# Finish and Flow

Why else do you tink you can’t keep up wit dem pesky sealz

that stuff is creating a boundary layer upon which you levitate.

I just took a wax comb to the bottom of my board…wanna race?

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I thought the use of riblets elongates the laminar BL, and sort of holds off the "amplification" (?) of TS waves, leading to turbulent BL.

Can you help me clarify this conceptually?

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i think we can get close.

when the BL is laminar, the flow is esentially steady state with no pertibations in the x,y,z directions.

however, the flow field is considered to be full of disturbances. if these disturbances are allowed to amplify they typically lead to transition to turbulance. if the disturbances are damped for some reason the laminar BL will continue.

T and S were 2 guys who conjectured about the presence of these disturbances and how they were manifested in the formation of wave structures in the BL.  They were given credit as the first to document this wave structure in a laminar BL that was in the process of transition.  The occurance of TS waves is a given.   Nowadays, the stability of the BL is defined by weather any particular frequency of TS waves are damped or amplified. Epirically, amplification of any TS frequency to a certain level has been found to be an excellant predictor for transition to a turbulent BL.

there are a few flow conditions which provide stability to the BL  and prevent the onset of turbulence.

the first is to go slow (big deal)

a very smooth surface is virtually always required (smoother than 400 grit)

another common way is to accelerate the flow .

the LE of a wing will accelerate the flow until you reach CP min (very stable and laminar) at which point the flow starts to decelerate (unstable and turbulence is soon to follow)

folks interested in making laminar flow wings try to keep the flow accelerating as long as possible by moving the thickest part of the foil as far aft as possible.

heating the BL  effectively reduces the viscosity in the sublayer (lowest part) of the BL. the effect is similar to a local acelleration of the flow and increased stability ( more laminar flow with delayed transition)

another method using localized suction of the sublayer can  also be effective in maintaining a laminar BL.

however, to the best of my knowledge none of the techniques used for turbulent BL drag reduction (riblets, microchannels, LEBUs, polymers) have been shown to actually delay transition.

they all seem to limit the spread of  turbulent  fluctuations in directions lateral to the flow. but  i dont believe the effectively dampen  the streamwise turbulence levels.

flow vis of a streamlined body  with  small levels of POLYOX in the BL  look very similar to laminar conditions.

additionally the drag (95% skin friction)  can be reduced by 30-40%

however close inspection of tne BL with laser doppler velocity measurements confirn that it does indeed have a turbulent boundary layer.

i hope that helps in some way.

but i know i've created more questions than answers and would  be happy to add more to the confusion if desired.

regards,

-bill

A  gloss coat completes the board and ads the last layer of strength. A lot of shops eliminate the gloss and to me it’s going to be a cheap board. I like to gloss the board and leave it as is with the waxy unpolished surface.Just rub out the rails.it is the strongest finish. Polishing is just for looks but also a sign of craftsmanship. If you like a sanded finish sand the gloss to whatever grit. My 3 cents

well said roger

**its its most important to seal the sanded surface to prolong the life of the board **

just a sanded finish is just poor quality work  & all these la de da speed finishes dont do one thing

if you want to belive you are going to go faster then rub out the gloss coat bottom only

**i would have thought the sways crew would be awake up as to why this is so popular with the likes of the big names  mentiond
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Howzit Mr.Clean, I love the look of a glossed and polished board and it does add that hard egg shell to the board. The sanded finish is faster though for those that think they need that extra 1 MPH or so speed. I did a test one time and took a clean polish rag and gave it a push along the bottom of both types of finished bottoms and the cloth didn't go very far on the polished board but it went right off the end of the sanded finish board, Don't really know how much faster it makes the board but it is a faster finish but give me the G&P finish any day. Aloha,Kokua

Will the paint finish on your VW Van make it drive more like a Race car? .....HAHAHAHaaaa!! Can't stop thinking about sanding down the bus and taking it out to collect some pink slips!

If I drive by too fast, you can always apply wax to the windows to slow'er down!

You’re waxing the wrong the side of the board. Next time trying standing on the deck, you’ll be amazed at the difference in speed.

At the cost of the benifits of reverse rocker. Idono.

Thanks for the explaination… that stuff is pretty fuzzy in my head by now… been a while since I studied any of that.

But bottom line, you’re saying that a smoother finish, smoother than 400 grit, maybe even a polished finish, should be faster than a standard sanded finish?

Going back a bit… you said many shapes of grooves work. I was under the impression that the grooves had to be V-shaped, had to change dimensions along the length of flow, and had to be perfectly in line with the direction of flow… which under a surfboard, is not exactly linear, but curved, relative to bottom shape. Any thoughts?

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But bottom line, you're saying that a smoother finish, smoother than 400 grit, maybe even a polished finish, should be faster than a standard sanded finish?

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yes, if you were trying to maintain laminar flow conditions.

however, in the real world there are only a few places where that is practical at elevated speeds.

if it was important to me to make any watercraft faster w/o the use of polymers i would also be one of those guys wet sanding with 400 grit. :)

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Going back a bit... you said many shapes of grooves work. I was under the impression that the grooves had to be V-shaped, had to change dimensions along the length of flow, and had to be perfectly in line with the direction of flow... which under a surfboard, is not exactly linear, but curved, relative to bottom shape. Any thoughts?

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there are lots of references regarding  the effectiveness of other groove shapes.

however i think the v grooves are/were popular because they add the least amount of additional wetted surface while providing  the necessary constraints to the BL.

i suspect they are also  relatively easy to manufacture compared to other choices.

I guess there are lots of times you'd like your board to be a bit faster, but i would think size, shape and trim generally have more potential for success.

I dont think there will be much of a market for BL drag reduction in surfboards until they announce the World Surfboard Racing Tour

:)

regards,

-bill

oops,

double post

…Rusty, seems that

all the money, time and those sanded finishes have not been helpful to you to learn how to surf fine…

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A  gloss coat completes the board and ads the last layer of strength. A lot of shops eliminate the gloss and to me it's going to be a cheap board. I like to gloss the board and leave it as is with the waxy unpolished surface.Just rub out the rails.it is the strongest finish. Polishing is just for looks but also a sign of craftsmanship. If you like a sanded finish sand the gloss to whatever grit. My 3 cents

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I agree 100% with Cleanlines.

I would like to hear comments from people that can "feel" the difference when they surf...people that have tested buffed out boards vs. sanded boards....

From what I've read on Swaylock's.....sailboat racers like sanded finished.....

We all know who the fastest surfer alive is........His boards are polished.......

now we can all argue who the fastest surfer is........Ha  ha

Ray

I know there is a lot of interesting science here but I can’t help but be reminded of back when I was a little kid. My brother and I would challenge each other to a “race” on little white-wash waves. Laying down vs. standing up, boogie board vs. surfboard it didn’t matter, the wave lets you go as fast as it wants.