I`ve been a believer for a long time... w/400 wet and dry using a linear pattern, back and forth between nose and tail. What I`ve done is to wet sand until the point at which water on the board`s bottom surface begins to form a sheet... an even, thin coating... almost slimy feeling to the touch. There is a difference.
I didn't ask permission from Greg Liddle to post this from his website - I don't think he would mind... you may not agree but there it is. Wet sanding I will always suggest that even the finishing bottom be wet sanded with #320 wet and dry sandpaper, using water and sanding in same direction as the stringer from nose to tail. The sanding should be done initially and periodically to create better laminar flow as the hull pulls itself through the water. I would suggest this to anyone wanting to maximize sensation and control.......it does really feel different and I believe goes quicker than a finish coated and sanded bottom. http://www.liddlesurfboards.com/index.html
John, Thank you! Something between 320 and 400 wet & dry would be ideal for this purpose. I`ve often tried using a worn 320 or a fresh 400 to approximate the effect. Note Greg`s comment about a "hull pulling itself through the water." The accelerating, "pulling" sensation of a good displacement hull across a unfolding wall is absolutely unforgettable!
Anon wrote: >The difference here is the grooves attempt to *channel* the laminar flow, not >trip it as is the case with dimples, which might put a hole in the theory of >the random-swirl sanded finish. If you do the final sanding straight from nose to tail wouldn't that be pretty close? regards, Håvard
I know everyone has their own opinion on this, and for good reason. It's hard to refute someones' personal experiences and trials. After all, I would agree that when running your hand over a wet surface that is glossed, it feels sort of *stickier* than that of a wet sanded surface. The wet sanded surface feels silky and sort of slimy. But I guess that's just because the surface can easily retain the water, whereas the gloss surface repels it. Who knows, maybe the *trapping* of this boundary water is the key to the performance of the wet sanded finish. One thing to keep in mind though. If comparing a sanded finish to a gloss, it really wouldn't be fair to compare a 7 pound sanded finish board to a 8 pound gloss finish board. A 15% difference in weight is a HUGE difference, and such a performance comparison just wouldn't be accurate. Ride a glossed board for a while, then sand it. That'd do the trick.
A perspective from avid sailor and engineer from Lockheed Martin. BTW, he sands his hull similar to what has been described above. One of surfing's never ending questions. One thing for sure is that no gloss coats sure do get dirty in the showroom. I've seen shopowners regularly Windexing boards (usually molded epoxy ones) in order to move the things out the door. To Wax, or Not to Wax An Engineering Perspective By Christopher VanEpps Which is faster? A waxed/polished hull, or a wet-sanded one? This is a question which “surfaces” (ha, ha) on a regular basis and quite often a wave of psuedo-science based debate swells and threatens to capsize scientific reason. While I don’t purport to be the Moses who will read from the tablets of speed and end all the arguments, I have, at least, seen the burning bush of science and would like to share my understanding with the reader. What follows are some basic aero/hydrodynamic principles and my observations based on same. I would like to thank Bill Mattson, Mike Fahle, Sonny Barber, Mark Michaelsen and several others (my apologies to those I haven’t named) on the Hobie Mailing List, for keen insights and anecdotes they’ve shared with me and I’ve paraphrased here, as well. They are, as a group, far more experienced sailors than I. Many thanks also, to Brenda Carpenter, MS in Aeronautical Engineering from MIT, for technical editing and BS detection. My apologies to the Mailing List subscribers, for many of them have already seen this information, in one form or another. Those who’d like a refresher and those of you with first-time curiosity, read on. Directions for accessing and subscribing to the Hobie Mailing List may be found elsewhere in OTW. The real answers can, perhaps, be found in the world of fluid dynamics and a discussion of laminar vs. turbulent flow and the associated boundary layer. Since an in depth study of this is about as fun as a root canal and so dry one must be hooked up to an IV just to read a text, I will take my references, in this first section, from a fabulous book entitled “The Illustrated Guide to