yeah using a ruler I'm getting the mid point of the Taekwando bulges at 80, 70, 60 and the classes are 58, 68, 80 so the violin plot is almost certainly smoothing (which I'm fine with as otherwise the chart wouldn't be visually pleasing)

Interested to know what the tiny bulge in Taekwando is just above the 58kg peak

as a simple example, you and your bf now swap boats

the lw boat your bf is in will be deeper in the water than when you were in it >> more drag

his boat that you're in now sits higher in the water >> less drag

the boats are designed for different weights but thats not really what we're looking at here, we want to know how much water is displaced as that determines the amount of water-boat surface contact surface area and hence the drag.

A scull rated to a heavier weight does this by being wider and/or longer, increasing the water-shell contact area [which is the key thing as by spreading your bfs weight over a larger area the boat 'sinks' less and has the right water clearance] and increasing the drag