While the M137 looks like the M136 above the water line, having continual saxboard height for improved longitudinal stiffness and higher rigger feet, it doesn't replicate this shape below the waterline. It's a new design, and the goal was to improve its carry for men of 90kg or more. Compared to the M30 it carries more volume in the centre resulting in less drag and has a less aggressive cross-section for additional roll-stability.