Aerodynamics” by H. C. Smith, 2nd Edition. This book reads more like a novel than a text and one doesn’t need a mastery of calculus and hieroglyphics to attain enlightenment. The following rules apply whether a fluid (air or water) is passing around a surface (sail or hull), or the surface passes through a static fluid. In essence, laminar flow occurs when a fluid flows over a surface in a smooth, layered fashion, in which the streamlines all remain in the same relative position with respect to each other. One must observe the phenomena of skin friction and boundary layers to understand flow. The viscous nature of air or water causes it to "stick" to the surface over which it flows; thus the velocity directly on the surface is zero for any velocity of the main air or water stream. Put into our terms, as our hull speeds through static water (water with no velocity) at 10 kts, the water molecule right next to our hull “sticks” to the hull and is “dragged” along at the same 10 kts. Proceeding above the surface, the velocity gradually builds up to free stream velocity (the velocity of the stream if the surface wasn’t present at all) at some distance above the surface. In our sailing case, where it is the surface that’s moving through the fluid, as one looks at molecules of water in increasing increments of distance from the hull, they gradually go from 10 kts, to 0 kts.. This area between the surface and the point where velocity reaches that of the free air stream is called the boundary layer. The reaction to the retardation of the flow velocity within the boundary layer is called skin friction drag. The thicker the boundary, the more drag. A turbulent boundary layer is thicker than the laminar. Turbulent flow is marked by streamlines that break up and become all intermingled, moving in a random, irregular pattern. Laminar flow goes through a transition region before becoming turbulent. In terms of efficiency/speed: Laminar = good, Turbulent = bad (in *most* cases, but not all). This transformation in flow can be seen in the smoke rising from a cigarette in calm air. The smoke rises initially in a laminar manner. Then, as it encounters the friction of passing through the surrounding air, it transitions to a turbulent flow. A scientist named Osborne Reynolds found that whether a boundary layer was laminar or turbulent depended on the fluid velocity, the distance downstream, and the fluid’s kinematic viscosity. The Reynolds number (Re)= ((fluid velocity*distance from leading edge)/kinematic viscosity) and is used to describe the viscous qualities of a fluid-surface interface. At low Re the flow is laminar and a high Re indicates turbulence. The point at which a laminar flow turns turbulent can be referred to as the Critical Re. In aircraft, since there is a change in Reynolds Number at each location on the wing as one heads down stream from the leading edge, it is customary to use a “characteristic” representative length from which to calculate the number. This keeps us out of calculus. The Reynolds numbers in most sailing applications however, (sail/air; foils/water) are orders of magnitude lower than those associated with aircraft. This is significant. It is important to note, that either an increase in speed, or, more importantly, a significant distance from the leading edge (bow of your boat) can greatly increase the Reynolds Number. It is also interesting to note that a sailboat presents a rather unique aerodynamic scenario, in that it has 3-part boundary along the water line. An airplane wing only has to worry about the wing-air interface. A submarine only has to worry about the hull-water interface. A boat on the surface, however, has to deal with both the hull-water and the hull-air interfaces. This gets tricky right at the waterline and it has been shown that the hull will literally drag air molecules below the surface of the water, against the hull, breaking up flow. This can be seen in the extreme example of the Hobie Tri- foiler where, at 35-40kts, there is so much air being sucked down the foils that the flow starts to cavitate. Cavitation is the rupture of a liquid or liquid- solid interface caused by reduction of local static pressure. Basically, water at the surface saturated with air molecules and other impurities form little nuclei and when the local pressure around them drops drastically, such as when a foil is dragged by quickly, they expand until equilibrium then collapse. Many factors determine the amount of energy released, but in certain cases it is explosive and can pit and damage hydrofoils and propellers, even those made from stainless and titanium. The Tri-foiler has the