The study presented in this paper is a contribution to the establishment of a reliable procedure for verification of numerical simulations of turbulent flow around ship hulls. In order to study the behaviour of the verification process under various circumstances, we have selected six test cases ranging from a 2D flow governed by a Laplace equation to the flow around the KVLCC2 tanker at model and full scale Reynolds number. For each case, solutions on much more grids are available than strictly required by the recommended procedures. Two types of power series expansions are applied to estimate the error, using unknown and fixed exponents. A procedure to estimate the uncertainty when the number of grids available is greater than the minimum required for Richardson extrapolation is also tested. The results indicate that the main sources of the scatter observed in the data for ship flows are the insufficient geometrical similarity of the grids and the use of numerical interpolation to obtain flow quantities at locations that do not coincide with grid nodes. In three-grid verification of ship flow computations conservative uncertainty estimates are therefore in place.