Like Tree0Likes

Bottom Sanding- what grit and direction?

Reply
Old 03-11-2007, 11:51 PM
  #11
Charter Member #232
Charter Member
 
Audiofn's Avatar
 
Join Date: Oct 2000
Location: Carlisle, MA USA
My Boats: 1979 Formula 302, 99 Formula 353, 81 Donzi 18 2+3 with 454
Posts: 18,379
Default From Sail World

Slip Sliding Away
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.


The Real World
Hulls in Waves--When sailing in real conditions, the shape of a hull or appendage works to enhance laminar flow. However, the condition of the water acts to destroy it.
Visualize water flowing past a sailboat hull. It is deflected outward by the forward part of the hull, and accelerates until it reaches the widest and deepest part of the hull. This acceleration creates a "positive pressure gradient" that stabilizes and prolongs the laminar boundary layer. The same effect exists on keels, and has led to foil sections and bulbs with their maximum thicknesses moved aft to delay the transition from laminar to turbulent.

The worthy opponent of these positive effects is the state of the water. Turbulence at the surface from waves, microorganisms, and contaminants can all be disruptive. Yacht designer David Pedrick, who has dealt with this question during several America's Cup efforts, feels that the imperfect sea state usually wins out. "We've used electronic sensors and microphones to test for laminar flow," he says. "You can get some, but not much."

The best chance for laminar flow is on the keel and rudder, both because of their convex shape and because they are immersed below much of the disturbance. Aerodynamicist and dinghy designer Frank Bethwaite questions "whether any surface is 'smooth enough' for a racing dinghy," when it comes to foils.

Smoothness vs. Fairness--So far, we have concentrated on the smoothness of surfaces, but not fairness. By fairness we mean whether the hull has highs and lows that deviate from its designed continuous curves.

On this subject, Karl Kirkman, a well-known hydrodynamicist with extensive tank-testing experience, found that hulls can be forgiving of gentle variations in shape as long as there are no sudden changes in curvature. "If there is a step or a dent in the hull," says Kirkman, "of course that has to come out. But hulls can be forgiving of a gradual waviness unless it is in a place where it could cause flow separation."

Pedrick agrees, but adds that, for keels and rudders, both smoothness and fairness are critical to performance.

Beading vs. Wetting--Any discussion of fast bottom surfaces eventually leads to the question of whether water should bead up on a hull or "wet" the hull so that it flows off in a sheet. "Beading has no relevance," says Kirkman. "It's a function of the surface tensions of the water and hull surfaces, and nobody has been able to explain to me how it has any relation to skin-friction drag underwater."

Even so, both Pedrick and Kirkman suggest that waxes are not a good idea because they seem to attract and/or react with contaminants in the water and can be hard to get as smooth as a finely wet-sanded surface.

What About Riblets?--Riblets were used on the bottom of the 12-Meter Stars & Stripes during the 1987 America's Cup. They are tiny v-shaped grooves that were applied to the hull on a vinyl tape (from the 3-M company). Soon after their televised debut, they were outlawed by the racing rules, which now prohibit "specially textured" surfaces that alter "the character of the flow of water inside the boundary layer."

Fine. But what if you are painstakingly wet sanding your new bottom paint to achieve a "hydrodynamically smooth" surface. You might wonder whether you could sand carefully in a fore-and-aft direction using 220-grit paper, and then "just launch the damn thing."

Fortunately for protest committees everywhere, this does not appear to be a smart approach. Early papers on riblets show that their effectiveness is sensitive to the geometry of the tiny grooves, and that rounded grooves are likely to increase drag.

They have also been shown to trip laminar boundary layers into turbulence sooner than smooth surfaces. As Kirkman points out, the optimum height of the riblets changes with speed, so any riblet choice is a compromise. So it seems likely that large scratches left in a surface from sanding with 220-grit sandpaper will increase drag rather than reduce it.


Bottom Finishes
By now we should agree on a few things. The foils should be mirror smooth. For keelboats and non-planing centerboarders, if you want to go fast in light air, the hull should be highly finished at least back to the midsection. Aft of that, a 400-grit finish is adequate for keelboats. Polish a high-performance dinghy from head to toe.
But what is the best finish? Should you use paint or gelcoat? And how do you maintain that finish? Here are some things to consider.

Magic Bottom Paints--We hear about paints that repel water, paints that bond water "because water sliding over water gives less drag," and paints with low-drag coefficients. When faced with such claims, remember that the chemistry of a paint can only reduce drag if it leads to a smoother finish -- either by allowing the paint to be sprayed on more smoothly, by creating a harder surface for finer wet sanding, or by preventing growth and contaminants from adhering. Any other claim runs into the no-slip condition. It may well happen that someone will figure out how to allow water to slide over surfaces, but until then, assume the no-slip condition is alive and well, and smoothness is what really counts.

