 Finally , in April 1915 , the British Admiralty agreed to a plan submitted by Drury @-@ Lowe the previous November , which envisioned attacking the German cruiser with shallow @-@ draft monitors , capable of navigating the Rufiji River . Two of the warships , Mersey and Severn , armed with a pair of 6 in ( 152 mm ) guns each , were brought from Britain . Königsberg had in the meantime been moved a third time , even further upriver . On 6 July 1915 , the two monitors crossed the outer sandbar and steamed up the river , despite heavy fire from German positions on the river banks . They stopped at a point they thought to be 10 @,@ 000 yd ( 9 @,@ 100 m ) from Königsberg , which would be in range of their own guns but farther than the smaller German guns could reply . Aircraft were used to spot the fall of shot . The monitors ' navigation was faulty , however , and after opening fire , they found themselves to be within range of Königsberg 's guns . She hit Mersey twice in the engagement ; one shell disabled the forward 6 @-@ inch gun , and another holed the ship below the waterline . Königsberg was hit four times in return , one shell striking beneath the waterline and causing some flooding . In the span of three hours , Königsberg forced both British ships to withdraw . 
