Golf ball boat - install a dimple plate?

[juiceclark]

Ok...I just have to ask:

Granted - a dimpled golf ball flies 30% farther than a smooth golf ball as the dimples create lift.

Then why doesn't the bottom of a big, wide and flat boat get some lift and relief of suction from dimples as well? Perhaps reverse dimples (pimples) would be better as dimples may cause more suction....like mini cupped hulls.

I Googled and cannot find anything about anyone trying a dimpled (or pimpled) area near the transom on a boat hull.

Tony in Sw FL

(sorry...have free time today because the stock market's up SO much)

[Chris Ostlind]

JuiceMan...

Take a look at the relative, speed through the air, of the golf ball and the relative, speed through the water, of a given hull design and make your relevant relationship models.

That should tell you the answer.

[Fanie]

Considering...

different media's, air is compressable and water is not.

The golf ball has dimples all round in all directions, the boat will have them on one side only - the under side.

The golf ball might travel slightly (but only slightly) faster than a boat.

You cannot tee a boat, but - a boat with dimples can be a handicap.

The golf ball has a shape that drags wind like a commet behind it. The boat has a sharp front for cutting throught the water. If you swap these two features, you'll have a golf ball you have to play in the water with with the sharp end towards the hole or in this case the well, and a boat that has to fly to work like the space shuttle does. Or similar, but you'll have to tack really fast.

The golf ball rotates when it is creating the lift. The only time a boat turns is when the beer is up and you have to head for the fridge on the land. A boat rotates after the third trip to the fridge when a gust of wind (could be from the golf ball) gets you between a broad reach and a close reach.

One is meant for playing golf, the other for FISHING

[PAR]

A golf ball's dimples don't create lift, but do maintain a better laminar flow around the ball so it's resistance is less. In fact a golf ball with bumps, (I was a friend of the son, who's father invented it, working at Dupont) flies better still, but the PGA banned them promptly, after they realized all golf courses would be too short with the new ball.

Bumps and dimples have been tried, as have thousands of holes which allowed air to fill the low pressure areas (try and keep all these clean and open in a race). Vortice generators and all sorts of things (like soap and oil injection) have been attempted, most with limited success (though oil injection works very well).

As an object subjected to flow, spherical things aren't particularly good (they suck actually), regardless of the viscosity in the flow medium.

You can increase the laminar flow qualities with some well designed surface protrusions, but what works good at some S/L ratios may be the feature that hurts you at other S/L ratios. If your design will operate within a narrow S/L range, then putting some bumps on the bottom may be worth the trouble. If you expect varying S/L ratios during general use, then you should consider suffering along with just a smooth bottom or possibly hanging an oil filled bag with a slow leak off your bow, so you can "slide" along a wee bit better.

[charmc]

possibly hanging an oil filled bag with a slow leak off your bow, so you can "slide" along a wee bit better.

Someone tried that recently, using an integral tank in the bow in lieu of a hanging bag. Couldn't control the leakage rate, though. :D

[Fanie]

The hole you see there is where you park boats in.

You could have something like a shark skin effect to slightly (only maybe) increase efficiency. One of those back to front things again. You need the boat to catch the sharks for their skin but you cannot finish the boat since you do not have the shark skins.

It's like the skippers license thing, just as back to dront. You cannot sail if you do not have a skippers license. You cannot get a skippers license if you do not sail.

[juiceclark]

I'm grateful for all of your comments. The golf ball is spinning and the dimples creates a vacuum above the ball for lift and less drag. Doh! I should have figured that one on my own. Instead I've wondered about it for years. (I warned you...we financial minds are usually challenged). I guess a spherical shaped boat that spins backwards is out of the question?!

I saw something online last year where they put crystal shaped thingys in the gelcoat and brushed them all the same direction to act like sharkskin. I haven't heard another word about it...so, apparently the results weren't Earth shattering.

Tony in Sw FL

[tom28571]

PAR set things straight about the dimples on golf balls. They delay the onset of flow separation, making laminar flow last longer. Injecting some materials into the stream is effective though and is deemed illegal by the yachting authorities while painting the boat with the miracle stuff of your choice is not.