sail plan for power to go much faster than 35-40kts, but they haven’t solved the problem of foil cavitation. Putting vertical fences on the surfaces helps, but doesn’t eliminate it. Luckily for Mr. Ketterman when cavitation does occur, section friction drag increases and lift decreases drastically, reducing speed and acting like a natural governor. Lucky also for us, our catamarans are traveling slow enough to not have to worry about hull cavitation, although we may get some pressure drag from small amounts of cavitation in our wakes. Still, it bears remembering that for a some vertical distance along the hull surface below the water line, the immediate substance touching the hull is air, not water. This distance will increase proportionally with hull velocity. In an unrelated, but hopefully interesting side note: If one looks at the air-sail interface, one must also consider the sail’s aspect ratio. Aspect Ratio (AR) is defined as the span divided by the chord of a sail/wing/foil. For a tapered sail or foil AR may be determined by dividing the square of the span by the whole sail area. While total drag has many components (parasitic, induced, friction, etc) and total lift relates more to angle of attack than aspect ratio, it may be generalized that a higher AR sail/wing/foil of the same area will generate the same lift with a considerable reduction in vortex-induced drag. Vortex-induced drag is caused by downwash (sideways wash in sails) causing the air stream to deflect at a different angle than the oncoming air. The lift vector actually gets tilted backwards and that component of lift, in the direction of the airflow, is induced drag. This happens most pronounced at the tips of foils. This is why a lot of the more high performance rigs are specifying square-topped mains. If you let your leach curl in to windward, you're killing your self here!. Unfortunately the longer span of a High AR rig places the loads farther out (or up in sails), resulting in greater bending moments. These must be countered by heavier support structures that add weight and drag of their own. One rapidly reaches equilibrium and thus the 218ft^2 of H16 canvas isn't a single sail 1 foot wide by 218' tall :-). Drag Queens are generally heavy, don't know much about sailing and their high heeled shoes can poke through your tramp. It’s best to minimize this type of drag on your Hobie as well. But I digress. Okay, so how does all this relate to and solve the original conundrum? Perhaps it would help to debunk some of the psuedo-science myths that people use when defending wet-sanding. I’ve heard the golf ball theory used. They say that a dimpled golf ball travels farther than a smooth one. They’re right. They then relate this to the rougher surface caused by the sandpaper, as compared to the smoothness of the wax/polished surface and claim this is faster. They’re wrong. A golf ball is spinning in an airstream caused by its forward motion. A perfectly smooth ball would suffer flow separation very early around its surface, a large wake and subsequently large pressure drag. Remember that Parasitic Drag = skin friction drag + pressure drag. A smooth ball has low skin friction drag, but really high pressure drag, because even though the flow is laminar, it separates from the ball very early. Now, if you put dimples on the ball to roughen the surface, the turns turbulent and the resulting higher energy flow can stay attached to the ball longer, delaying separation, making a smaller wake and reducing the pressure drag. You have traded off the increased skin friction drag against an order of magnitude drop in pressure drag. Thus, the total drag drops and your drive goes farther. Spheres (golf balls) are very special cases, from an aerodynamic point of view. See Figure 1 & Figure 2, below. Figure 1. Golf Ball Aerodynamics Figure 2 . Basic Drag Equation Incidentally, tennis balls are fuzzy for the same reasons golf balls are dimpled. I can also explain how the stitches on a baseball are necessary to make them curve, some other time, if you wish. Don’t even get me started on a Whiffle™ ball’s aerodynamics. That’s really scary. Great. Now, here’s the difference between the golf ball and your boat and it’s subtle, so try to stay with me. You’re not sailing a spinning golf ball through air at 120 mph. You’re dragging a cigar shaped thing through water at 10-20. If you guess that this makes a difference in the total drag picture, give yourself a cigar. You’d need some pretty big dimples ?. At the speeds your hull(s) is(are) traveling combined with the “roughness” caused by 600-1000 grit sandpaper, you aren’t getting squat for lift