In a U. S. Maritime Administration program to up-grade the vast number of World War II Liberty Ships now in the Reserve Fleet from 10 knot to 15 knot ships various new powerplants were tried. This paper is an evaluation of the free piston-gas turbine powerplant built by General Motors Corporation and installed in the converted Liberty Ship ‘William Patterson’. The machinery is briefly described and the shipboard operation is discussed. The difficulties encountered and performance results are given. Conclusions on marine application of this type of machinery are made.