Rough vs Smooth which Bottom is better?

General Discussion
1

Ozzy and I were having a PM discussion about bottom finishes and thought you guys might want to join in. Ozzy mentioned something about air bubbles being trapped in the texture and grooves.

BILL SAID - "I remember an ASR trade show 10 or more years back when everyone had super fuzzy sprayed finishes and the push had come up from Australia and everyone was calling them “speed finishes” and promoting them as better, faster, etc. Thinking back to sanded finishes I could understand where some would (wrongly) think they were faster from sailboat examples. But the BUBBLES thing mystifies me. I have no idea where the Bubbles idea came from. I don’t think there is any emperical evidence for this.

OZZY SAID - "The way it was described to me many, many moons ago (15 yrs), is the sanded finish creates or holds tiny air bubbles on the bottom of the board in the sanding grit scratches from the sanded finish, creating some sort of air cushion for a boundary layer and making your board faster because it had that air cushion under it instead of just water. We even discussed the grit of finishes. Rougher was better is what was told to me. 80 - 120 grit is sooooo gnarly duuuuude! Yep. I believed it 'cause I didn’t know any better. Well, that’s my story and I’m sticking to it. hahaha I think out of that whole thinking came the dimple bottom boards, venturi hulls, etc. Trying to take the air cushion thing to it’s extreme. But I do think that in time some of those extremes will run their course, and very subtle forms of them or something similar will be very useful as subtle refinements like the concaves and stuff we use today. But as usual, I could be wrong. REAL wrong if I say it right. haha "

What do you guys think. Especially you old timers ha! who go back far enough to remember where these design theories and rumors starter.

2

Sailboat technology is definitely not the entire picture, that’s for sure. Most boats are displacing and bending water flow but surfboards are (in part) skimmers. Our crafts are between two media, the water and air, and this presents a special challenge.

I see the bottom as having zones. Some places we need attachment of flow for stability and control while others promote dynamic lift only and improper attachment yields friction. I got some info years ago on US Navy studies on skimming devices.

Basically, we must realize that the surfboard has a boundary layer. When water flows smoothly across the bottom of the board there is a thin layer of water that travels with the board as it goes across the wave face. (If you are going 20 mph across the wave, believe it or not, there is some water coming along with you at 20 mph).

I did some basic calculations and found the layer to be as thick as 3/4". This layer has a flow regime which varies in speed (relative to distance from the board). Right next to the board the flow speed is near zero, and as one moves the reference point away from the board the speed gets closer to the vehicle speed.

The importance of having a smooth surface, as you’ve hinted, becomes dependent upon where we want attached flow. Also, the opposite case, where we do not want attachment (yet avoid the killer turbulent flow) must also be decided. Once zones are understood and defined, then surfaces can be prepared accordingly.

Lastly, there is the case where reduction in the thickness of the boundary layer becomes beneficial. Surface textures and objects like “turbulators” will thin out the transitional flow and reduce friction by maintaining attachment and avoiding turbulence.

So, the real question may not be if we want an entire bottom to be one style of finish, rather what texture goes where…

PS. I’ve always enjoyed your blank designs from way back Bill, and want to thank you for those as I made some real gems from them;)

3

Thanks for weighing in George.

Good stuff you have shared, from it I know you understand the function of these different surfaces and their effect on the boundary layer. All good stuff which will no doubt get expanded on by others here. But…

What about this air bubbles imedded in the texture thing. Ever heard of it and where it came from?

Thanks for the nice compliment about my blanks. Blanks at that time were terrible. Too thick no rocker. Mine were the first ones to delve into the “close to shape” zone. I may have mentioned it before, if so, sorry. But the 7’9" was my first one and They wouldn’t mold it for a long time as they were affraid it was too thin! Ha!

Keep it coming guys and gals…(are there girls here? I hope so!)

4

Aloha! I learned a lesson years ago about the difference between a polished gloss, and a sanded surface. After riding a polished board for long enough to get it wired, I wet sanded it back down to 400. The difference was instantly obvious. Where the water on the polished surface would bead up, the wet sanded finish would hold the water in a thin film. The film of water seemed to eliminate the friction and make the board feel slick and lively. Other grit finishes seemed to have more or less the same effect, but 400 felt to me to be the most ‘lubricated’. Aloha…RH

5

Hey Bill, hey Rick,

I think the broad concept to the air attachment to the board surface has a technical name called, “Free Surface Aeration”. I am short on time at the moment but a lot of research is being done right now in the field of Hydrodynamics with regard to pipe and spillway flow. There is a commercial push to be able to flow more liquid through pipes (eg. oil) and spillways (eg. irrigation) by introducing air/gases into the liquid flow by self-induced means and applied means.

I hate to say it but the ‘Russians are ahead of us’ when it comes to applying these ideas to water vehicles (see Krylov Shipbuilding and Research Institute). They are using gas injectors and trapping compartments to reduce friction. Of course this stuff has been tried for over 100 years and really the most simple solution for surfboards will be the best. I really don’t see some sort of James Bond pressurized gas cell coming to the rescue.

