The project described in this thesis was undertaken m order to gam a better understanding of hull-waterjet interaction. The study has investigated interaction effects between a waterjet featuring a flush type intake and a planing hull, 7.4 metres in length. Full and model scale resistance and propulsion tests have been conducted to accurately measure the thrust deduction fraction and interaction forces. Various techniques for measuring the waterjet thrust are discussed. Graphs of the thrust deduction fraction for a variety of towing positions are reported. Hull static pressure measurements are also reported. A free body, vector approach was used to develop empirical, analytical models for both the towing and propulsion situations. Both models are based on the Savitsky planing performance equations. The propulsion model draws also on Hadler's work on propeller-hull interaction. The analytical towing model was validated against full scale towing tests and small correction functions added. The adjusted model was found to compare favourably with model tests. Using the correction factors developed for the towing model, an analytical propulsion model is constructed incorporating the waterjet momentum forces. A method is described whereby a vector, called the interaction vector, accounting for remaining interaction effects can be found. Two mechanisms are proposed which would account for the interaction vector. By comparing the full scale propulsion tests with the propulsion model, the interaction vector is found. Its general magnitude and position are shown to be consistent with the combined proposed mechanisms.