How U-boats weakened Japan's postwar economy

Did German U-boat damage to British trade help explain Japan's later economic weakness?

Yes; Japan depended heavily on British merchant shipping and lacked the enlarged postwar transport fleet that helped the United States recover.

The United States emerged with a greatly expanded merchant fleet and transport infrastructure, while Japan lacked the same reconstruction base. Removing the war therefore preserves more of the commercial system that supported Japanese growth and naval spending.