As for advertised test results showing the drag reduction of a bottom paint, common sense says to be skeptical. For instance, a 10-percent drag reduction would lead to a huge speed advantage in a one-design fleet. If that happens, you'll know it.

Gelcoat--Sailors with production boats often worry if their gelcoat hulls are fair enough. Even if reflected light seems to "flutter" as you move your head to look at the hull, chances are that your surfaces are adequately fair. Your eye tends to be over-sensitive to this, and glossy surfaces show everything. Another boat with a duller, wet-sanded bottom may look perfectly fair; but if it were glossy, chances are that reflections would dance a bit on its surfaces, too. Unless you can actually feel roughness or unfairness, your efforts are better spent on your keel, centerboard and rudder.

Sometimes, excessive "orange peel" or "print through" is visible in the gelcoat. This means that, although the surface is often fair enough, it may not be smooth enough. If you decide to wet sand a gelcoat hull, it's best to have a boat shop refinish the underwater surfaces with another layer of gelcoat, or an epoxy barrier coat.

Don't sand gelcoat without good reason, however. Untouched gelcoat has a thin, resin-rich layer on the outside that helps to protect it from weathering. Removing this layer will not only cause quicker fading, it may expose porosity that is trapped in the gelcoat. This porosity is not much of a drag problem (tiny protrusions such as road dirt are much worse), but it will leave the gelcoat less effective as a water barrier to the laminate and core.

With gelcoat, the best way to maintain the finish is with soap and water. Many sailors put a layer of liquid soap on a hull before launching to keep the bottom clean while sailing out of a polluted harbor. While this is effective, the soap also adds to the pollution. As an alternative, some sailors polish the bottom. Do whatever it takes to keep the bottom free of contaminants. Remember, wax is not recommended.

If the bottom is finished in an epoxy primer, you can also wet sand it to maintain a clean, smooth finish. But a wet-sanded bottom will get dirty more quickly than a polished or shiny gelcoat surface. Here's how they do it in the America's Cup:

The Perfect Hull--Perhaps the best way to discover what the experts do is to check the hulls of America's Cup contenders. These boats are drysailed, and thus do not need antifouling protection or paints that can be left immersed for long periods. However, there are still lessons to be learned.

At least among the American syndicates, there seems to be little variation from what Pedrick describes: "We start by getting the hardest surface possible. Since Courageous in 1974, we have used Awlgrip on the boats because the catalytic urethane chemistry yields an ultra-hard surface. We wet sand this to a 600-grit finish, and finish it off by sanding in the streamline directions -- just to do the least amount of harm. Before launching, we put detergent on the hull to keep any oil or contaminants off."

In the end, it's not that complicated. Your appendages should be as smooth and fair as possible, and your hull should be just as flawless in the forward areas. Aft of where you expect transition on the hull (certainly by amidships), the surfaces need only be "hydrodynamically smooth." Finally, when faced with the myths, remember the no-slip condition.

Paul Grimes was a Collegiate All-American sailor at Brown University, an engineer for Tillotson-Pearson, and is currently owner of Fusion Composites in Middletown, R.I.
__________________
Put your best foot forward!
Audiofn is offline  
Reply With Quote
Old 03-12-2007, 12:00 AM
  #12
Charter Member #232
Charter Member
 
Audiofn's Avatar
 
Join Date: Oct 2000
Location: Carlisle, MA USA
My Boats: 1979 Formula 302, 99 Formula 353, 81 Donzi 18 2+3 with 454
Posts: 18,379
Default

If you guys want all the science that came in the beginning of this artical I can get it for you.

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.
__________________
Put your best foot forward!
Audiofn is offline  
Reply With Quote
Old 03-12-2007, 12:39 AM
  #13
Charter Member #232
Charter Member
 
Audiofn's Avatar
 
Join Date: Oct 2000
Location: Carlisle, MA USA
My Boats: 1979 Formula 302, 99 Formula 353, 81 Donzi 18 2+3 with 454
Posts: 18,379
Default

This is a picture of an America's Cup boat. These guys are going for ever .000001 MPH that they can get. They have mirror smooth bottoms.
Attached Thumbnails
Bottom Sanding- what grit and direction?-americas-cup-boat.jpg  
__________________
Put your best foot forward!
Audiofn is offline  
Reply With Quote
Old 03-12-2007, 08:01 AM
  #14
Charter Member #927
Charter Member
 
Payton's Avatar
 
Join Date: Jan 2001
Location: IN
My Boats: 1991 34' Super Hawaii
Posts: 4,817
Default

Quote:
Originally Posted by Audiofn View Post
. They have mirror smooth bottoms.

