Austrian navy experimenting with new river gunboat design in 1856

An item reported that the Austrian government ordered to experiment with a new type of gunboat especially for the defence of the Danube shores. After several successful tests near Freibad was a row flat-bottomed vessel designed armed with guns of a very heavy calibre.

Source
Verhandeling en berigten betrekkelijk het Zeewezen, de Zeevaartkunde en de daarme in verband staande wetenschappen, 2nd part. Jacob Swart. 1856.