On April 18, 1998, at 0930 eastern daylight time, a Stewart 51 Inc., S-51D, N51S, collided with the ditch on the departure end of the runway , according to the pilot, during an attempted forced landing on runway 29R at the Vero Beach Municipal Airport in Vero Beach, Florida. The personal flight operated under the provisions of Title 14 CFR part 91 with no flight plan filed. Recovered weather data disclosed that visual weather conditions prevailed at the time of the accident. According to an official of Stewart 51 Inc., the operator, the airplane sustained substantial airframe damage. The commercial rated pilot, and his passenger were not injured. The flight departed Vero Beach, Florida, at 0925. According to the operator, the flight was on climb out when the pilot experienced a loss of engine power. The pilot radioed the air traffic control tower and reported the engine problem. The pilot was cleared for a downwind emergency landing on runway 29R. Reportedly, the winds were 130 degrees at 14 knots. The airplane landed on the first 1/3 of the runway, and rolled off the departure end of the 3500 foot long paved surface on to the rough and uneven grassy surface. According to the operator, the engine never quit running throughout the emergency, but there was a reduction in engine power. The pilot also reported that the propeller blade pitch went to an uncommanded high pitch position. The uncommanded high pitch is normally associated with the loss of oil pressure in the propeller hub. The examination of the engine assembly disclosed that the engine oil tank was ruptured, and there was no oil recovered from the engine lubrication system. The examination of the oil tank assembly disclosed that a plastic plug blocked the oil tank breather line. According to the operator of the airplane, the oil tank had been removed, and was painted several days prior to the accident. During the painting operation, all holes in the tank, including the oil tank breather line, were covered with plastic plugs.