I always keep breakdowns/sabotage on for realism sake. On one of my outings, my engine broke down mid mission. Suddenly surfaced in my VIIB I could only do about 5 knots at flank and submerged, 1-2 knots at flank.
Of course this occured when I was all the freakin' way in the middle of the ocean west of Ireland.
And.. to top things off, I ran into a completely un-escorted small convoy of like 10 ships. Even surfaced with pristine weather, I couldn't keep up with them !