequentily from a single four cannon battery. This effect is not trivial and was a critical part of the sucesfull US and British artillery tactics.

The 2 rpm cited above is appropriate for suppresive fires intended to keep someones head down for a hour or more. The acceptable rate of fire for the French 75mm gun for short periods was well over ten shots perminute. Bursts of 20 shots perminute were practical. US artillery crews using this weapon had a saying "Three in the air and one in the breech" which refered to a round in the breech and three empty propellant cases simultaneously thrown to the discard pile.

Unfortunatly I dont have any documents on fire standards or ammo recomended for target types for the French artillery of 1940. From the French and German eyewitness descriptions at hand I'm estimating six rounds per gun would be on the lower side for a battalion concentration. This is in line with the lower end for British practice with their 85mm 25lbr cannon.

That sort of density can produce casualtys amoung tank groups. However in the case of Crehen it was a case of combined arms. There the artillery fires forced the German tank leaders to keep their heads inside the tanks. This made it much more difficult to manuver and spot the French tanks & AT guns concealed about the town. The exposed German tanks, unaccompanied by infantry repeatedly were destroyed by the French tank & AT guns. For three hours a single French infantry company with four AT guns and a squadron of tanks halted Rothenbergs regiment of the 4th Pz Div. It was only after The German artillery was brougt up and begain efective suppresive fires on the French that the German tanks passed the villiage. Less than two kilometers up the road they were halted again for the afternoon by a similar combination of artillery and AT guns. The final attack on Wasines late that evening ended when the command tank was hit by French artillery fire.

One final note. Supporting the Crehen/Thisiens/Wasines postions there were two 75mm and one 105mm battalions of the 3rd DLM and two 75mm battalions of a independant artllery regiment in range. All were linked on a common command network. German accounts suggest multibattalion concetrations were the norm there. Ditto for the continuation of this battle near Merdorp the following day. Three days later the Moroccan divsion defending the Gembloux gap were supported by between sixty and eightyfour cannon, depending on when in the battle the artillery fires occured.

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If you look at the design of the fortifications, most do not have a local entrance. Instead, they are connected by tunnel to other blocks and in turn to the support blocks far to their rear. Typically, the only blocks with local entrances are infantry blocks. These are more common as interval fortifications than in the Gros Ouvrages.
A surprise glider attack on several Petite Ouvrages along a section of the line would appear to have an excellent chance of success. These smaller forts typically had just three to five blocks and usually only one turret block, typically a machinegun turret. A quick attack like that at Eben Emael would likely have caused such a fort to be completely disrupted for some time until the defenders were able to orgainze some sort of infantry foray as the Belgians did hours after the initial attack.

The numbers I gave are accurate. Some examples:

The Gros Ouvrage at Fremont had a single twin 75mm turret with all-round fire and another 75mm casemate battery of three guns sited to fire east along the line. Other than that, this fort had just 81mm mortars, machineguns, and automatic rifles (an a couple of 25mm AT guns) for defense.
The interval forts between this Gros Ouvrage and the next had just 81mm mortars and machineguns as their primary weapons.

The Gros Ouvrage at Rochonvillers had two 135mm turrets, two 75mm turrets, and one casemate with four 135mm guns firing along the line only.

Schoenenbourg has just two 75mm turrets while Hackenburg, one of the largest Gros Ouvrages has one 135mm turret, one 75mm turret, a casemate with a single 135mm gun, and two casemates each with three 75mm guns sited to fire along the line.

As I stated, offensive firepower in the Maginot Line is very limited in numbers at any point.

Yes, HE rounds could cause tanks considerable damage or even destroy them in direct hits. A 75mm round will generally go through between 25 and 40mm of armor depending on shell type. A heavy wall round might go through a bit more. More than enough to destroy most tanks in 1940.
Against this, direct hits in shelling are unlikely. But, even splinters could have been dangerous to a 1940 tank and damage to running gear, etc. is very possible. In addition, just forcing a tank force to move or avoid artillery fire could often be effective in stopping an attack.

Unlike Eben Emael, the Maginot Line had supporting infantry units both within it and along its rear. The forts themselves unlike Eben Emael are very, very well supplied with close in weapons to defend against infantry.
Many of the blocks, particularly if they are manned and ready would be difficult to take even with paratroops landing on top of them. The French could start using their automatic rifles, machineguns, small mortars, and grenade chutes / launchers to sweep the top of a block with a hail of gunfire and shrapnel. Only by silencing the cupolas on the roof of the block would any relief be gained. Not an easy proposition for a parachute force.
The various blocks could also call on neighboring ones and neighboring forts along with field artillery to the rear to simply fire on the fort itself. This happened at Eben Emael also. The fort is immune to such fire while the attackers are not.

A glider assault on the supply / admin block to the rear of the main fort is possible but this would leave the defenders cut off from ground forces. Gliders might be possible to take out interval or Petite Ouvrages where the size and ability of the defense is far less than in the Gros Ouvrages.

The Belgian forts (there were four the size of Eban Emael, for extra credit name them) were susposed to have infantry posted on the surface and adjacent, and there were several MG turrets covering the surface. But the Belgians were not informed about any war starting that morning. Only a few sntrys were about. There were several companys of infantry posted across the cannal, asleep. The remainder of the garrison was either below in the gallerys, or in barracks a kilometer or two away asleep as well.

Had the garrison been on war alert there would have been three to four MG and up to a dozen automatic rifles covering the surface of the fortress.

There was also a store of barbed wire and stakes, that would have been set out on a days notice.

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I'm not denying what you are saying, even tghe snips I'm taking at your quotes are only for space saving purposes, as I recognise the validity of what you are saying.

Nevertheless, I was thinking more of the artillery assets component of the fortifications themselves, which as described before seem to be rather sparse, certainly not the multi-batallion concentrations you mention. Even at higher rates of fire, a tank unit hopefully for it is not quite a static target (unless it bogs down in front of an anti-tank obstacle - ditch, minefield, etc.) so I very much doubt an artillery unit of whatever size can target a tank unit in movement in indirect fire, it would rather fire on pre-registered targets such as certain geographical features (transit locations) and as I said in defence of AT obstacles. Otherwise the tank unit would be hard to catch immobile.

Of course if you have instances of this happening I'd be indeed curious

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The spacing between the forts would allow for another points f the fortress to fore on the section under attack. You would face a fierce barrage if you decided to attack the Maginot Line. And the germans were stoped at some points!


Cheers...

How fierce? How many guns within say 10km range? What anti-tank obstacles?

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It varies quite a bit. A rough estimate is that there is about 5 to 7 km between Gros Ouvrages with something like one to two km between interval blocks in between. So, one could expect long range fire from two Gros Ouvrage typically, and possibly three at the very most. At closer ranges two Gros Ouvrage and one or two interval forts could engage targets.

One possible method I haven't discussed would involve use of alot of smoke rounds coupled with a general bombardment by field artillery on these forts. They are completely dependent on the observation cupolas for effective return fire. If these are blind by smoke and possibly uninhabitable due to concussion from the artillery fire (not an uncommon thing) the Germans probably could have closed fairly quickly on a fort to assault it.
There are no defensive minefields so only the wire and antitank obstacles would prevent it. Unaimed defensive fire probably would slow things down alot but it might have been possible to still enter the fort perimeter and take on the above ground portions without prohibitive casualties.

I found this perhaps it can bring the maginot line from words to life with the pictures and map references. Some nice movies too.

The maginot line Lorraine Tourism Horizon Hotel

and these.