Talk about being counter intuitive, eh? What am I really up to here? The short answer is: ‘‘Challenging you, the reader, to think.’’ First off, is drag, on a surfboard a bad thing? Sometimes, but not always. It depends on the kind of drag, and where it is, on the surfboard. In this case, on the surface of the fin. In particular, a rough, or texrured surface. This concept is only valid, in the dynamic environment of a wave. In the calm still water of a pond, where the water is not energized, the surface drag on the fin,will slow the surfboard. The opposite is true, in a wave, where the water molecules are really moving. It is those energized water molecules, in contact with the textured surface of the fin, which will drive the board at a higher speed. How does it do it? In a phrase: ‘‘By tapping more of the kinetic energy of the wave.’’ Having a hydrodynamicaly foiled fin is also a great help too. The foregoing is not theory, but the experience of numerous people, with boards and fins that have only had the surface of the fin modified. Thier collective experience was that the board was faster across the wave, and through the turns. This is one of those ‘‘dirty little secrets’’ about how a surfboard fin works. Make the change on the fin, on a board you think you know well, before you dismiss the concept out of hand. All that said,‘‘HAVE A HAPPY NEW YEAR.’’
rougher surface doesn’t create drag as much as disrupt surface tension adhesion = less hydrodynamic resistance=more speed…why pro boards are sanded, America Cup hulls as well…am I close?
Yes, close. Spot on, for something moving through the water. However, a surfboard is moving over the water, at the air/water boundry layer. There, the ideal is a highly polished, water repellant bottom. Some folks may disagree, and that is thier privilage. A sanded pro board, has more to do with weight savings (IMO), than any other consideration.
“The Counterintuitive Concept of…”
…
How much savings in weight for a 5’8” x 19 1/2?
indubitubly Mandrake!
happy new year.
Raney said,at K38 1/2
to us fry said 180 on the deck
220 on the bottom
scratch pattern
from nose to tail
full passes.
deck ,no wax refresh weekly…
…ambrose…
whatabout static line
standing lunge
no paddle takeoffs?
with a sashimi appetizer?
Drag reduction is greater in energised (turbulent) water conditions with a rough surface versus a smooth surface - compared to undisturbed (laminar) water conditions.
The rough surface alters the patterns of near surface turbulence and reduces their effect and the magnitude of the associated drag. A smooth surface is unable to affect the near surface turbulence in the same way, and as a result has more drag.
I’m not the expert on fluid dynamics, but if I read your post, another idea came into my mind:
during the ride, we face different influences on the bottom. the upfront rather touches the water, from front to mid, we have different conditions, from out of the water to slight touches, to deep touches, this is definitely an air water boundry layer zone, Whereas the tail is almost sunk, there is just a water/surface layer. Why not polish the front of the bottom and roughen the tail aera, at least around the fin zone? This should in theory combine the effects.
What do you, as the real experienced shaper and surfer think about this way. (Be aware, that my experience is auite poor and I never will be able to reach yours…)
Well, in my mind, the operative word there, is theory. At one point I had the same thoughts. I never went down that path, so didn’t discuss it. I prefer to talk about my experiences and observations. That said, I think what you’ve put forth has some real merit. It’s the sort of thing that should be done on an existing board, well known to the rider, so that the change of surface texture on the tail area, is the only thing that is changed on the board. What I had thought about, in that regard, was a change in texture beginning at each rail, near mid length, and angleing back to just before the fin. As i said, I never did it, so didn’t talk about it. Kudos to you, for out of the box thinking!
surfboards, the singular, unique, exception to all the rest of the fun hog gear
The medium itself virtually impossible to replicate on screen or in tank
The tool to plane the medium subject to endless imposed and generated non linear forces
Tom Carrol once said he could tell if a new board was going to work within his first half dozen strokes paddling out on it
On the north shore, broke an 8’0 at Sunset that had been working well, scouted around and found a used Willis almost identical in dims, rocker and fins, only it had the Willis unique golf ball sized dimpled bottom on it
Desperate to get back in the lineup, bought it and hustled back out that afternoon, and the board worked better then the one I had broke, faster through that mid section so easy to get shut down on
unaware of a Cray supercomputing algorithm or hydro tank tests that posit golf ball dimples on the bottom of a board will make it go faster
what we do have is this mobius loop of shaper and riders…shaper design tweak, rider feedback…rinse and repeat, an endless continuum of empirical based advancement
Hope to still be here in a couple of decades, if for nothing else just to see what the surfboard has become by then…
Ask Liddle . He’ll give you the full drill on sanded finish. Believe me he has one and it is completely logical.
for me the contact surface drag of surfboards skin is a negligible parameter. guys experiments dimples others shark skins, some made a kind of wax like for ski that was try by pro surfer while ripcurl pro france 20years ago. nothing seems to give a noticeably change.
What is the net surface area of a fin?
What is the net drag of that surface area with a gloss finish?
What is the net drag of that surface area sanded to 400-600 grit (or with shark-skin riblets)?