and even if you did it’d be in the vertical plane and wouldn’t help forward velocity unless it caused a planing situation, which it won’t. (No this lift would not help you to windward!). All you’re getting is the parasitic and induced drag components. The pressure drag component of the equation is small enough compared to the other components as to be ignored. Airplane wings would be completely smooth, waxed and all, in an ideal world. However, engineers put turbulators, vortex generators and other devices on them to "roughen the surface" of the wing. Why? Not to reduce drag and make them faster. It’s to increase lift, or improve low-speed stall characteristics and/or to try to reattach turbulent flow before it completely detaches. Any roughening of the surface increases parasitic drag and decreases top speed potential in a foil type surface. The engineers just put up with this cause they have to. Keep in mind all of this theory is in reference to a particular body's Reynolds Number. A Hobie Cat will be operating at a very low average Reynolds Number, so allot of this talk is rendered moot. Just because one has a low Re doesn’t mean one experiences laminar flow. Thus the importance of determining the critical Re, which is no small feat. And all talk of fluid dynamics pivot on the viscosity of the fluid involved and water and air aren’t even close in viscosity. The bottom line is whatever one has to do to improve the flight of spinning golf ball really has no bearing on a Hobie hull(s) being dragged through the water by its sails. Some point to the fact that water beads on a waxed surface, while exposed to the air, and this is due (correctly) to a phenomena called “surface tension”. They drown in the shallow end of logic pool by thinking (incorrectly) that this “tension” must “pull” on the submerged hull, slowing the boat down. I must pause to laugh here. If this were actually true, wouldn’t it hold that if you gave that same tenacious water thousands of little scratches to hold onto, it could pull that much harder? Motor oil will bead on a waxed surface and I’ll bet that’s pretty slippery too. Water has a surface tension property, but it’s dependent on the interface. Surface tension may also be broken, as you may be demonstrated by observing the meniscus disappear from a thin column of water, in a graduated cylinder, by adding a drop of liquid detergent. Hmmm, did I say surface tension can be broken “chemically”. Think about that. Discuss amongst yourselves. Luckily for us, what the air/water interface is doing has nothing to do with how it reacts, submerged at the hull/water interface. Not to mention that static and dynamic states have markedly different characteristics and we only care about dynamic flow. Same equations, but drastically different values. Water and air boundaries are completely different. Different densities, different viscosities, just different. The fact that water is beading in the air has no relevance to the discussion. Why do you think your daggerboards, used to balance forces are so much smaller than your sails? Different interface and mediums. The difference in drag, along a 16-20 foot hull, between a wet sanded hull and a waxed hull has never been experimentally confirmed, to my knowledge. That's how minute is the difference. Until someone drags a 600 grit sanded hull through a tank of water, with transducers attached to measure drag, then drags the same hull after waxing and proves a significant drag increase, I must insist that the lower drag will be attained with the waxed (smooth) hull. Frank Bethwaite, on page 263 of his brilliant book “HIGH PERFORMANCE SAILING”, states "...at practical yacht or dinghy speeds, only the bow area of the hull can hope to run with a laminar boundary layer. Under this area the surface should certainly be highly polished. But beyond this zone the flow will become turbulent (remember the Reynolds Number equation and the relation to distance behind the leading edge/bow) and under turbulent flow a highly polished surface will not be any faster than some rougher surface, provided always that the roughness is less than some small fraction of the boundary layer thickness." Central to this is that roughness. I believe the boundary layer thickness to be extremely thin at the hull water interface and while Mr. Bethwaite does not concretely recommend waxing/polishing the entire hull, he doesn’t preclude its success and distinctly promotes leading edge treatment. The key is to have your hull as aerodynamically smooth as possible to keep the flow attached for as long as possible, keeping the transition from laminar to turbulent flow as far