Welding is very difficult and dangerous for manual welders. The double hull structures found in ships are very hazardous environments, and as such the shipbuilding industry demands a safer, autonomous system to perform the welding rather than deploy manual welders. This paper describes the design of a new mechanism, called the 'Rail Runner', which is able to autonomously travel within the double hull structure. The design of a 3P3R serial manipulator for welding is also described in this paper. As an application of the 'Rail Runner' mechanism, we combine the 'Rail Runner' platform and the 3P3R serial manipulator for autonomous welding. The mechanical system of this robot is composed of a six-axes (3P3R) manipulator for achieving the welding function and a six-axes mobile platform for traveling within the double hull structure. This robot is able to autonomously travel between longitudinal structures, with transverse direction, and is capable of welding in double hull structures. The 'Rail Runner' can raise the efficiency of the welding process, as compared to manual welders. This in turn raises the international competitiveness of the shipbuilding industry.