Based on the investigation of large length measurement and alignment techniques in 3D construction, a laser diode alignment system for boat assembly is presented. In the system, a horizontal laser diode beam and a vertical light plane formed by a rotating pentagonal prism are used as a measuring datum. The system is of low cost reliable, safe and convenient to be used on-line in small boatyards with reasonable accuracy. The synthesised error of the system is analysed. The application of the system in boat assembly and some considerations of the system design are described.