Electric power plants and other units with closed loop water systems often treat the water with long chain polymers to reduce friction in the boundary layer and increase efficiency. Britain Chance did build a sailboat that had a series of small holes near the bow where a long chain polymer was injected into the flow. It was promptly outlawed.

If I'm looking at it properly, a backwards spinning golf ball creates down force not lift.

[juiceclark]

Hmmm. You'd think someone would have brought wet exhaust to the front of the hull to ad lubrication from, for example, a filthy pair of Detroit Diesels.

In any case, I'm going to figure a simple and effective way to add air under this wide hull...eventually:
http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/22690

[marshmat]

Just to clear up why the dimples work on a golf ball, and not on bigger slower things like boats....

See the attached image. This is a classic system found in all fluid mechanics texts, and one that an amazing number of students fail on their Fluids II midterms.

The dimples induce a slight bit of turbulence in the boundary layer- the very thin film of air that is right against the surface of the object. With a laminar boundary layer, the flow would travel in smooth layers, and would separate from the ball very close to the leading edge in order to maintain these smooth layers- leaving a huge, turbulent wake behind. A slight bit of turbulence in the boundary layer delays the separation, allowing the flow to remain in relatively smooth contact with the ball until somewhere along its trailing face- thus, when the flow does break from the ball, the turbulent wake is much smaller.

It's not a complicated system, but it is important to distinguish between turbulence in the boundary layer and turbulence in the wake. A slight increase in the former, for a golf ball, greatly reduces the energy dissipated in the latter.

The spin on a golf ball is not intended for reducing drag. What it does is to induce a slight circulation around the ball, which results in a net upward force (a lift) on the ball. This effect is not nearly as intuitive as it sounds and some reading on potential flow theory is required to understand why it works. But in the end, the lift comes at the expense of the rotational inertia of the ball, and so while the spin slows down in flight, it spends more time in the air and so goes farther.

Now to a boat. Unless you're sailing a harbour tug, you do not have separated flow on the underside of the boat to begin with. And you're not in the same Reynolds number regime as a golf ball. So the increase in skin friction you get by forcing a turbulent boundary layer is not offset by a decrease in the energy dissipated in a separated wake, because there is no separated wake to begin with.

Predatory sharks, AC yachts and other high-tech marine systems sometimes use tiny ridges and other structures on the skin of the animal/boat. Exactly how they work is the subject of a lot of research right now; essentially, they are manipulating the boundary layer to provide the right combination of laminar and turbulent properties for the pressure gradient in a particular area. Simply slapping the stuff on has no benefit without knowing what it's doing to the boundary layer in which area.

[Chris Ostlind]

...If I'm looking at it properly, a backwards spinning golf ball creates down force not lift.

You are right, you were looking at it backwards.

Observe how a pitcher in baseball gets the ball to curve or sink. The rotation applied to the ball from the hand release and wrist snap, imparts a powerful forward rotation from the top of the ball rolling downward over the front of the ball. The ball, literally claws its way in a downward arc much more drastically than a simple, decaying parabola of arc.

A ball given the opposite rotation, such as a really hard fastball with lots of reverse spin, is called a riser. Golf balls hit with lots of backspin climb higher and also stop quicker before rolling backwards, hopefully on the green.

[Chris Ostlind]

There has been some thought on entrained air benefits on Viking long boats. In more modern times, there has been fairly extensive study on the effect by a Danish boat designer, who's name escapes me now, in which he deliberately formed air entrainment channels in his hull surfaces in order to facilitate the same effect he was seeing in the Viking boats.

I saw this guys work on the Internet and was truly interested in his work. I have since saved it to my computer in such a fashion that I can't find it anymore (sunspots, or maybe brain spots) His boats were in the Faering size of things and looked to be working quite well.

If anyone has a memory jog from this suggestion, I'd really like to have a current URL for the design work of this gentleman.

Chris

[kach22i]

As the price of fuel rises the more these things (dimples and bumps) will be revisited.

Maybe there will some progress made on the topic in the near future.

