In heavy weather, with battery recharging on, in a Type II you may only pull 8 knots.
Check to see if you're at standard propulsion or recharging mode.
If on standard propulsion, still only getting 8 knots, then yes probably sabotage or malfunction.
Unfortunately due to game limitations this cannot be repaired at sea.
But hell, at least return to port and start a new patrol before you consider a reinstall.
