In this paper, some of the architectural and tracking aspects of the software engine used in our on-going MARIN (Mobile Augmented Reality Tool for Marine Industry) project are described. The purpose of the project is to construct an augmented reality tool for shipbuilding. The future target usages of the system vary from inspection to actually improving the efficiency of some of the repeating procedures in shipbuilding. The system also includes means for indoor localization and access to the pre-existing CAD model of the ship. In our case, the tool is hosted by Unity 3D, which allows the interfaces between the inertial sensors and the visual tracking thread. Some of the implementation issues related to these interfaces are described, while some challenges related to visual tracking are also discussed.