While /u/Saelyre has covered the fact that she was already making for port before the torpedo hit that jammed her rudders, they didn't explain why she didn't make it back after the torpedo hit. Bismarck had a long, unarmoured stern structure, which attached to the armoured main hull, and contained an armoured steering compartment (containing the machinery needed to turn the rudders). When the torpedo struck the stern, the explosion caused the ship to 'whip', vibrating heavily up and down. The armoured hull, the unarmoured stern and the steering compartment all vibrated at different speeds, basically causing the stern to tear itself apart and collapse on top of the rudders. This was a pretty common occurrence on German ships with similar sterns, with torpedo hits in similar locations on Prinz Eugen and Lutzow resulting in the same outcome. With the stern collapsed, the rudders became jammed at 12^o to port, where they had been at the time of the torpedo impact. This meant she could only travel in circles. Several things could have saved her from this: either manual control over the steering could be established, she could be steered using the engines, or damage control could repair the rudders. Damage control teams could not work their way through the mangled stern to the steering compartment, as they would need to do to turn the rudders by hand. The collapsed stern also prevented damage control from freeing the rudders; to repair the ship, much of the stern would need to be cut away, and the capability to do so was not available. Finally, while attempts to steer the ship using the engines were made, these had limited effect. Bismarck had three propeller shafts - one on the centreline, and one to either side. This layout was hydrodynamically efficient, but meant that any attempt to do differential steering would be held back by the central propeller, compared to a design with two or four shafts. The damaged stern also hampered these attempts. There were suggestions that the stern could be blown off, but this was avoided, thanks to the risk that doing so would damage the propellers. With the rudders damaged, the ship was effectively out of control; making it back to port was impossible. All Bismarck could do was sail around in circles, and await the arrival of the British fleet.