The boat mounted a 140 mm deck gun with a theoretical range of about 15 kilometres and a radar credited with detecting aircraft at 80 kilometres. That radar figure depended on a target flying high enough to rise above the horizon. A submarine's aerial starts close to sea level, so the curvature of the Earth restricts what it can see. This contrast separates catalogue performance from the geometry that governs real operations.
Notable Point · 38:21-39:23
Impressive gun and radar figures met physical limits
The I-400 carried a substantial deck gun and aircraft-warning radar, but a submarine's low height sharply limited the radar's practical horizon.