The pilot reported that he was landing on runway 18 when the airplane started to yaw to the right. Rudder correction was not sufficient to stop the yaw, and the brakes did not seem to help. The runway consisted of about 8-inch tall grass at the time. The pilot was ultimately unable to maintain directional control and the airplane rotated approximately 180 degrees before impacting a fence located about 30 feet from the runway. Wind in the vicinity of the accident site was from 210 degrees at 8 knots. The pilot noted that there no failures or malfunctions associated with the airplane prior to the accident. The airplane sustained substantial damage to the left wing and aft fuselage.