What is the net drag for that fin’s surface area at various velocities?
At what velocity do shark-skin riblets provide the greatest reduction in drag? What is the size and shape of those riblets?
Figure it out for yourself, kook, by actually riding a surfboard.
The answers have been provided by many people with appropriately credible qualifications, backgrounds and experience (pros).
You kinda got the why right, but are totally correct on the result. The layer of molecules between the fin and the freestream flow is called the boundary layer. The fluid dynamic term Reynolds Number (Rn) is an indication of the characteristics of the boundary layer. Our fins operate in a transitional flow regime where the Rn is fairly low. Basically, the molecules have a hard time negotiating around the curve of the fin. We therefore need fairly thin foils on our fins. To help molecules get around curves better, turbulating the boundary layer by roughening up the surface is a common practice. We dimple golf balls to reduce the drag. High performance gliders put zigzag tape at specific locations on the wing to transition the boundary layer from laminar flow to turbulent flow as specified by the aerodynamicist. We put ‘grit’ at specific locations on sub scale wind tunnel models to simulate the higher Rn flow of the full scale aircraft. Our fins are notorious for laminar separation bubbles at all angles of attack - basically they have little partial stalls all over the place. To fix it, we need to roughen up the surface of our fins a bit. Mostly just from 10-30% chord on the outside of side fins and both sides of center and single fins.
McDing- thanks to Mr. Liddle, I always have some Wetsand sandpaper on hand to treat the bottom of board. If I don’t, I will use fine grain beach sand before I go out…
The sanded bottom thing has been around for a long time now, having come first (IIRC) from Hobie cat racing, perhaps later from America’s Cup boats, back in the days when such AC boats were traditional monohulls. And there was graphite paint, too.
But… there’s always a but… If you can get your watercraft 0.1% faster over a miles-long sailing course, you might win your race by a handy if not considerable distance. On a surfboard, 0.1% extra something on a ride less than a minute long, is like, almost nothing.
My grandfather made my first board in the early 1960s. Ten years later I was making my own and he gave me static for sharp edges on fins… I said “if it saves me once, it’s worth it”. Now I have scars, one of them on just behind my shoulder, and lucky it wasn’t much worse. “So what?” is left to the reader to determine.
Glossy-bottom boards are popular because shops exist to make money. Sanded-bottom boards are harder to sell, thus poor business practice. If a board maker believes he/she deals with knowledgeable customers who understand, tolerate, accept sanded bottoms, go with it. Most will not.
Dimple-bottom boards… I’ve seen a few, decades and decades ago. They are now rotting in some landfill under tons and decades of waste. True that breaking laminar flow can help, but dimples aren’t the way to do it. Golf balls fly farther you say… but golf balls are not in the same flow regime as surfboards. The Reynolds Number is a ratio of inertial forces to the viscous forces, and tells you the flow regime you’re operating in. Froude Number compares inertial force to mass of the fluid in which one is moving. Dynamic similitude is important. It’s a subject not understood my most, who will be satisfied by their current boards and who for the most part are not sufficiently sensitive to realize actual improvements in the tiny range that a deep study might yield.
If anyone finds this is insulting, think… if there were a significant tech-based advantage to be found, why have surfboards been essentially the same since invention of the thruster, or earlier? THERE IS NO MAGIC, just around the corner, that will suddenly rise into view and change everything.
At age 67 (oh the horror) I’ve seen a lot of gibberish come down the road, as have many of the older members of this list. Usually it’s sponsored by this or that shop, trying to sell product. Sometimes several shops will jump on the bandwagon, if they think “whatever” will sell boards. The sh*tloads of marketing hype never stop, folks, and it continues to this day, regardless of what some old bitter mental-case might say. You gotta be able to strain out the BS, even though sometimes you’ll want to believe it. It’s a well crafted marketing effort that builds upon (extrapolates) a small truth to “something totally new and you gotta have it!”
Rant over, for now. Ya wanna sand the bottom, go for it, it’s a lot less trouble than glossy.
Oh but one last thing: OP, it would be more honest if rather than making veiled suggestions, you just come out and say what you intend us to receive.
I’m with Lemat and Honolulu here.
The effect of surface is negligible on fins.
The reynolds number is too low for dimples or other textures to have any noticable influence other than surface drag, surface drag is negligible compared to all other forms of drag on a surfboard.
I highly respect thraikill’s experience, love his twin setup and do fully agree that the lessons learned are very valuable lessons. However it is risky to explain them with hydrodynamical principles especially on molecular level without proper scientific validation. Rules of thumb are great for condensing years of empirical evidence into a simple set of rules, but the “physical principles” behind those rules are often incorrectly understood, just like Newton’s laws of gravity have been proven to be a severe simplification by Einstein.
Hans,Lemat & Honolulu all true. I will say that when I do a nice glossed board; I want it to have a nice glossed fin. Aesthetics not function. Preferably a “fringe” from Rainbow. But on my every day rider, I like a sanded raked fin.