downstream from the leading edge as possible. Conclusions? An individual who has sailed with and around Dennis Conner (of America’s Cup fame) related a quote to me in which Mr. Conner was asked why he wet- sanded his cup boats. He replied that he had absolutely no idea, but that if he didn’t, he was sure the other teams were and by God he was going to as well, if for no other reason than to level the playing field. It is also postulated that his teams wet-sand to promote team unity and to assure as “fair” as possible a hull form, more than a scientifically based attempt to gain speed. I believe he should wet-sand, then follow up with a silicone-based polish. It is interesting to note that when Dennis lost the Cup to New Zealand and subsequently took the Catamaran (yeah, boy) “Stars and Stripes” to get it back, he was not only wet-sanding, but using a controversial coating (polish) that I believe was called “Shark Skin”. It’s amazing to watch how in their desperation to go 0.001 knot faster, it’s even easier to suck the best sailors into trying every bottle of juice from every snake oil salesman on the globe. The fact is any possible difference that “Shark Skin” could have made, as compared to wet-sanding, or wax, or silicone polish, is so miniscule that it can’t be measured from the noise. Go ahead and wax your hull. It will protect it from UV damage, keep it looking shiny and, thanks to Billy Crystal, we all know it’s better to look good than to feel good. A Hobie Mailing List subscriber made another salient point that, unless you are doing your wet-sanding, on the beach, just before a race, you can rest assured your tow vehicle and trailer will be throwing all manner of road filth on your bare sanded hulls and it ain’t as smooth as you thought once you get to the race. Just a few streaks of tar and all your bets are off. Ever done a comparison of road grime removal between wet-sanded and silicone polished hulls. I have. No contest. Cleaner hulls are faster hulls we can all agree, N’est-ce pas? It’s also been postulated that wet-sanding on the beach the morning of a regatta is used as a psychological tactic. That is if someone sees a competitor paying that close attention to the details of his boat, he/she may begin to question his/her equipment preparation and any doubt one can place in an opponent’s mind on the beach translates to inches on the water. There may be some merit to that argument. Wet-sanding vs. Polishing is also moot the first time you blow a start or a tack, miss a shift, fail to cover a closely matched opponent, or foul someone and have to do a penalty turn(s). So go ahead and wax your boat. If someone beats you and they wet-sand instead of waxing/polishing, they were a better sailor, not a better boat prepper. Even if it was Dennis Connor. Even the best can be scientifically misled. We won the spring A-fleet series on Cayuga Lake (NY) in a heavily waxed J33 this year. I have race-prepped boats from Sunfish® to Cats for people, including waxing and silicone-based polishes and they have finished no worse and sometimes better (psychological advantage?) than ever in regattas. I believe the most important part of this debate, whether you personally decide to wet-sand only, or follow up with polish/wax, is the attention to detail either process brings. This is where gain can be achieved. By this I mean that a sailor/crew that expends the time and energy to painstakingly go over every inch of his/her hull(s) in preparatory obsession will necessarily be in tune with all his/her vessel’s nuances and idiosyncrasies and, I believe, this attention naturally flows to the rest of the sailing experience and the entire experience is heightened. Kind of Zen-like? One will also spot potential trouble spots sooner. That’s why most motorcycles you see are always spotless. A rider’s life may depend on mechanical integrity and a good way to stay in touch with that is to clean and preen. This has nothing to do with “having a faster boat”. Adjusting the “nut on the end of the tiller” through conscientious, contemplative time in the butt bucket is the only real way to do that. I admit there are arguments to be made in opposition to mine that can sound pretty convincing. The only “fact” in this debate is that it is still just that. Most aerodynamicists will admit it’s still as much of an art as a science and the more we learn and understand, the less we realize we understand and the more we have to learn. The bottom line is that, as long is there is argument and the differences are microscopic anyway, I’m willing to err on the side of making my boat prettier and easier to maintain and, at