The skipper carefully and slowly maneuvers the tanker into the lock chamber, because the 100 meter long RO-VER has no brakes.

There isn't much space, and side winds could endanger the ship.

While the crew secures the tanker against drifting off, the hydraulics close the heavy lock gate.

Then flaps are opened.

The water of the Main flows out of the lock chamber without the need for expensive pumps.

In total – depending on the water level – around 15,000 cubic meters.

The ship sinks with the falling water level.

Seven minutes later and three meters deeper, the level of the undercurrent is reached.

Oliver Bock

Correspondent for the Rhein-Main-Zeitung for the Rheingau-Taunus district and for Wiesbaden.

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The traffic light jumps to green.

The tanker has been released and can continue its voyage towards Rotterdam without further interruption on the Main and soon afterwards on the Rhine.

On the other hand, the next lock is waiting for oncoming traffic upstream after about 13 kilometers near Eddersheim.

It is also remotely controlled from the control center in Kostheim, as is the Frankfurt-Griesheim lock.

The waterways and shipping administration has a total of eight such control centers for lock control on the Main.

There are exactly 34 locks on the 384 kilometers of the Main course up to the entrance to the Rhine-Main-Danube Canal and a further 14 to the Danube.

But the Kostheim lock at Main-Kilometer 3.2 is a special feature.

After Iffezheim am Rhein, it is the busiest inland lock in Germany because it is passed by every ship that changes from the Main to the Rhine.

Or vice versa, and it doesn't matter how far the journey goes.

It would take at least four days from the mouth of the Main near Kostheim to the Danube.

On average 45 ships per day

Only around a third of the ships that enter the Main from the Rhine and stand in front of the Kosthheim lock gate after only three kilometers want to go as far as Bamberg, for example, and very few go further to the Black Sea.

Many sail just a few kilometers further up the Main to the ports of Raunheim or Flörsheim to unload gravel, fertilizer or fuel.

On average over the year, 45 ships passed through the Kostheim lock every day in 2021.

Its name is deceptive, because the lock, which was first built in 1886 and built a second time between 1930 and 1934 in the architectural style of the New Objectivity, is located in the district of Ginsheim-Gustavsburg, while the hydroelectric power plant "Kostheim" on the opposite side of the Main belongs to Hochheim .

This is confusing at first sight.

Also for the hydroelectric power plant, which was built in 2009.

With the exception of two, all locks on the Main are also hydroelectric power station sites.

The two large lock chambers, each approximately 340 meters long, have been in existence for almost 90 years, making uninterrupted shipping possible.

"These chambers would probably not be built for that long today," says Hans-Joachim Scheid, the manager responsible for the Kostheim lock at the Main Waterways and Shipping Administration, which was reformed and reorganized in 2021.

Because the long towing trains for which these chambers were designed at the time no longer exist.

Today it is up to 135 meters long individual drivers or almost 190 meters long pushed convoys that want to be channeled as quickly as possible.

Another special feature is the southern chamber, also known as the banana lock in the jargon of inland waterway skippers.

Because the two lock gates are not arranged in the middle and are only twelve meters wide, the actual chamber is around 20 meters wide.

Inland skippers who are traveling with a ship that is 11.45 meters wide prefer the 15 meter wide entrance to the north chamber, which can be divided into two segments.

This saves overly cumbersome and careful maneuvering.