Numerical modelling of the effects of trim on a ship’s form factor and viscous resistance

Predicting ship resistance with high accuracy is essential to reduce fuel consumption. An understanding of the factors affecting the resistance of a ship is needed to enable the development of strategies to that end. A well-known way to reduce the resistance is to optimise the trim angle of the ship. However, uncertainty is an obstacle to optimisation as the total resistance can reduce by about the same order of magnitude as the overall modelling uncertainty at full-scale while model-scale approaches suffer scale effects. Understanding the scale effects and devising strategies to account of them is therefore essential. The dependence of form factor on the trim angle is investigated using Reynolds Averaged Navier-Stokes modelling. While trim is known to impact predominantly the wave resistance, its effects on the form factor are less known, which is particularly important for slow ships where the viscous resistance accounts for the vast majority of the total. The present paper demonstrates a drop of 3% in viscous resistance when the ship is trimmed of 2° by bow with a fixed sinkage. The optimal trim angle of 2° is independent of scale effects as we found the same results for Re = 106 and Re = 108 . The CFD form factor shows a high stability with differences of less than 1% between trim angle of 0° to 3°. The minimum form factor is also found for a trim angle of 2°. Scales effects are, however, observable on the form factor and on the trim moment and vertical force. These scale effects will have an impact on the ship resistance when sinkage and trim will be degrees of freedom.