The relation between blade slip and oar shaft flexibility dictates how the blade loads up during the drive. So, blade shape should be taken into consideration when choosing the oar shaft flexibility.
The feeling of how an oar loads up during the drive depends on the motion of the hands relative to the blade motion in the water. For example, at the catch, a more flexible oar shaft allows for a higher handle speed relative to the blade - providing a lighter loading. Similarly, a less efficient blade at the catch would slip (towards the stern) more, leading to a higher handle speed relative to the blade - also resulting in a lighter loading.
Because a more efficient blade (Fat2) – which has a better lock in the water – has less negative slip than an inefficient blade, it should be fitted with a more flexible shaft than a less efficient blade (Smoothie) to produce a similar loading profile for the rower.