Rick also points out that general skin textures can “feel” faster based upon empirical methods. The direction of sanding should be explored as some sort of alternating rail-to-rail “X” pattern should keep flow attached along the bottom of our boards. (Kind of like a basket weave, over and under look). The texture could be made to cause bubbles to attach in some large areas of lift and other textures could keep control at a low drag penalty. The correct angle(s) for this pattern would be critical

I believe identifying the zones where control is important versus large supporting lift areas must be defined, then some clever surface texture solutions could be tested. Fins are also subject to this analysis, as well as their interaction effects with surrounding surfaces.

If just a few “small” surface changes are made in the right places and in the right ways, then I believe a surfboard could be made with improvements to speed and control which would be significant…

6

Bill,

Did you ever glass any of the Willis Bros boards? I had an 8’6" gun PHAZER with the tennis ball size indents in the last 1/3 of the bottom tail. This is a roughened bottom on a gross scale. I really couldn’t tell the difference to be honest. My phazers took on pressures dings in the phazers I think cuz the indents got into the lesser dense PU foam.

Also I recall this wipe on towelette of environmentally safe slickery stuff to increase waterflow efficiency on surfboards/boats. The kayakers swore by the stuff.

isn’t the sanded finish pretty much standard for the majority of boards these days? Has anyone ordered a polished glossed bottom for hydrodynamic reasons??

I like to see what’s on the bottom of water skis and tow boards. I think the single concave plays more of a role than the bottom finish.

I am sure the boat racers have it figured out also. When I saw on TV Lance Armstrong in the wind tunnel to determine where to put the dimples on his lycra body suit to increase in efficiency by 7%. PlusOneShaper is probably right on. He just needs to tell me where exactly to put my rough bottom so I can improve during the tour de Jacko’s this winter.

7

I definitely had the same experience as you RH. And from discussions with many people over many years, the trapping of water against the surface reduces the boudary layer as you describe.

As far as air being trapped, the closest thing I’ve seen, not used on surfboards, is the swimsuits the olympic swimmers are using. That’s just my observation on tv, slow-mo etc.

I once glassed a board and only sanded the deck and rails, including about 10mm onto the bottom to get edges. The bottom was left like we used to do textured decks, hot coat squeegeed off. It went really fast with no apparent drawbacks.

8

im with rick and wildy on this one…a hydrophillic finish with a thin film of water attached is the slickest…if the water beads or wiks off fast on the bottom it goes to the workshop for rework…

strangely enough, you see so many guys with all kinds of crap on the bottom of their board…wax, scratches etc etc…they seem to be the same guys struggling for waves and speed…i never let anything touch my bottoms and try to minimize hand prints…kind of OCD on this one…need help!

9

this one’s been in the fav’s for a while now…

http://www.coachesinfo.com/category/swimming/146/

oh and regarding sanding direction…i take mine to 600 and i follow the same line as my front fin toein angles on each side of the board…altho the X pattern makes sense too…X patterns is what’s used in cylinder wall honing of piston engines…

and a quick comment on the thread’s title…

rough v smooth doesnt seem fair…howbout

ultrafine sanded v polished/glossy

or

hydrophillic v hydrophobic (what!?!)

PS - i emergency surfed a board once with 100 grit bottom and it felt like i was draggin an anchor

10

Top end slalom water skis currently all appear to be sanded down to about a 400 grit surface or finer with a linear pattern parallel to the length of the ski. The finishes are very smoothe, but not glossy. There was some experimentation in the past with different bottom textures, but it was mostly to see if it could help the ski decelerate coming into a turn.

The bottom texture of alpine racing skis is a very big deal now and has been studied quite a bit to determine which “grind” is fastest. The grind that works best for warm, wet snow (closest to actual water) is a cross-hatched pattern that’s fairly coarse (to the eye it looks pretty fine though unless your used to looking at a lot of different ski bases prepped for different snow conditions). The purpose of the coarse cross-hatching is to break surface tension (suction) with the water in the snow. Seems like something similar would work on a surfboard bottom, too.

11

The windsurfers are having this same discussion http://www.star-board.com/forum/starbulletin/read.asp?ID=5468

12

I don’t want to diminish the value of this discussion. I’m enjoying it a lot 'cause I’ve been watching all these trends since the early 80’s. But, there’s a fashion spreaded outside with guys painting their boards with all kind of stuff, mainly on the bottom of their boards. And I can’t see they strugling to get speed and control, on the contrary, they’re ripping and breaking rules. So, what’s the matter. Is it speed our real problem? It was told me once that in surfboard riding speed is something more related to be engaged with the wave, and not ahead of the maneuver zone. So, it’s more related to control than speed (or high speed). My experience showed me that in the great majority of the cases, when somebody complains about speed, it generally is related to the balance of different features of the board (fins setup, rocker, rails, foil, etc). How many guys do you know who usually spank their boards on the bottom and follow surfing without problems because of the “multiphasered” bottoms.

