Abstract. The 19 channel rotating shadow band radiometer GUVis-3511 built by Biospherical Instruments is introduced as an instrument which is able to provide automated ship borne measurements of the direct, diffuse and global spectral irradiance components without a requirement for stabilization. Several direct sun products, including spectral direct beam transmittance, aerosol optical depth, Angström exponent, and precipitable water can be derived from these observations. The individual steps of the data analysis are described, and the different sources of uncertainty are discussed. The total uncertainty of the observed direct beam transmittances is estimated to be 4.24 % at 95 % CI for ship borne operation. The calibration is identified as the dominating contribution to the total uncertainty. A comparison of direct beam transmittance with those obtained from a Cimel sun photometer at a land site and a manually operated Microtops II sun photometer on a ship is presented, yielding relative deviations of less than 3 % and 4 % on land and on ship, respectively, for most channels and in agreement with our previous uncertainty estimate. These numbers demonstrate that the instrument is well suited for ship borne operation, and the applied methods for motion correction work accurately. Based on spectral direct beam transmittance, aerosol optical depth at 510 nm can be retrieved with an uncertainty of 0.0032 for a 95 % CI. Only minor deviations occur due to the different methods used for estimating Rayleigh scattering and gas absorption optical depths, as implemented by AERONET and in our processing. Relying on the cross-calibration of the 940 nm water vapor channel with the Cimel sun photometer, the column amount of precipitable water has been estimated with an uncertainty of +−0.034 cm. More research is needed to estimate the accuracy of the instrument for low sun (solar zenith angles larger than 70°) and during periods with strong swell.