[tom28571]

What I was looking at improperly was which direction backwards spin was.

http://en.wikipedia.org/wiki/Magnus_effect

I plead dislexic observation.:confused:

[PAR]

Don't feel bad Tom, I suffer from a very mild dyslexia sometimes, usually when the one that must be obeyed asks me to do something. I pray to Dog every night, that it will go away.

[Chris Ostlind]

Toma and Paul,

The only reason I could speak to the baseball thing is because it is in a brain section that is reserved for memory clarity. Probably due to self-delusional desires about being a major league catcher at one time in my life.

Everything else floats on a sea of luck.

It looks like I'm in good company... ;-)

Chris

[Pericles]

Dimpled bottoms are available. :D

http://www.clacka.com/gulfStream.htm

Pericles

[DanishBagger]

I have read this thread from the get-go. But only when you (Perry) posted that picture do I find words for what might be a practical problem (I really don't get all the mathematics involved): That bottom will be a b*tch to keep clean. I guess you could teflon coat it, though?

[Pericles]

DB,

Drift boats are one way only, flowing downstream with the river current. Then, back on the trailer. :D

The text mentions a European car maker who has used dimples, Any ideas who?

Pericles

[DanishBagger]

DB,

Drift boats are one way only, flowing downstream with the river current. Then, back on the trailer. :D

Somehow I missed that very good point :!:

The text mentions a European car maker who has used dimples, Any ideas who?


Volkwagen? (for their Golf …)?

[Pericles]

:D :D :D :D :D :D :D :D :p

[DanishBagger]

Found one (and not the bad joke about the Golf):

http://www.familycar.com/RoadTests/LexusLS430/Index.htm

However, it's a Lexus – not what I'd consider "european" …

Other than that I can only find people talking about BMW who used dimpled panels on the undercarriage, but they did so to reduce the noise.

[ted655]

Shark skin is dimpled.

[tom28571]

Why would a drift boat be helped by any drag improvement? Sounds like the last reasonable application.

[Chris Ostlind]

You're right, Tom. I get the feeling that it's more of a marketing tool than a functional improvement. The "tunnel" also strikes me the same way. Enhanced wetted surface to what end?

Now, if the frictional release actually makes the boat spin more easily, it may have limited value for the guy with the oars as he tries to point the dude with the rod in the right direction. The tunnel, on the other hand, would seem to actually slow the rotational movement. A zero sum game?

[juiceclark]

Pericles,
Thanks for that link...now I know I'm not completely irrational.

One would think those widespread dimples would cause suction - maybe to hold that boat in wild waters? I won't be adding them to my sculling shell anytime soon.

If the reduce air noise, I'm writing a letter to Ford. My Lincoln Blackwood's rear view mirrors create a lot of noise...better dimple 'em.

[whipper]

Dimples on a low speed application may reduce drag powered by oars. In a high speed application theres many reasons why that would not be desirable. The first one that comes to mind would be the effect of the turbulent water has on the drive. there would be to much aeration introduced to the propeller for efficient slip percentages. It is how ever desirable not to have a smooth bottom on the running surface were the hull planes at speeds that start to free the the hull approaching maximum velocity. Most Drag racers for instance will cross wet sand the aft portion of the hull with 600grit and there gear case also. This has been prov en to shave precious 100th of a sec in the liquid quarter. You will also never see a painted gear case in the pits they are all premiered only. Paint is to smooth. I guess its like what was mentioned above like shark skin. To much though like the size of the perforations or indents on a gulf ball would be to large.The theory is correct but the size comparison of a gulf ball isn't. Having said that all thing relative i suppose if you took the roughness of wet sanded 600 grit on a 19-21 ft boat and optimized that same scale relative pattern on say a 100fter than the perforations or sanded areas size would be more resemble that of a gulf ball? Perhaps the multiplied scale would only be like using 100 grit though also? I'm not doing the math Right now.:o You get my drift though. Speed has always been attributed in less wetted surface. How this is achieved is many different ways. Other than foils and hydro planes its buy less hull at the most aft portion of the hull. Any other ways and the aerodynamics take over from the hydrodynamics and you may end up with a wig.:D Don't get me wrong I'm not saying a new way to kite a hull isn't on the horizon but I see to many holes with the dimple theory the size of gulf ball indents is all.

