Polished vs. Sanded Finish: revisited

General Discussion
1

I just had an interesting conversation with my cousin who is an aeronautical engineer for the U.S. Air Force. I picked his brain about the “Slick Skin vs. Sanded Skin” discussions that come up every so often on Swaylocks. He had some interesting information.

There is a new jet being developed by the Air Force called the F-22 that has a slick skin. He said that everything on the plane is ground down and polished to get a surface that is as smooth as possible. The result in his words: “The jet gets up to speed, somewhere over the speed of sound, but when they shut it down it takes forever to slow the thing down. It just keeps going, and takes alot longer to slow than a conventionally skinned jet.”

I asked about the “Rough skins is faster” discussion. His answer was that rough skin on planes is used in certain areas for control, not for speed, because the rough areas in effect cause more air drag.

Then the golf ball came up. Dimples in golf balls cause a back-spinning ball to create low pressure on top of the ball, giving lift, causing the ball to stay up longer, making it travel further but not faster.

Jets travel fast. Surfboards go relatively slow. Does the technology equate?

He was a little vague. His answer was," Some of the principles are very close to being the same, and can be applied to surfaces going through water".

Apart from the skin question, he definitely agreed that plane wings and rails on surfboards share the same principles: Soft rails that allow water to wrap around them will cause the rail to be pulled into the wave. Sharp rails plane.

I just wanted to throw this info into the Swaylocks bag of knowledge. I’m going to keep on polishing my boards. But maybe leaving part of the board with a sanded finish would allow for added control. Something to think about for the next board. Doug

2

So what about fins? Should they be polished or sanded? Hmm.

3

I don’t know if this would be applicable, but I saw a test a few years back that demonstrated how texture made an object drop/move through water. I don’t remember who was doing the testing, but what they found through the use of high speed photography was that a ball (a bowling ball) that had a 2” or 3” patch of sand paper attached to the bottom, the side to hit the water first, would let the water wrap around the ball much like you see in the golf ball dimple tests. You could see the laminar flow of water around the ball with very little separation which I believe equated to less drag as the ball moved through he water. The smooth ball would push more water out of the way and create more turbulence/ drag. I have no idea how that would apply to a surfboard, but thought I would throw it out here anyway. Someone might be able to make use of it. The conclusion of the discussion was that adding a bit of grit ( in the form of metal flake) to the shear line of the of the boat could make the boat slide into the water better giving more control, depth, and length to the ride by reducing turbulance and drag.

I saw this on a kayak message board in a discussion about how to make a squirt boat flow and sink better into the water where you want the water to flow around the boat with as little resistance as possible (ie. to maintain laminar flow to reduce drag) and use a “wing affect” to take your boat under water using the AOA of the deck and hull with the current for control. If you want to see that at work check out the link in my signature block. It’s a video of a person doing a mystery move about 5’ deep into a river “seam” and doing a triple loop as he disappears out of site. Very cool and difficult to execute.

Hafte

4

Interesting, sure.

Anyone (besides me) bothered to look into terminal velocity and momentum equations IRT surfcraft?

Some interesting things come up.

Here’s a tip. The idea of “true area of contact” has a critical impact on drag at the interface of object/air/water. because water is about 1,000 times more dense than air. Momentum (thus mass) also has an important role.

Rambling, rambling… Shut up Doug!

5

Then the golf ball came up. Dimples in golf balls cause a back-spinning ball to create low pressure on top of the ball, giving lift, causing the ball to stay up longer, making it travel further but not faster.

Havent heard that one Doug. The dimples create turbulence, which keeps the flow attached much better (reduced wake) resulting in lower drag and longer ball flight.

The other stuff is interesting tho.

6

…all the boards that I did in 1991 sports a mix of gloss, sanded and “speed finish”

normally sanded on top and bottom speed finished (clear)

or all sanded and I d put (bottom) a tape from the stringer to the rails in form of V in fron t of the fins, and spray finish the clear to the tail…

sanded is not the same like speed finish, where you obtain water on water surface

past year I did some eggs an minieggs with gloss on top and speed finish on rails and bottom…may be one is in the resources I dont remember

7

What is “Speedfinish”?

8

Hey DS.

Craftee is right, the dimples are introduced to generate turbulent flow, which seperates later from the ball, reducing drag.

But i agree about the pressures around the ball, and the backspin generating pressures.

I wrote a paper on the Hydrodynamics of surfboard fins.

If you follow this link http://www.iinet.net.au/~livanos/CFD_report/ you can score a copy of the report.

Under the hydrodynamics section, i’ve talked about drag and other forces. Have a read, let me know what you guys think.

Point being, is that in terms of drag there are more components than just skin drag. But in terms of skin drag, one major idea that hasn’t been mentioned ( i think Dougirwin was sorta on the right track with momentum) is the fact that skin drag (smoothness or roughness) depends largely on the fluid (air or water) viscosity.

