Determination of service speed has always become a primary concern when designing a ship. However, a ship designer may not consider its effect on the use of energy and seakeeping performance of the vessel. The current paper discusses the determination of the actual and proper speed of a ro-ro ferry, operated between Tanjung Perak (Surabaya) and Lembar (Lombok), based on resistance estimation and seakeeping performance. The investigation is carried out using commercial software MaxsurfTM and further compared with computational fluid dynamics (CFD) approach to capture the effect of detailed wave drag, which is not quantified by the design software. The discussion and analysis include the determination of optimum speed based on the size of main engine and the seakeeping performance of the ship covering the aspects of heave, pitch, and roll.