On May 17, 1995, at 1749 Alaska daylight time, a float equipped Maule, M-5-235-C airplane, N6248M, registered to and operated by the pilot, experienced an engine power loss after takeoff and was forced to land on the muskeg near Anchorage International Airport, Anchorage, Alaska. The personal flight, operating under 14 CFR Part 91, was departing Lake Hood Seaplane Base, Anchorage, for a local flight. No flight plan was filed and visual meteorological conditions prevailed. The private certificated pilot, the sole occupant, was not injured and the airplane received substantial damage. According to the pilot, he drained the fuel sumps and gascolator and found no water or contaminants. He had 20 gallons of fuel in each wing tank. The engine run up was normal and the fuel selector valve was set on the left tank. The engine operated normally until the airplane reached an altitude of 100 feet above the ground. The engine then lost "substantial" power, surged to full power, and then lost power again. He stated he did not try to keep the engine running because of his proximity to the ground. He landed the airplane on the muskeg approximately 1/2 mile from Lake Hood. The pilot stated this was his first flight of the year, and the airplane's first flight since its annual inspection. Examination of the airplane and engine showed no external damage to the engine, accessories or fuel system. The engine was started, warmed up, and accelerated to 1800 RPM. The engine ran smoothly and continued to operate from both the left and the right fuel tanks. The engine could not be accelerated to full RPM due to the damage on the airplane. The fuel caps were removed from the left and right fuel tanks and they were vent checked. Both caps were labeled as vented caps and found to vent properly.