The original coal-fired machinery could spray fuel oil into the furnaces for extra heat while retaining the ability to steam on coal alone. Replacing that arrangement with oil-fired machinery helped the rebuilt Kongo-class ships retain useful speed despite their added weight. The change also removed a measure of fuel flexibility. A successful performance upgrade therefore deepened Japan's strategic dependence on oil logistics.
Notable Point · 37:54-38:59
The strategic cost of Kongo's oil-fired speed
More efficient oil-fired machinery preserved Kongo-class speed but increased the Japanese Navy's dependence on secure petroleum supplies.