A programmable boatlift system having a cable extending up from one end of a boatlift cradle, across to a shaft attached to a motor, and down to an opposite end of the boatlift cradle. The cable passes through a hole in the shaft. A position sensor having an idler sheave is placed on the cable. The motor winds or unwinds the cable around the shaft to cause the cable to move across the roller sheave to raise or lower the boatlift cradle. The position sensor produces an output signal proportional to the distance the cable travels over the idler sheave as the cable causes the idler sheave to rotate. An electronic control circuit uses the output signal to infer the vertical position of the boatlift cradle and to position the boatlift cradle as desired.