the same time, will spend more time on the water, practicing skills. For those who may be curious at this point, here is what I do to my boat (H16): I wet sand, by hand, all the way up thru 500, 600, 800, 1000, and on to 1200 or 1500 grit 3M papers. I rub each grit of sandpaper in one direction only. Obviously you don’t want to sand through the gelcoat, so prudence is essential here. Then, as I switch to the higher number paper, I rub at a 90 deg. angle to the previous. In this way, I can easily see when the tiny scratches from the previous paper have been removed. I keep alternating as I go up. Then I apply 3M Fine-Cut rubbing compound with a 7” orbital buffer. This removes the rest of the sanding swirl marks. I proceed with a good quality polishing compound (3M, or Turtle Wax) and finish with a silicone- based polish (Starbrite Boat Polish, McGuiar’s #53/#53 Boat Polish, or Eagle Poly-1; whatever I have in the shop). I generally don’t wax/polish the topside of the decks because it’s virtually impossible to get the white residue out without gasoline and a flame thrower. I have discovered a silicone-based product from Black Magic®, called Professional Protectant™, that when applied to the decks, leaves an ultra-glossy, non-fading, UV protecting shine that lasts for weeks and makes the non-skid look like new. It’s a simple spray on/wipe off process. Yes, it’s possible I have to much spare time and, after all this, I still get my head handed to me on the race course, but it’s not the hull prep of my boat that’s to blame. Now, if you’ll excuse me, I’m going to go put some time in on the water. See you there. Cheers, Christopher H. VanEpps Aeronautical Systems Engineering Lockheed Martin. http://www.catsail.com/archives/v3-i2/feature4.htm
thanks Rob, that was very interesting. I belive his hydro- and aero-dynamics are accurately stated, although he fails to mention that Reynolds numbers are also dependent on the size of the foil being considered (since the relative density of air/water molecules will change). Eg what is true of a 50 foot wing, sail, or hull will NOT be true of a 5 inch fin or 6 foot surfboard...I think he hits the nail on the head when he says its basically still a debate! How about this: Gloss the deck and sand the bottom! Rails are going to be tricky though...
roger- i've always done it parallel to stringer-that's what Liddle showed me to do.can definitely feel the increase in speed-it also cleans off any accumulated gunk that might slow you down.if no sandpapaer available, i'll use dry, fine beach sand.Later! matt
Sanded is still smooth to water. Read the section on "laminar sub-layer." This could answer this debate for a while. Unless someone disagrees. -Rob A smooth bottom is a fast bottom. Is the bottom of your boat smooth enough? By Paul Grimes You can't be a sailor and not be a scientist. Every time you leave the dock, you become part aerodynamicist, part hydrodynamicist -- even part cosmologist (when you get that urge to bang the left corner!). Sometimes it's enough just to know that something is fast, without knowing why. But with more complex questions, such as how to minimize "skin-friction drag" on underwater surfaces, you will be bombarded with different theories. Some people will tell you to wet sand, others will say to polish. Stars & Stripes had micro grooves in '87, but maybe you need Teflon in '96. When faced with such a variety of opinion, the only solution is to charge ahead, fearless of the complex science involved, and try to discover the truth. You'll be surprised to find that the subject isn't that complicated after all. Basic Science The No-Slip Condition--The term "skin friction" is misleading when used to describe drag on hull surfaces. When we think of friction, we normally think of two surfaces sliding against each other. This does not happen underwater. Fluid dynamics textbooks usually begin discussions of this topic by explaining the no-slip condition. This stipulates that the fluid molecules against a moving surface do not slide (slip) over it. Instead, they are pressed against it and adhere to it. This occurs regardless of the type of surface (gelcoat, paint, plastic), how smooth the surface is, or whether water beads up on it. In fact, if you could figure out how not to adhere water to a hull, getting it to slide by, you would have discovered a major scientific breakthrough. Boundary Layers--To visualize what happens as a result of the no-slip condition, imagine yourself in a scary situation: an overcrowded subway station. You are part of a crowd pressed up against a train which is now full and starting to move slowly. The people who are