This article concerns modelling and controller design for elastic ship-mounted cranes with the Maryland Rigging. Three inputs are employed to control the vibrations in the plane of the boom; the luff angle is utilized to ensure the controllability of the elastic boom, and the total length of the upper cable in conjunction with the position of its lower suspension point are used to guarantee the controllability of the payload. The dynamic behaviour of the crane is described by a multi-model approach depending on the current values of the cable length and boom luff angle. Consequently, a variable-gain observer and a variable-gain controller are designed to control the vibrations of the crane; the numerical values of the gains are updated according to the current operating region, which is determined by a region finder. Simulation and experimental results show that the expressed control strategy performs well and has a significant effect in controlling the vibrations of the crane for different operating conditions and payload masses.