The Körting engine produced thick white smoke that was visible for miles at sea. And at night the stack emitted a continous shower of sparks that were equally visible. In addition, the engine produced a series of "pos and bang" that sound much like a gasoline engine back-firing, whci was audible for long distances. The U-1, shown here, was a one-of-a-kind boat. The subsequent designs were much larger.
U-3 & U-4
The U-2 is not included in this thread because it was equipped with a Daimler engine. So, the next boats in the series were U-3 and U-4. These wre considered ocean-going submarines that could operate with the fleet. The problem was that the Körting engine's fuel consumption caused a drastic limitation to the boats' operational range. This colorized photo of the U-3 appears to have been altered because there should be a type II stack where the rectangular contraption is between the sailor near the stern and the group around the after mast.
U-5 to U-8
The U-6 shown in this photo postcard is equipped with the type II stack, that was considerably taller than the rimp stack used on U-1, but still too short. Crews compained that with a following wind, smoke obscured the conning tower and made keeping watch difficult.
U-9 to U-12
The U-9 series featured the type III tall stack that became standard for all the following boats. Even with improvements in the Körting engine and the practice of installin up to four engines in a boat, the operating range remained badly limited. From Helgoland, a Körting boat could reach the mid-east coast of Britain or the eastern area of the English Channel. In both cases the time on station was limited to five days.