Polish Navy's Riverine Flotilla, popularly known as the Pinsk Flotilla, operated in the Pripyat basin in eastern Poland 
(today's western Belarus), and was meant to act against the Soviets. In September 1939 its vessels had no chance to fight against the Germans, and  the ships were scuttled after the Soviet invasion on 17 September.*
In "what if" scenarios they can be used against the Germans on the Vistula.*
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There were used several sidewheel armed steamers armed with two 75 mm guns; two of which were rearmed with 13.2mm AAMGs in late 1930s. *
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There was also a number of smaller craft, classified as armed cutters (KU, 'kuter uzbrojony') or light armed cutters (LKU), armed usually with 37 mm gun and machine guns.*
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Several small craft were used in 1939 against the Germans in the Vistula Detachment of the Riverine Flotilla (KU-4, KU-5, KU-6). Among them was the river chaser KU-30, being the last Polish vessel on the Vistula afloat, scuttled on 28 September 1939.*
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_____________[description by PIBWL]*