This makes the comparison between skins on aircraft, and skins on watercraft quite difficult… although some properties are the same. For example, a smoothed skin on the aircraft reduces the drag, allowing it to take much longer to slow down. However the yachts in the americas cup, introduced a rougher skin on their yachts to effectively hold a boundary layer of water close to the skin of the yacht, and since water / water drag is less than water / boat surface the yacht had a relatively small decrease in drag (1-5%) i cant remember exactly… but if you think about it, 1-5% drag over such long distances as in the america’s cup equivilates to a large distance.

Now just throwing this out there… since the amount of board surface contact is lets say around 2/3rds of the bottom and some of the rails… and with such interupted flow (turns, bumps, lumps in the wave surface so a lot of air pockets are flowing over the board)… does a 1-5% increase in skin friction really make that much over a difference over 50-100 metres?

Interestingly, the army has created a torpedo, with a blunt head, which moves so fast, that the blunt head pushes air out and over the body, effectively enveloping the torpedo in an air bubble, and since the friction between air and water is much less… the torp is pretty much ‘flying’ through the water at incredibly fast speeds. So, how can we get surfboards to “float” on the wave? Already been done in a way - hydrofoils.

L

9

Have you guys seen this?

Greg Griffins Air Board

it uses a cowl effect to suck air from the deck to the hull underneath. It’s very much debated but a damn cool idea if it worked. Gliding on air.

Cheers,

Rio

10

I think i have seen something similar about sucking air under the board… I like the idea, makes sense, probably does reduce drag. But also you have to look at what you loose… say control.

In that photo, i guess its the metal plate doing the job… but right infront of the fins??

Having too much air and no water contact running past your fins would reduce control and drive badly…

Everything is a balance.

The more you learn, the more you realise you dont know.

L

11
Quote:

I don’t know if this would be applicable, but I saw a test a few years back that demonstrated how texture made an object drop/move through water. I don’t remember who was doing the testing, but what they found through the use of high speed photography was that a ball (a bowling ball) that had a 2” or 3” patch of sand paper attached to the bottom, the side to hit the water first, would let the water wrap around the ball much like you see in the golf ball dimple tests. You could see the laminar flow of water around the ball with very little separation which I believe equated to less drag as the ball moved through he water. The smooth ball would push more water out of the way and create more turbulence/ drag. I have no idea how that would apply to a surfboard, but thought I would throw it out here anyway. Someone might be able to make use of it. The conclusion of the discussion was that adding a bit of grit ( in the form of metal flake) to the shear line of the of the boat could make the boat slide into the water better giving more control, depth, and length to the ride by reducing turbulance and drag.

I saw this on a kayak message board in a discussion about how to make a squirt boat flow and sink better into the water where you want the water to flow around the boat with as little resistance as possible (ie. to maintain laminar flow to reduce drag) and use a “wing affect” to take your boat under water using the AOA of the deck and hull with the current for control. If you want to see that at work check out the link in my signature block. It’s a video of a person doing a mystery move about 5’ deep into a river “seam” and doing a triple loop as he disappears out of site. Very cool and difficult to execute.

Hafte

Hafte:

That’s a pretty good empirical test, and it doesn’t seem like it should be extremely difficult or expensive to emulate. I’m visualizing using the “cannonballs” used in fishing downriggers. Uniform size and weight, cheap enough, and fairly easy to coat with a variety of finishes. A real “quick and dirty” could be a variation of the “Galileo test”: drop identical balls with different finishes through 12’ of water simultaneously; measure how long each takes to hit bottom. A possible follow-up might be to put some kind of “tail” on each ball to prevent rotation, and compare results to the first test. Do you have any pointers to more information about this test, or recall any more of the particulars?

-Samiam

12
Quote:

…

This makes the comparison between skins on aircraft, and skins on watercraft quite difficult… although some properties are the same. For example, a smoothed skin on the aircraft reduces the drag, allowing it to take much longer to slow down. However the yachts in the americas cup, introduced a rougher skin on their yachts to effectively hold a boundary layer of water close to the skin of the yacht, and since water / water drag is less than water / boat surface the yacht had a relatively small decrease in drag (1-5%) i cant remember exactly… but if you think about it, 1-5% drag over such long distances as in the america’s cup equivilates to a large distance…

L

Lavz:

I’m always skeptical of comparisons between aerodynamics and applied hydrodynamics for another reason. Air is compressible, water is not. Perhaps this does not materially influnce the results, but I have never seen a theoretical or practical demonstration indicating that it does not. Until I do, I must assume that it is significant.

-Samiam

13

Hey Samiam,

I totally agree with there being significant differences and behaviours of air / water, making comparisons hard.

Yet as always, there is some similarities, they are both fluids, and consequently behave in similar manners. So some stuff will be the same.

In relation to specific examples and so on, it varies from example to example and its much more complex than just guesstimations.