[BOATMIK]

What I was looking at improperly was which direction backwards spin was.

http://en.wikipedia.org/wiki/Magnus_effect

I plead dislexic observation.:confused:

You were probably looking at the page upside down!

But you would be right if you came here to OZ. We have to topspin our balls to keep them in the air.

MIK

[Village_Idiot]

I remember reading an article in Nature magazine sometime around 1993 about researchers using surface treatments to enhance water flow. For their research, they used water flowing through a pipe, rather than a hull (pipe) flowing through water. What they found was that random protrusions in the pipe increased water flow something like 17% (or maybe it was 14%, I don't remember clearly as it's been ~14 years). Uniform protrusions did not have the same effect, maybe a few percent improvement.

The theory was this - as water flows along a smooth surface, the water that contacts the surface flows slightly slower due to friction. Meanwhile, the faster adjacent water is trying to hurry it along, and ends up "rolling" the slower water up into vortices (much like you might roll up a rug). These vortices (or "rolls") become larger and larger and finally break away from the near-surface area and introduce friction and turbulence to the overall water flow. However, by introducing small random protrusions, these vortices are broken up before they can "roll up" and become very large. The end result is a more laminar flow overall with less turbulence and greater hydrodynamic efficiency.

As far as automobiles trying the golf ball technology, Chrysler tried it on one of their NASCAR race cars back around '69 or '70 on a vehicle with a vinyl roof. However, the vinyl roof tore off of the car at race speeds before they could develop enough conclusive evidence to determine its efficacy. At the time, many of their innovations were being outlawed by race officials, so they likely didn't pursue it any further. :(

One thing the racers did learn was that their engine air intake scoops needed to be several inches above the hood to do any good as there was too much static air (or perhaps turbulence) right at the surface of the hood. :eek:

[kach22i]

FYI:

Whale propeller thread:
http://www.boatdesign.net/forums/showthread.php?t=17382

Tail tube has fins inside to uncling air: The kulikovair website with detailed picture is down for an unknown amount of time.
http://www.aerospaceweb.org/question/helicopters/q0141.shtml
http://www.aerospaceweb.org/question/helicopters/helicopter/notar.jpg

[intrepid71]

For what its worth, I did my senior thesis at Webb Institute on this subject. At the time (early 90's) there were boogie boards and windsurfers on the market with dimples, claiming improved performance. The boogie board had tightly packed, about 3/8" diameter. The windsurfer had larger, shallow oval shaped dimples, about 1-1/2" x 1". We were able to get dimpled and undimpled versions of identical boards. We towed them for resistance in the model towing tank. We also towed at higher speed behind a powerboat. The results were the dimpled boards showed higher resistance. We were unable to find conditions where they were superior. The Clackacraft ad is the first time I have seen dimples associated with a boat. The fact that they would be put on a *drift* boat is slightly mind boggling. I am very skeptical that there would be any benefit.

[juiceclark]

An ol' friend of mine died this week and was looking at a video of the "Bristol Beaufighter" he flew in WWII. If you look at the video below at about 20 seconds, you'll see the nose of this fighter was dimpled. I would guess this gave some lift to the nose of this very heavy fighter.
http://www.youtube.com/watch?v=EkdFVa-wC-E&eurl=http://video.google.com/videosearch?q=bristol+beaufighter&emb=0

So, after I've sat on this for almost a year, I think my post was upside down! Dimples below the waterline would create suction - the opposite of what we'd want. But what if you put them them on the bow above the waterline?? On a fairly high speed boat, this would give a little lift right?

Also, this post never really properly addressed the idea of pimples below the waterline to break suction. My Dad went broke on a company that made duck decoys...but the "Suc-Duck" was their best seller. The one, big dimple created suction and held the duck on the water to look more real. (link below) I just seems a bunch of pimples under there would break suction easier than a step.
http://www.cabelas.com/reviews-cdn/8815/226994/product.shtml?bv=product-_-8815-_-RLP-_-226994-_-description_link

[Tcubed]

The concavity in the bottom of the duck is for hydrostatics not hydrodynamics. It will increase the small angles righting moment by increasing the moment of volume about the CG. Much like a catamaran.

