On May 31, 1993, at 1130 Alaska daylight time, a wheel equipped Cessna 185 airplane, N185TD, registered to and operated by the Pilot in Command, over ran the runway at the Glacier Bay Country Inn Airstrip located near Gustavus, Alaska. The personal flight, operating under 14 CFR Part 91, last departed the Glacier Bay Country Inn Airstrip for a local flight. No flight plan was filed and visual meteorological conditions prevailed. The Private Certificated Pilot in Command and the passenger were not injured and the airplane received substantial damage. According to the Pilot in Command, the grass strip was wet and he landed approximately 400 feet beyond the threshold of the 1200 foot long airstrip. The airplane was STOL equipped. He stated that there was no braking effect on the wet grass and the airplane slid off the end of the airstrip, struck a ditch and collapsed the gear and damaged the wing. According to the Pilot Operating Handbook Supplement (POHS 10-6), furnished by Robertson when an airplane is modified with a Robertson STOL kit under Supplemental Type Certificate SA1441WE, the Robertson NORMAL landing data at a gross weight of 3350 pounds, 40 degree (full) flaps, 58 knots at 50 feet above the ground at sea level pressure and 59 degrees Fahrenheit temperature, the ground roll should be 310 feet and 890 feet to clear a 50 foot obstacle. The note section states that the distances are based on zero wind, power off, and heavy braking. It also states that for operation on "dry, grass" runways, to increase the distances for both the ground roll and total to clear 50 foot obstacle by 20% of the "total to clear 50 foot." According to the Pilot in Command, he was using full flaps, and an approach speed of 65 knots indicated. He touched down 400 feet beyond the threshold of a 1200 foot airstrip which left 800 feet remaining. The grass was wet and he stated the braking action was "nil." By applying the POHS information, the airplane should have had a ground roll of 362 feet. However, the POHS does not show what increase in ground roll should be applied when the grass is wet or when a higher approach speed is used.