Jon, that really is more than we needed to know.

Thanks for posting the article though.
Payton is offline  
Reply With Quote
Old 03-12-2007, 08:11 AM
  #15
Platinum Member
Platinum Member
 
Mentalpause's Avatar
 
Join Date: Nov 2000
Location: IL...LOTO
My Boats: Nor-Tech 340 CC
Posts: 3,536
Default

Quote:
Originally Posted by Buddy OO View Post
John, what did the sunsation guys say about it and do you think it helps your speed, see ya at loto
They feel they pick up a slight improvement with it. Ended up I just liked the looks of the graphite on the bottom.
Mentalpause is offline  
Reply With Quote
Old 03-12-2007, 10:45 AM
  #16
Registered
 
Back4More's Avatar
 
Join Date: Feb 2002
Location: Lake Michigan
My Boats: 353 Fastech
Posts: 6,624
Default

I read all that and I still don't know what to do.
I think for a stepped bottom boat, one should not worry about trying to get more out of it....but for a non step, it moght be worth it.
Next time I see RF at a Boatshow or Poker Run I will ask him, he should know.
Back4More is offline  
Reply With Quote
Old 03-12-2007, 10:55 AM
  #17
Registered
Gold Member
 
DoTheMath's Avatar
 
Join Date: May 2002
Location: Ma. / Lake Winni., NH
Posts: 1,803
Default

So, here is my .02 - and I have maintained this for years (since my college days in mech. engineering) - if you could "dimple" the bottom / running surface of a boat like a golf ball, would that work to increase speed? I had a wake board that had big "dimples" in the bottom of it - prob. 3" in diameter - and they said it was to relieve surface tension so the board would leave and re-enter the water easier. A golf ball is dimpled to enhance it's "flight" characteristics, correct!? So, if you could apply that to the running surface of a boat, and do so in a "proportionate" state, (the right sized dimples for a given area / weight / speed / etc...) could you do away with the inherent "issues" steps have and have a surface that gives less resistance than a traditional smooth suface. And I'm not talking about 3" dimples here, I'm talking very small dimples, just enough to effect the laminar flow of water over the hulls surface...
DoTheMath is offline  
Reply With Quote
Old 03-12-2007, 11:19 AM
  #18
Registered
 
hammer01's Avatar
 
Join Date: Feb 2006
Location: NORTHERN ILLINOIS
My Boats: SOLD!!!
Posts: 1,008
Default

My buddy has a boat that sat in the water in Lake Michigan for 3 summers with no bottom paint that is now "pitting" from the delamination aspect. Will this do the same without having to run a sander? Just playin, but scary that is represents similar physics in appearance.
hammer01 is offline  
Reply With Quote
Old 03-12-2007, 11:38 AM
  #19
Charter Member #232
Charter Member
 
Audiofn's Avatar
 
Join Date: Oct 2000
Location: Carlisle, MA USA
My Boats: 1979 Formula 302, 99 Formula 353, 81 Donzi 18 2+3 with 454
Posts: 18,379
Default

The golf ball treatment has bee tried on things from windsurfers to huge ships and I believe that they were found to cause suction if I remember correctly. I will try and find the artical.

Jon
__________________
Put your best foot forward!
Audiofn is offline  
Reply With Quote
Old 03-12-2007, 11:48 AM
  #20
Charter Member #232
Charter Member
 
Audiofn's Avatar
 
Join Date: Oct 2000
Location: Carlisle, MA USA
My Boats: 1979 Formula 302, 99 Formula 353, 81 Donzi 18 2+3 with 454
Posts: 18,379
Default

Quote:
Originally Posted by Payton View Post
Jon, that really is more than we needed to know.

Thanks for posting the article though.
Just wanted people to realize that it works different the most poeple thought that it should. It is not all about Laminar flow.

There are WAY bigger increases to be had with working with props, trailing edges of the hull and steps and removing hull imperfections and waves.

The sanding has been proven over and over and over again to be a psycological gain, or if there was a gain there was a reason for it like colder temps giving more HP to the engines........ Even Dennis Connar said that he did it for psycological reasons. They also have shown that the problem with it is that you get dirt and growth MUCH Faster with a sanded bottom and the growth very quickly turns into a loss.
__________________
Put your best foot forward!
Audiofn is offline  
Reply With Quote
Reply

Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Pure Energy
Superboat
4
06-13-2006 12:29 PM
HPJunkie
General Q & A
2
03-22-2004 11:00 PM
HPJunkie
General Boating Discussion
2
03-22-2004 06:29 AM
Tom McCann
General Q & A
7
02-16-2003 09:41 PM
kook
General Q & A
3
07-12-2002 05:52 PM



Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 08:40 AM.


Copyright 2011 OffShoreOnly. All rights reserved.