The pilot reported that while loading the airplane he noticed the airport manager installing sprinkler heads along the entire length of the grass runway. The pilot requested that the airport manager delay turning on the sprinklers until he departed the airport; however, the sprinklers were turned on prior to departure. The pilot stated that he remained on the right half of the runway to keep clear of the sprinklers during the takeoff roll. As the airplane approached midfield during the takeoff roll, the pilot determined that the airplane was not "moving fast enough" to continue the take off. The pilot stated that he aborted the takeoff by pulling back on the throttle and applying the brakes. He added that during the aborted takeoff, the airplane began skidding on the wet grass and he shut the engine off. Subsequently, the airplane exited the departure end of the runway and struck a sign and a boulder, which resulted in substantial damage to the right and left wings. The pilot reported no mechanical malfunctions or failures with the airplane prior to the accident. According to the Northwest U.S. Airport/Facility Directory, runway 35, a grass runway, is 3,400 feet long and 150 feet wide. At the reported weight of the airplane, in the weather conditions of the airport at the time of the accident, the airplane had a calculated takeoff ground roll of about 2,000 feet, with a landing ground roll of 880 feet. These numbers are calculated for a dry, paved, level runway, with two notches of flaps used for the takeoff. The airplane took off on a wet, grass, down-slope airstrip, with only one notch of flaps. The Performance Section of the Pilot Operating Handbook for this airplane states that the “effects of conditions not considered on the charts must be evaluated by the pilot, such as the effect of soft or grass runway surface on takeoff or landing performance….”