Kaiyō No.1-class oceanographic research ship

The Kaiyō No. 1-class oceanographic research ship (第一海洋型海洋観測船,) was a class of oceanographic research ship/weather ship of the Imperial Japanese Navy (IJN), serving during World War II. The IJN official designation was 200-ton oceanographic research ship (二〇〇瓲海洋観測船,).

Construction
In 1938, the IJN wanted to investigate the weather and ocean currents in a potential future battlefield. Kaiyō No. 1-class ships were built for the purpose. The IJN made Hokkaido Fisheries Research Institute San'yō Maru a sample for the Kaiyō No. 1. All ships were deployed in the Hydrographic Department, and they were directly controlled by the Naval Ministry. Therefore, being government ships, they raised civil ensign, not the naval ensign. The crew were civil servants except for several warrant officers.

All sister ships were built by Mitsubishi Heavy Industries at Shimonoseki shipyard.