Uncertainty affects estimates of the power potential of tidal currents, resulting in large ranges in values reported for sites such as the Pentland Firth, UK. Kreitmair et al. (2019, Roy. Soc. Open Sci., 6: 180941) have examined the effect of uncertainty in bottom friction on tidal power estimates by considering idealised theoretical models. The present paper considers the role of bottom friction uncertainty in a realistic numerical model of the Pentland Firth spanned by different fence configurations. We find that uncertainty in power estimates resulting from bed roughness uncertainty depends on the case considered, with relative uncertainty between 2\% (for a fully-spanned channel with small values of mean roughness and input uncertainty) and 44\% (for an asymmetrically confined channel with high values of bed roughness and input uncertainty). Relative uncertainty in power estimates is generally smaller than (input) relative uncertainty in bottom friction by a factor of between 0.2 and 0.7, except for low turbine deployments and very high mean values of friction.