The pilot, who was also the builder of the experimental pusher-configured airplane, stated that he made a normal departure from his base airport. As the airplane reached about 250 feet above ground level (agl) he transitioned from the best-angle of climb configuration to the best-rate of climb configuration. The airplane emitted a loud "bang" behind the pilot and the engine subsequently experienced a total loss of power. He established a shallow turn back toward the airport and landed in a field just south and adjacent to the runway. The airplane touched down in soft, wet grass and the right main landing gear collapsed. The fuselage was substantially damaged. After egressing the airplane, the pilot noted that the propeller had separated from the airplane. Upon closer examination, he discovered that the metal bracket he had constructed to support the propeller as part of the speed reduction system had fractured. He estimated that the bracket had amassed about 10 hours of flight time on it.