Abstract: An auxiliary propulsion system on a boat trailer having a fixed frame and a displaceable frame that is slidably adjustable along the fixed frame. The fixed frame rotatably supports ground-engaging wheels on opposite sides of the trailer, each having a chain on the inside of its rim. The displaceable frame supports a pair of electric motors at the front end of the trailer driving a pair of longitudinal drive shafts through gear reductions and slip clutches. The longitudinal drive shafts drive respective transverse drive shafts with pinions on their outer ends for driving engagement with the chains on the wheels. A handle on the front end of the trailer is coupled to the displaceable frame through a mechanical linkage for selectively positioning the displaceable frame along the fixed frame to put the pinions in or out of driving engagement with the wheel chains.