13

The purpose of the coarse cross-hatching is to break surface tension (suction) with the water in the snow. Seems like something similar would work on a surfboard bottom, too.

thank you…thats good relevant information and jives with my earlier point…water beads are an obvious dead giveaway of too much water tension and a clear signal of a hydrophobic surface…no beads for me thanks…

hey Jeff, i suppose adding skin friction is an easy way to add some control to a too slippery surfboard (good travel trick) but it wouldnt be my first choice…funny, but when i said it felt like i was draggin an anchor i was referring to paddling! theres more to it than just on-wave performance…

“Theory + Experience = Magic”

hmm…

“theory + application + LUCK! = majic”

14

I can only talk from my experience , In my last 2 long boards I have had both polished and protec, the protec is a lot faster however it may not be from the finish but the weight compared to the polish.

There have been many so called speed finishes in Australia, one that springs to mind is shiny seal, had it on a board in the late 80’s couldn’t tell the difference.

A rougher texture should break surface tension so be faster,maybe, sorta kinda.

15

Prior to making plastic pieces, I ran a boat yard. We specialized in sled race bottoms. I worked with Allen Andrews, Bill Lee, Jim Reichel and Gino Morelli on surface finishes and appendage transition zones. I still race aboard some of the top boats on the west coast. Earlier this month I was prepping Pyewacket for Orange Coast College, now that Roy Disney is donating it.

Anyway we had a schedule of sanded finishes that worked best at different intended maxium velocities. The minimum grit was 400 and for the really big “fast boats” would bring certain areas down to 1200 grit. But, it was always very important to understand what mode you are optimizing for. When we are paddling we’re in displacement mode and minimizing turbulence is huge for ease and efficiency of getting up to hull speed. But, when we are surfing we’ve transition into planing mode. And, under certain circumbstances a small amount of turbulence or features like turbulators may help trip a flow up onto plain slightly earlier.

Last week Bill was talking about how roughthe surface can be surfing the North Shore and how much chatter a Southern California style surfboard can experience. With the extrapower and surface texture, tripping a board up on to plane is probably a good thing. But, if your into ease and glide go nuts with the super fine sanding.

16

Okay, so on the latest board, which will see it’s first water tomorrow, I sanded 400-1500, wet. Water sheeted and stuck nicely. Then went throug the Macguire 3-step. Washed off with soap and still have mostly sheeting. Applied the Teflon boat polish +1 and Wildy referred to in the “high-performance” post; beads like a Mofo. I’m planning on taking a sheet of 1500 with me in case I run in to skatey problems, but here’s your chance to weigh in on what I’ll experience with the ultra-repellent finish before I can report back with results.

Meecrafty; none of the hydrophilic stuff in Vancouver after much time on the phone yesterday. Looks like it’s another “mail-order from the states” candidate…reluctant…my last shipment pretty much doubled in price after working in duties and a customs broker. OUCH.

17

Interesting questions, guys. I’ve been pondering similar things for quite a while now, especially after learning when I was a kid obsessed with R/C model aircraft design that a fabric covered surface, as in old pre-WW2 aircraft, actually suffers from less drag than a modern smooth-skinned, fully metal-covered surface due to the tiny surface variation in the fabric breaking up the airflow. I know that the same ideas have been applied to surfboards, but what I wonder about the whole question is if surfboards, with the exception of tow-in boards, ever achieve enough speed to really make any difference noticable. I can see that windsurfers or kitesurfers might get enough sustained speed going to really be able to tell, but I would think that it would take a large, powerful wave to allow for enough constant speed and flow over the planing surface for any real difference to be noticeable, and there would be very little difference in day-to-day riding, unless you live on the North Shore or somewhere where large, powerful waves are the norm. Then again, I’ve never tried it out myself, so who knows?

18

I also wonder about the effect of a chord flapping around or the random toe too close to the rail, or that stylish drag of the hand in the face, or the little salty scabs that build up on the bottom that don’t completely dissolve. Too many variables in the whole act of surfing to get too worked up over what; an additional 0.5 ft/sec of maximum speed. On the other hand, back in the 60’s, shaving down for a swim meet was thought to be more a psychological boost that physical. Fun topic though.

19

My $0.02 in here too… I’m not a great surfer, and I can’t really contribute too much here, but take it for what it’s worth. On bottom finishes, I’d stick with a polish. I use a product called Race Glaze. Water beads up and falls off real slick. My rationale behind this is what I’ve learned from the kayakers(my little bro used to race) in my area. Where I live, we have a 2x world champion kayaker and about 3 members of Team Canada. They are all into racing slalom kayaks. These have to perform in virtually the same conditions as any board(some slick unbroken water and some foamy messy sections.) The boats also ride for the most part on top of the water and a have a hull made up of the same materials as surfboards (carbon fiber and epoxy). If this is what they all swear by, and claim to be able to feel a difference in the ease of achieving speed. if it works for them, then it likeyl does the same on a board.

Corran… what do you use? Same stuff? or regular bottom?

for what it’s worth…

20

Problem with using yaks or boats is that thems vehicles, the rider doesn’t shift his weight to affect surface area!

Surfers do, like trim, lean back, forwards, unweight.

That’s why no one knows! Just like in wndsurfing, no one knows!

Didja foresee that one?