During his forced idleness Admiral Yi Sun-Sin prepared for the future; he had his men make salt by evaporating seawater, and used it to pay local workers for building ships and barracks and to trade for materials his navy needed. His energy and patriotism were so contagious that many worked for nothing. Having heard not only of Yi's military feats, but his contributions to the navy as well, the king conferred upon him the admiralty of the surrounding three provinces.
For a successful invasion of Korea, the Japanese knew that they would have to eliminate Yi Sun-Sin. No Japanese fleet would be safe as long as his turtle boats were prowling the sea. Seeing how the internal court rivalries of the Koreans worked, the Japanese devised a plan. A Japanese soldier named Yosira was sent to the camp of the Korean general, Kim Eung-Su, and convinced the general that he would spy on the Japanese for the Koreans
Yosira spent a long time acting as a spy and giving the Koreans what appeared to be valuable information. One day he told General Kim that the Japanese General Kato would be coming on a certain date with the great Japanese fleet, and insisted that Admiral Yi be sent to lie in wait and sink it. General Kim agreed and requested King Son-Jo for permission to send Admiral Yi. The general was given permission, but when he gave Admiral Yi his orders, the admiral declined. Yi knew that the location given by the spy was studded with sunken rocks and was very dangerous.