Treaty weight and Fourth Fleet lessons

The comparison moves from outward similarity to the practical differences created by machinery layout, funnels, island position, and hull strengthening. The Tomozuru and Fourth Fleet incidents forced Japanese designers to revisit ships that had been squeezed hard by treaty limits and aggressive requirements. The storm damage created a readiness crisis and imposed a large burden of yard time, industrial output, and reconstruction across the fleet. British observers interpreted the evidence as proof that Japan had built treaty ships too lightly, while surviving Japanese records do not reveal the full repair bill. Hiryū's extra displacement reflects a navy learning that a fleet built too lightly could be beaten by weather before an enemy arrived.