Free stream tidal turbines are rotating bodies in fast flowing tidal currents, and as such are exposed to fluctuating loads from the surrounding fluid. These time varying forces will cause the blades to deform dynamically, potentially deflecting the blade shape away from the optimum orientation as well as exciting resonant responses that may enhance fatigue loading. It is important to understand this hydroelastic response for all but the stiffest blades. A loosely coupled, modular approach to fluid structural interactions (FSI) has been developed for the analysis of horizontal axis tidal turbine blades (HATTs). This paper discusses the methodology behind the FSI process and illustrates the technique through a case study of a 20m diameter, three bladed, horizontal axis tidal turbine, in which the deflection of the blades is examined through the iterative procedure.