I saw some experimental and simulation results of flow in the micro grooves. Interesting, and yes they have to be tailored to the velocity and flow direction to have any useful effect. It's not really about detaching flow, the grooves actually modify the flow structure at the wall, and reduce the overall viscous friction production.
Even if the grooves were 100% straight down the length of the hull, or even better, if they were somehow aligned with the mean flow direction at every point on the hull, the truth is the direction of the flow over a kayak hull changes enough while paddling to make the grooves effectively out of alignment most of the time.
Pat is right that smoothing a hull will reduce drag - the amount depends on how bad it was before and after, obviously. And Willi is right that just smooth is best, no additives, wax, etc.
Kayak Motor Kit
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