n order to assess the manoeuvrability and controllability of fast non-planing monohulls, knowledge on rudder induced static and dynamic forces, including interactions, are of key importance. The complexity of the flow around the aftship, the influence of the propeller and mutual interaction between the rudder – propeller – hull are all judged to be significant. When the rudder is in its correct location behind the ship hull the flow in which it is operating is complex. The rudder is working in water which has been deflected and accelerated by the presence of the hull and propeller. The position of the rudder can have a large influence on its effectiveness. In this paper the influence of the transverse location of the rudder on lift, drag, stall and on the interaction phenomena is discussed including the effects of the wake field and the propeller induced velocity field on the flow conditions in the rudder plane. Analysis on the relevant frequency range of rudder motions for fast non-planing monohulls operating in waves will be presented. The dynamic effects of steering on the rudder forces will be studied.