43.6% Drag Reduction

[Dave G 9N]

This is a little rough. Approximately 38 to 53 microns. So says a clickbait article from Wired.
Distributed Micro-Roughness (DMR) is not riblet film and the effect is not directional. All testing was in air, not water, but so was riblet film at first.

The rivulet (sic) process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
...

Yakino and his team precisely measured the total drag coefficient on smooth and DMR-coated surfaces over a wide range of Reynolds numbers (ratio of inertial to viscous forces acting on the fluid) (Re = 0.35 x 10⁶ to 3.6 x 10⁶).

Two types of DMRs were used in this experiment: A convex pattern made of glass beads with diameters ranging from 38 to 53 micrometers (μm) and a concave pattern applied by sandblasting. The height of the DMR coating is only 1 percent of the thickness of the boundary layer and is classified as a “smooth surface” from a hydrodynamic point of view.

Experimental results showed that the critical Reynolds number at which the turbulent transition begins increased from approximately 1.9 × 10⁶ to 2.2 × 10⁶ for the DMR-coated model, and drag was dramatically reduced by up to 43.6 percent in the transition zone. Furthermore, the DMR-applied surface consistently showed a drag coefficient lower than that of the smooth surface up to the highest measured Reynolds number (3.6 x 10⁶).

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112026115201
https://www.t-technoarch.com/data/anken_en/T22-118.pdf

armbeat — Drilling Intelligence https://www.armbeat.com/news/tohoku-university-discovers-436-drag-reduction-via-micro-roughness

43.6% Drag Reduction Achieved: Japan's Microscopic Roughness Discovery Overturns 80 Years of Fluid Dynamics https://kantenna.com/topic/dmr-microscopic-roughness-43-percent-drag-reduction-tohoku

(If Wired won't let you read without a subscription, refresh the page, and quickly hit CTRL A then CTRL C before it finishes loading and blocking and paste a copy into Word.)

P.S. As I recall, one of the major drawbacks of riblet film was that the drag reduction was short lived because riblet film made an ideal substrate for fouling. Trailer sailors have the advantage here.

 

[gonzo]

It works well on masts to limit the size of vortices.