The recent award of licences by The Crown Estate for ‘Round 1’ marine energy development sites in the Pentland Firth and Orkney Waters (PFOW) has focussed the attention of stakeholders on the need for a vastly improved understanding of the local and regional marine environment. The potential environmental intervention effects resulting from ‘extensive’ installation of marine energy devices, necessitates rapid, accurate, baseline environmental assessments to be undertaken. This paper addresses the implementation of the SUNTANS model to simulate the complex flow characteristics in the Pentland Firth. Validation is achieved through comparisons made with field sourced data. A sensitivity analysis of the bottom friction coefficient, which is a parameter required by the model directly related to how much energy is dissipated by seabed friction, is presented along. Results show that the model outputs fit better to field data using a higher value of this parameter than typically assumed, which is sensible because of the size grain of boulders located in the Pentland Firth.