Vessel motion is an aspect of design that requires a high degree of consideration with regard to passenger comfort. Within the last two decades, extensive research work has resulted in development of numerical and analytical methods for the prediction of heave and pitch motions of catamaran hull forms. However, in the recent past, there appears to be a strong interest in the development of trimaran hull forms. Investigations have shown that little research has been conducted on such hull forms to reduce their motions in heave and pitch. In this article, we investigate the effects of the (longitudinal) stagger of the sidehulls on the motions in heave and pitch of a representative trimaran hull. To quantify the effects of longitudinal stagger of the sidehulls (outriggers) with respect to the centrehull, experimental investigations were undertaken at the Australian Maritime College Ship Hydrodynamic Centre. A round-bilge high-speed hull form model of the Australian Maritime Engineering CRC systematic series was constructed and subjected to extensive experimental analysis as well as computer simulations (HYDROS) for four different longitudinal stagger positions. The investigations demonstrated that this variation and the resulting variation in the radius of gyration could have a significant effect on the heave and pitch motions. The literature survey indicated that, to date, investigations on trimaran hull forms have been confined to determining the effects of transverse and longitudinal positions of the sidehulls only on the resistance characteristics. The investigations undertaken within the scope of this article provide a starting point to investigate the effect of the trimaran's sidehull position on the motions of the vessel.