touching the train have no choice but to move along with it, and they push those pressed up against them as well. You are 6 feet back from the train, but you, too, are bumped and pulled along, though not as fast as the train and those against it. Somewhere behind you, farther out from the train, the effect ends, and the train's motion does not cause people to move. What you just imagined (except for the panic) is what happens to the water molecules against the hull of a sailboat. The region of water pulled along with the hull is known as the boundary layer, and it can take one of two forms -- laminar or turbulent. When the boundary layer is "laminar," it's thin and presents little drag. It's also fragile, so it quickly breaks up into a thicker "turbulent" boundary layer as it flows aft on the hull or foils. When turbulent, the boundary layer pulls more water with it, creating more drag. Therefore, the first goal of bottom preparation is to extend the laminar boundary layer as far aft as possible on your hull and appendages by creating practically perfect surfaces in the areas where it can exist. The second goal is to minimize drag aft of the transition to turbulence, and this is a little easier to do. Most of the turbulent boundary layer consists of chaotic, swirling eddies, but there is a thin layer next to the hull known as the "laminar sub-layer." Any surface roughness small enough to be immersed in this layer is "hydrodynamically smooth." In other words, making it any smoother will have no benefit. This means that the hull does not have to be as smooth in the aft sections, where you know the boundary layer will be turbulent, as it does in the forward sections, where you hope to preserve laminar flow. Let's take a look at two examples: a boat traveling at 2 knots and one moving at 12 knots. Laboratory experiments with flat plates indicate that the transition from laminar to turbulent flow in the boundary layer should occur in the first 6 feet at 2 knots, and within the first foot at 12 knots. Boats are not flat plates, however, and they don't sail in calm test tanks, so we need to search further for evidence of the true transition point. What the lab results do teach us is that the greatest opportunity for laminar flow is at low speeds. Note that this is also when most of the total drag of a hull is due to skin friction (as opposed to wave drag in heavy air). So the smoothness of the forward sections of your bottom and foils is most important when sailing in light air. As for what is "hydrodynamically smooth" aft of the transition point, when sailing at 2 knots, it's any scratch smaller than 4 mils (thousandths of an inch). At 12 knots, the "admissible roughness" reduces to under 1 mil. A human hair is approximately 2 to 3 mils in diameter, and a bottom finished with 400-grit sandpaper should have a hydrodynamically smooth finish aft of the transition point for speeds up to 7 knots. So, for most keelboats, a bottom which is finished with 400-grit sandpaper in the aft sections is adequate. For planing dinghies, which sail faster, the aft sections of the bottom need to be smoother. http://old.sailingworld.com/grimes.htm
"As for what is "hydrodynamically smooth" aft of the transition point, when sailing at 2 knots, it's any scratch smaller than 4 mils (thousandths of an inch). At 12 knots, the "admissible roughness" reduces to under 1 mil. A human hair is approximately 2 to 3 mils in diameter, and a bottom finished with 400-grit sandpaper should have a hydrodynamically smooth finish aft of the transition point for speeds up to 7 knots. So, for most keelboats, a bottom which is finished with 400-grit sandpaper in the aft sections is adequate. For planing dinghies, which sail faster, the aft sections of the bottom need to be smoother." From what I've read the laminar sub-layer occurs only within the turbulent zone (which would not include the very front of the board) and is very thin - a few hundredths of a millimeter. The laminar sub-layer appears to be partially caused by the fact that in that area random movements of the fluid are to some degree prevented by the presence of the "wall" (surfboard, hull, pipe wall, whatever) So any roughness in the bottom of a surfboard needs to not exceed that depth in order to avoid destruction of the laminar sub layer. -- The math to calculate such things appears to be known but over my head. I suppose it's arguable that the entry point of a surfboard might need to be so smooth as to be polished, while the remainder of the board could (or should)be sanded... but to what grit? How fast ARE surfboards anyway? 15-20 knots? tow-in boards are even faster...