The Riqueval Tunnel is a 5,670 metres (3.52 mi)-long tunnel on the St. Quentin Canal, near Bellicourt. Napoleon ordered the tunnel’s construction in 1801. It was completed in 1810 as part of the St Quentin Canal. An electric tug was added in 1984. The Riqueval tunnel is the second longest tunnel in the world, after the Biassa II tunnel in La Spezia, Italy.
Here a two videos showing us being towed.
You have to steer the boat through the tunnel to remain in the center. I wrongly thought the chain would somehow center the boat. After we bumped on the side for a few minutes, I was reminded. I took the helm. It actually took quite a bit of minor course adjustment as the tow creates a bit of turbulence that bounces off the walls of the narrow canal.
There were just a few air shafts. Overhead lights are evenly spaced and light the tunnel well. The chain is a bit noisy. The journey took us 1 hour and 50 minutes. We were the only boat. With additional boats it would take longer.