From Flexspoon.com: “Bottom speed-- the bottom finish-sanded (nose-to-tail) with 400 grit wet-n-dry provides optimum surface texture. Too rough or too water repellant (slick) creates turbulance between laminar flow layers beneath hull. NO wax of any kind! The edges above waterline also need this treatment.”
In your experience, does sanded finish really make a difference for surfboards, such as fishes? I have seen somewhere here on Swaylocks notes on the benefits of sanded finishes but i thought to ask you guys directly about it. A placebo effect or the real deal? I would have thought that sanded finish creates more drag as compared to gloss and polish…
Nice observation. however, nature is full of suboptimal mutations. Thus, although a shark has 120 grit skin surface it does not mean it is an optimal trait.
the technology is real. but the magnitude of the effect is relatively small.
a search for "riblet (or micro channel) drag reduction" will give you a good idea of what folks have measured in controlled experiments.
riblets or shallow lengthwise grooves can reduce the turbulence level in a turbulent boundary layer.
when sized correctly to the boundary layer of interest they can reduce skin friction levels by a few percent (i'm guessing , maybe as much as 5%)
but skin friction is likely responsible for only 30% of the drag of your board ( guessing again) but the bow wave and tail wake and fin drag are big contributers.
I doubt a 400 grit hand sanded surface is really optimal for any reynolds number . But i do know 3M fabricated plastic films (with riblets) for one of the Americas Cup challengers the last time Australia owned it.
For 2 identical boats, 1% difference in drag for a 1 hour race could be huge.
Nice observation. however, nature is full of suboptimal mutations. Thus, although a shark has 120 grit skin surface it does not mean it is an optimal trait.
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On the other hand, evolution, natural selection etc has generally resulted in species becoming optimal - as those that are not are deselected.
But anyway, I'm an engineer - and I can confirm that a gloss finish is slower than a matt surface. The extent of roughness of the surface will behave optimally at different speeds though a fluid, hence golf balls have dimples on them (because they leave the tee at around 150mph for a 100mph club head speed). Something moving much slower, like a surfboard, will require smaller "roughness" such as a matt finish, to get the same effect
Its all to do with boundary layers and wake turbulence, a google search will shed some light if you like maths
Shark skin being rough is definitely not a "suboptimal mutation", it will help it "slip" through the water, as counterintuitive as it sounds!
...........- and I can confirm that a gloss finish is slower than a matt surface. The extent of roughness of the surface will behave optimally at different speeds though a fluid, hence golf balls have dimples on them (because they leave the tee at around 150mph for a 100mph club head speed). Something moving much slower, like a surfboard, will require smaller "roughness" such as a matt finish, to get the same effect
Its all to do with boundary layers and wake turbulence, a google search will shed some light if you like maths
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not exactly.
in fact, two completely different mechanisms.
riblets (length-wise) grooves are designed to limit the lateral spread of turbulence in the boundary layer.
this effectively reduces the skin friction of the surface yet has no impact on the wake or pressure drag.
dimples on golf balls are intended to intentionally trip a laminar boundary layer and create a turbulent boundary layer which unfortunately has a higher skin friction. However all sperical objects have a high drag coefficient because of their large wake ( separated flow ) The singular advantage of a turbulent boundary layer is that, compared to a laminar boundary layer, separation is delayed and the resulting wake is smaller (less drag)
so the dimples increase skin friction drag but reduce the dominant pressure drag effects from laminar separation.
Maybe in a test tank going absolutely straight you might be able to tell the .000000120 of a difference.
All surfboards would be shiney if they didn't cost more. Spend more time on attributes of dragging as leash vs no leash. then leash drag with 1 pcs of kelp vs a 10 ft stringer of the brown stuff.
Then the attributes of trimming out a board properly.
heck 400 grit...just take it down to 600 and polish it out
Red boards are faster than white ones....hands down
I had a surfboard with golfball dimples on the bottom.....it sucked.
Maybe in a test tank going absolutely straight you might be able to tell the .000000120 of a difference.
All surfboards would be shiney if they didn't cost more. Spend more time on attributes of dragging as leash vs no leash. then leash drag with 1 pcs of kelp vs a 10 ft stringer of the brown stuff.
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I enjoy the finer points of the dynamics, but I also hear what you're sayin'. My favorite example is the pro guys who would complain mightily if there was a zit on the bottom of their board but don't hesitate to put add-on sponsor stickers all over the drive section of their boards. I think it slows 'em down; but hell, for all I know, the raised edges of the stickers act as vortex generators and make 'em go mach 3......
(note to any of the pros I've shaped for over the years: I'm not talking about you, it was those other guys)
Oh, and for the record, Coil does a 400 finish on all of our boards.
been surfing for over 20 years now, and still think there is more to the overall package than the type of finish when it comes to drag… not to mention rider input, ever changing conditions, fins, wax stuck on bottom, and the list goes on…
No, no, no the golf ball dimples were about 1 1/2 in concaves, all over the last 3rd of the board. It must have been a glassers nightmare. Made the board do all sorts of strange things, If going slow, it would spinout on a mushy bottom turn. Worked ok in well shaped bigger waves........but then again what board doesn't work well in good formed bigger waves?
Make'm red with flames & pointy and polish out to 2000 grit. That's fast.
I randomly ran into a guy who builds racing hulls about a year ago. He told me they sand the hulls to 400 along the waterflow direction. That's the fastest from his experience in boat racing.
but many factories dont gloss the boards not only for the time (hours+money) but for the dents
that easy showed in a 4oz potato chip and look bad.
also, speed finish is the way to go if you wants to sand the board to 400
because what you sand is a double hot coat
so better to sand the hot coat and the speed finish it
in that way you save the gloss weight and seal the board + water on water glide due to rough surface + if you make a clear white board, it will be clear for long time and in the future the tan will be whiter than with resin
due to fine UV filters (and the resin and foam filters too)
think of skis and snowboards. Texture is fully knoticeable on a good tune up vs. a polished flat ski/board. It can make a huge difference that can subtract up to seconds when racing. No wonder pro ski tune techs for world cup make a ton of money when win margins are hundredths of a second
Optimal for who? Male sharks might prefer a female that’s easier to catch!
IMHO A shark’s skin may not be optimal for speed, but that is not their skin’s only function - disease prevention, attack prevention, sensory input etc make it a compromise.
Perhaps the light, fast flat bottom boats are closer. Also, anyone ever notice that few military or commercial boats have gloss coats? Governments don’t need shiny, they need effective
Most surfboards still have a lot of “form over function” in them in order to get them off the racks. Cool Thread!
I’m with Mike Stickers make you go faster.But they have to be the right sticker. Quicksilver will slow you down but a nice vulcom will make your board faster and hand longer in the air. It’s true i saw it onece in a surf movie.
