On May 5, 2002, about 1330 eastern daylight time, a Blanik L-13 (glider) N711KR, was substantially damaged during an off airport forced landing, near the Saratoga Springs Airport (5B2), Saratoga Springs, New York. The certificated flight instructor and student pilot were not injured. Visual meteorological conditions prevailed for the local instructional flight. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. According to the flight instructor, he and his student boarded the glider, and departed. Once airborne, the glider was towed to 1,500 feet agl, and then was released from the tow-plane about 1-mile east of the airport. After release, the glider continued to climbed to 3,000 feet agl. The student then maneuvered the glider to the south to work several thermals, and then to the northeast. After flying for approximately 1/2 hour, the instructor told the student to proceed back to the airport, and enter the traffic pattern. While en route and approximately 1.4 miles from the airport at 1,300 to 1,400 feet agl, the glider encountered "heavy sink." The instructor checked the variometer and noted a 1,000-foot per minute descent. He then told the student to increase airspeed in an attempt to fly out of the sink. When the glider was approximately 0.7 mile from the airport, the instructor realized they were not going to make the runway. He took the controls, and because of insufficient altitude to maneuver, setup for a downwind landing to a softball field. The glider touched down in the field, and the instructor executed a ground loop to the left to avoid hitting a fence straight on. The right wing struck the fence, and the glider came to a stop. Examination of the glider revealed about 5 feet of the wing had broken off from the impact. The instructor added that the glider had a 28:1 glide ratio and was capable of flying 6.8 miles from an altitude of 1,300 feet agl. A weather observation was taken about 9 minutes before the accident at the Albany International Airport (ALB), Albany, New York, which was located 21 miles to the south of the accident site, and 285 feet msl. According to the observation, the wind was variable at 4 knots, visibility was 10 miles, sky was clear, temperature was 68 degrees Fahrenheit, dew point was 34 degrees Fahrenheit, and the altimeter setting was 30.21 inches of mercury. Another weather observation was taken about 7 minutes before the accident at the Floyd Bennett Memorial Airport (GFL), Glens Falls, New York, which was located 24 miles to the northeast of the accident site, and 328 feet msl. According to the observation, the wind was variable at 5 knots, visibility was 10 miles, sky was clear, temperature was 66 degrees Fahrenheit, dew point was 36 degrees Fahrenheit, and the altimeter setting was 30.19 inches of mercury.