Respectfully, I submit that this is an incorrect statement. To fully appreciate the positive effect a textured surface has on fin performance, (and by extension, board performance) the textural change must be applied to a fin and board combination that is WELL KNOWN to the rider. Then any improvement in performance can be readily experienced, and must be attributed to the one thing that was changed. The textured surface of the fin. My statement is rooted in actual rider experience, in the water, on equipment well known to the rider. In my world, experience and observation, always trumps theory. A surfboard functions at the air/water boundry layer, planing OVER the surface of the water. At the same time, the fin is fully submerged, and moving THROUGH the water. Two completly different environments, only inches apart.
When I was a teenager, long before engineering school, after I had a new board for awhile, after dinging the (single) fin a few times, I’d have to sand out the nicks and chunks, but didn’t sand to a polish, and it rode better afterwards. When I started shaping/glassing my own boards, I left the fin(s) rough sanded. It was all emperical. It wasn’t until many years later in engineering school that I understood the physics of why that was.
Don’t get me wrong, I agree with what you say. My point is that the skin friction drag of the fin (not the form drag) is order of magnitude lower than the total drag, which is mostly board and leach drag. Unless when you reach really high speeds then fin’s skin friction drag starts to contribute significantly, which might be the case for big wave riders, but that’s not where my experience is.
This is indeed a massive difference.
Good science designs experiments/research that can be quantitatively replicated and that eliminate(s) placebo effects.
At what speed would you expect to see this effect?
Very True. Theory proven, not value judgement.
Hard to say, depends on multiple factors.
- Do you use a leach, and how low is it?
- How precise is your board control (the more precise the earlier you'll feel it)
- How draggy is your board
- How large is your fin(s)
From my windsurfing experience, I’d say it starts to become noticable somewhere between 40 and 80 kph (25 and 50 mph). Please note this is not backed by hard numbers, just my personal feeling.
Note what the application of some Scotch tape on the upper surface does to an Eppler 193 airfoil at Rn in our ballpark. The strip of Scotch tape trips the boundary layer to turbulent.
First image - credits - ©1980 - same year when I dropped a semester of college to go surfing at Hanalei Bay
Second image - wonky lines with smooth surface - high drag and non-linear reduced lift.
Third image - less wonky lines with Scotch tape at 10% chord.
Forth image - all cleaned up with Scotch tape at 23% chord.
Making the surface rough with sandpaper will product the same results. I also use aluminum tape.
Is this measured data? What size foil? Planshape? At what speed? Air or water? If water, fresh or salt?
Does tape = sanded surface? What type of sanded surface? Have you measured actual drag of a sanded surface (what grit)?
Many uncontrolled variables. Predictions are modeling.
This would require plunging wave height greater than 7.5 ft for 25 mph and plunging wave height greater than 30 ft for 50 mph.
I understand that Reynolds number are medium independent.
I have heard arguments that the laws of fluid dynamics/mechanics do not apply well to small foils (e.g. fins). Macro, meso and micro scales? Do modeling predictions apply equally?
Replicated data with validating statistics make comparisons more reliable/credible.
Seems to be data in air using RC airplane sized wings, probably very high aspect ratio.
Laws of fluid mechanics still apply very well to fins, it is the much lower Re numbers where problems occur AFAIK (insect wings, … )
Measured or modeled data? If measured how (methodology)? How many replicates? Sample size? How many treatments?
Can a taped surface be taken to be representative of a sanded surface?
Ok, to answer some of the questions regarding Dieter Althaus’ test data I posted this morning…
It is test data.
It came from the Institute for Aerodynamics and Gasdynamics at the University of Stuttgart.
It is a properly run high-fidelity wind tunnel test of 2D flow - no wingtips - the test item fills the test section.
More information about how the test was run is in the book - it is all in German - I am not going to post those pages much less translate it for y’all.
I posted it to show actual real world results given so many of the comments after the initial post about this phenomenon.
Whether by a rough sanded surface, dimples, grit, Scotch tape, zig-zag tape (my favorite) or whatever else, these things cause the boundary layer to transtion from laminar flow to turbulent flow sooner than it would on a smooth surface.
A good primer to understanding the boundary layer is: Drag on a Sphere and Cylinder - it has pictures that show things actually happening - take note of the dates on some of the credits.
Real world example:
Wen, Weaver and Lauder (2014)
Journal of Experimental Biology 2014, 217: 1656-1666; doi: 10.1242/jeb.097097
Greatest biomimetic shark skin drag reduction occurred in a velocity range of “0.1 to 0.6 m s−1.”
“There is a critical flow speed (U*=0.345 m s−1) where a transition from ‘drag decreasing’ to ‘drag increasing’ occurs.”
“Most sharks do not swim at high speed for the majority of their daily activity pattern, and common cruising speeds for many sharks, including those species classically considered to be high-speed specialists, fall in the range of 0.5–1.0 body lengths s−1.”
_____
Interview with Lauder:
“It’s during the steady, long-distance migrations that you’d really begin to see the benefits,”
Also:
Oeffner and Lauder (2012)
The Journal of Experimental Biology 2012, 215: 785-795. 1242/jeb.063040
Yes, that is definitely the case.
My point is rather that these percentages are really nothing to worry about if we’re still using the wrong profiles, are dragging a leach and using high friction boards (the boards are lifting us out of the water, fin lift only comes in that range when turning heavily).
Also note that those differences are made in the AoA range below stall and that no significant difference is made close to stall. IMO it’s especially important to improve the stall characteristics to get a noticably better fin. The Thrailkill twin is a perfect example of improving the stall behavior while sacrificing on the lower AoA ranges (due to significantly larger total fin area needed).
Anyway, just my theories that have great effects in reality