 Shōkaku was the first carrier in the IJN to be fitted with radar , a Type 21 early @-@ warning radar , mounted on the top of the island around September 1942 . The date of Zuikaku 's installation is unknown , but both ships received a second Type 21 radar in a retractable installation adjacent to the flight deck after October . Before June 1944 , a Type 13 air @-@ search radar was installed on the light tripod mast abaft the island . The Shōkaku @-@ class carriers were also fitted with a Type 91 hydrophone in the bow that was only useful when anchored or moving very slowly . 
