Dear Sir and all!
Just found this in one book, Zvonimir Freivogel's "Austro-Hungarian Submarines in World War I".
The Austro-Hungarian Navy had four "Havmanden" class submarines, Ub. 20, Ub. 21, Ub. 22 and Ub.23. Two of them were build in Pula, the other two in Rijeka.
They were laid down in 1915, and commissioned in 1917.
The submarines were outdated and not very useful, they never sunk any ship during their service.
Ok, the following extracts are from the book:
- "the submarines were having an 3-ton keel that protected the hull when the submarine was sitting on the sea-bed, and could be detached for emergency surfacing.", and
- "in the case of sinking, the conning tower was to serve as rescue chamber, the keel was to be dropped-off in necessary and there was an telephone float for the communication with the surface, with an 100 m long line.".

So, not just the CSS Hunley had dropping keel, but also fout Havmanden-class Austro-Hungarian submarines.
By the way, the Havmanden-class submarines were built for Denmark, before WW1, three in Rijeka and another three in Denmark, and stayed in service until the end of 1920's and beginning of 1930's.
Those four in Austro-Hungarian service were built just because Austria-Hungary needed submarines urgently, they know very well the submarines were outdated.
Two of the four submarines were lost in action.
Best regards!
Funkmeister