The Type IXD2's quoted surface speed is about 19.2 knots, although higher figures may apply in favorable weather or under different operating limits. A navy can publish an accurate figure for normal peacetime conditions without revealing the maximum output available when machinery wear is less important. One rule of thumb equates a wartime year with six peacetime years of engine strain for destroyers and three for capital ships. Speed comparisons therefore depend on weather, operating procedures, and how much accelerated wear a navy is prepared to accept.
Notable Point · 2:46:54-2:49:19
Why published speed figures conceal wartime machinery strain
Weather and machinery limits can reconcile different speed figures, while wartime operating practices impose much greater engine wear.