Why the scenario keeps geared diesels

Diesel-electric transmission could eliminate mechanical gearing and was already a plausible marine option, as American practice demonstrated. It is excluded from the main calculation because German surface-warship projects were using geared diesels. Adopting diesel-electric drive would add a second technological divergence to the propulsion change. A later aside suggests that mature diesel-electric machinery might suit Scharnhorst and Bismarck, while still withholding that change from the worked scenario. Keeping the historical German transmission choice makes the range and power estimates deliberately conservative.