The question adressed in this project is whether modern hull lines with wide, box-shaped transoms, that seem to originate from extreme racing machines, are also beneficial for modern performance cruisers from a hydrodynamic performance point of view. A new 41 ft (12.3 m) hull was designed as an average of contemporary performance hulls in the 40 ft segment. The aft part of the hull was stretched stepwise and cut at constant length overall in order to systematically increase the transom size. Six hull variations with box shaped transoms were created in this way. To investigate the influence of the transom shape, the six hull variants were redesigned with a more rounded transom. The resistance was calculated for all twelve hulls in the upright and heeled conditions at Froude numbers 0.35 and 0.60. The computations were carried out using a Reynolds-Averaged Navier-Stokes (RANS) solver with a Volume of Fluid (VOF) representation of the free surface. The solver was validated against several hulls in the Delft Systematic Yacht Hull Series (DSYHS). Very different optima are found for the low and high speed cases. In an upwind-downwind race the round transoms performed best for the three fastest transom sizes. The fastest hull around the course has an immersed transom area to midship area ratio of 0.16 and it has a rounded shape. It is 1.9 % faster than a more box like transom. Overall the round transoms are faster around the race course.