L

14

Samiam, its been too long ago. I remember a couple of pictures showing the ball as it entered the water, and how a “film” of air clung to and wraped around the ball with the textured end. The message board I read this on does not archive. After a month or two old posts just fall off the bottom (for lack of a better way to put it).

All that I can recall is that the tests were done in what looked like a shallow tank. I don’t call if there was a article attached to the pictures giving more detail. It was available on the internet. Maybe a search would reviel more info.

Hafte

15

An aeronautical engineer freind made a comment during one of our discussions that the behavior of high speed air, and low speed water was VERY similar. An observation worth considering.

16

shouldn’t it be more a discussion about hard(stiff) versus soft(pliable) relationships between opposing forces?

air injection(Brewer(the owl board)/Morey/Griffin/Alexander) are designed to physically seperate the board from water contact.

Pliable surfaces with core flex (porpoise skin studies/Surflight/Sunova) are designed to allow the water to travel without resistance where it naturally wants to go by removing the conflicting relationship between the hull and the water allowing you to basically tap into and follow the lines of energy already created with the formation of the water energy into something called a breaking wave. Soft accomodates the energy flow while flex snapback mimics muscle memory tapping into the energy provided by the natural flow, kind of like a power dam. George’s flexspoons. Dales mats, and Tom’s sponge have demonstrated the natural speed available from taking advantage of the energy versus fighting it. Kind of like the indigenous peoples versus the enlightened peoples way of thinking of how to deal with nature.

The effects of the appearance the thickness of the skin layer presents of something like a surfboard seems inconsequential compare to the actual shape and how that effect how the object presents itself to the various ways the water wants to move as it forms into a breaking a wave in which goes from lateral to circular forces.

I think a good aerospace comparison would be to take the worlds fastest fancy hi-tech fighter jet without it’s engine and a hang glider or glider and drop both off a cliff and see how they tap into and manage the energy in the natural flow of air to to translate that into movement.

17

I hunt ducks in the marshes of the Sacramento delta. Me & all the guys I hunt with drive flat-bottomed boats out the sloughs & into the ponds. A duck boat is about as flat as a boat can be - 12’ long, 4’ wide (square) with a 16" transom and about 12" of nose rocker :slight_smile: Not all that different from Manoa & Uncle D’s fish boards, really, just more weight capacity and designed to keep a couple guys and a dog out of the water.

Over the last couple years, most of the guys have had the bottoms of their boats sprayed with truck bed liner. Line-X is the most popular, but probably because there’s a camper shop nearby that does it on Saturdays while you wait. I don’t really know where the idea started, but most guys began by doing it because the algae & barnacles won’t grow on it (we’re in brackish water).

But get this - the guys who’ve done it are using a lot less fuel than the rest of us. My friend Billy lives on the marsh and makes a few bucks by fueling up everyone’s cans on Friday evenings, ready for Saturday morning. He fuels us all up every week…and knows who’s been out hunting. He says fuel consumption is about a 60% with the spray on, than it is without. I don’t hunt often enough these days to make the $300 spray pay for itself, but lots of the guys do. We all run between 7 & 10 hp 4-strokes.

Some guys say their boats are faster with the liner too, and that could be, but I haven’t driven a boat first without & next with it, so I couldn’t say. But I think the fuel comsumption says something interesting about adding texture…

18

3M makes an adhesive-backed film that, under a microscope, looks like the “hook” part of their Velcro. There are tiny hairs molded onto the film. This stuff was developed to coat the bottoms of America’s cup boats. The idea is to break up the surface tension of the water and allow a water-to-water interaction istead of water-to-sticky-surface deal. The fillm feels like sharkskin and is really expensive.

I don’t know how well it works - I just know they make it, and they say it works.

19

What I have always thought about is that the world’s fastest fish aren’t perfectly smooth. Sharks etc, thier skin is rough in a way.

20

Actually…

Personally I used to religiously use the Surfco “Ultra Glide” product below till they discontinued it. I believe the original formula was a invented by Paul Strauch.

http://www.surfdirect.co.uk/shopping/shopexd.asp?id=25

I now use this religiously to keep not only keep my bottoms clean from dirt and wax but also to restore the seal of the gel coat on the outside of the board. And to reduce UV induced yellowing.

http://www.303products.com/tech/index.cfm?fuseaction=product.display&Product_ID=492

It shouldn’t matter I guess

But it does seem to make a minor difference in my mind using this stuff

probably not good for the environment though…

I guess no better than the early 70’s when I was spraying on silicone lubricant and WD40 as well as Morey’s alkaseltzer crushed into a powder mixed with a tad of leftover resin thinned with styrene and acetone and brushed on the bottom of your board and then sanded a bit once you paddled out. Even waxed the bottom of my board with a bar of soap(another morey suggestion). Once even painted on a thin 1/16" coat of clear silicon sealant all over then bottom of my board as well as glued on that finely ribbed rubberpad some medicineman was selling back then.

30+ years later I don’t know if I’m a better surfer because of it…

same thing as the endless search for the ultimate diet or fountain of youth I guess…