U-boat speed depended on staying surfaced

Second World War U-boats could not outrun the escorts hunting them, although they could overtake slow freighters while running on the surface. Once submerged, conventional boats lost most of that mobility and could outrun very little. The late Type XXI and Type XXIII represented a different underwater-performance standard, but they were not typical of the campaign's operational boats. The contrast shows why forcing a U-boat below the surface sharply reduced its freedom to maneuver even before an escort delivered an attack.