van Oortmerssen in 1971 published the results of a regression analysis of the resistance of ninety-three models of tugs and trawlers which had been tank-tested at the Maritime Research Institute Netherlands. This was in the form of an expression for the residuary resistance of the vessel based on parameters available at an early stage of design. His method remains one of the few suited to such hullforms and quickly found favour with those involved in prediction of their resistance. The method is included in many commercial resistance-prediction packages, and is still widely used.However, there was a number of errors in the original publication. Depending on how these errors are treated, it is possible to come up with different values of the total resistance, and almost every known implementation gives differing results for the resistance. Many of these errors have been resolved by correspondence with the author and with MARIN, and it is the intention here to record corrections to the known issues.In addition, it has been found that there are some combinations of parameters which give anomalous results. For these combinations, the resistance does not increase monotonically with speed as might be expected for this type of displacement hullform, but shows a distinct “hump” as might be expected for planing hullforms. The anomalous results have been investigated to determine the combinations of parameters for which they are produced, and a method of dealing with the results is proposed.