Servomechanism synthesis for a two degree-of-freedom robot arm assembled on a vessel and affected from sea disturbances is considered. Dynamic equations of the robot arm with unbalance and of an approximate base disturbance model are derived. Direct linear controller synthesis based on a linearized model is then studied. It is shown that, although the plant model is nonlinear, linear controller that is synthesized based on the "internal model principle" provide good performance and perform better than the commonly used PID controllers. © 2012 IEEE.