The Clackacraft dimples are a joke, otherwise known as marketing gimmick.

The point of the golf balls dimples is to create better streamlining around a poor aerodynamic shape by inducing turbulence ahead of the separation point. This is highly dependent on the Reynolds number. The golf ball happens to operate at Reynolds numbers where turbulation is an advantage. There exist other Reynold #s for which the golf ball would be better off without the dimples.

In other words, dimples are only of value when trying to maintain attached flow on an object with sufficiently severe adverse pressure gradients and/or operating at problematic Reynolds numbers.

There is also the special aspect of an object moving upon a fluid interface, which is that of fluid entrainment for possible drag reduction, where the fluid with the lower viscosity is introduced to separate the surfaces from contact from the higher viscosity fluid. This was mentioned above in the case of the viking ships and indeed all clinker built boats. This has been demonstrated in the case of the "Dragon" class which were built carvel or clinker and raced together. Neither seemed to have an advantage, implying that the effect of air entrainment about offset the resistance of the clinks.

Earlier there was mentioned the idea of holes to inject fluid into the stream. In fact, in almost all cases the holes are to suck not blow. This again is a technique for preventing flow separation, and typically with much less of a drag price than the turbulation method.

Then there is also the micro vortex generation method which is very effective at injecting energy into sluggish bottommost layers of the BL.

The dimples on the front of that airplane, if i may hazard a guess, is just a riveting artefact. That aircraft has a lot of parasitic drag, so i don't think they would have been too concerned about turbulating an area which is not prone to flow separation anyways. If you look at the very end you see in a patch of reflected light on the top of the wing how rippled the plating is. It is pretty normal when riveting thin sheets of metal. I'm no expert at that particular aircraft though, so i may be wrong on this.

[keith66]

Regarding entrained air in viking stlyle boats, i had direct experience of this last weekend. I had just sold a 1938 Rye beach boat to a Gentleman in Kent and we were delivering her across the Thames estuary to Conyer. "Billows" is a very pretty clinker built centreboard open boat. She was still a bit leaky after 3 years ashore and we kept hearing a funny fizzing noise. eventually after lifting the floorboards i found the culprit, one frame rivet near the garboard that was leaking but what was surprising was the amount of air coming in as well. This was near the stern of the boat and she was ghosting along in a force 2 with the barest ripples on the water. Wish i hadnt sold her now!

[FAST FRED]

"You'd think someone would have brought wet exhaust to the front of the hull to ad lubrication from, for example, a filthy pair of Detroit Diesels."

This was patented long ago by a naval guy named John Paul Jones.

No not the Origonal JPJ , just the same name, maybe related.

The Patent office has plenty of schemes to make boats go faster cheaper , just most dont work.

FF

[rwatson]

An ol' friend of mine died this week and was looking at a video of the "Bristol Beaufighter" he flew in WWII. If you look at the video below at about 20 seconds, you'll see the nose of this fighter was dimpled. I would guess this gave some lift to the nose of this very heavy fighter.


"Bristol Beaufighter RD 147 (PLU) represented the Mark X version of Frank Barnwell’s design, combining a "thimble" nose cone containing Air to Surface Vessel (ASV) radar "

http://glostransporthistory.visit-gloucestershire.co.uk/JetAgeRMCbristol.htm

It looks like the dimpling was done on purpose to support the Yagi type Radar used in night fighting. I would guess that a round nose would deflect incoming electromagnetic pulses, and the dimples were a compromise with the need for streamlining.

[FramerDan]

I'm not sure of how directly comparable the two are, but you might look at Seaplanes for an answer.

For a specific example, a Grumman HU-16 Albatross is a twin engine aircraft with a 96 ft Wing Span. Like most planes it's rivets are ground down smooth before painting, but (according to legend) an owner did a repair to the underside of his aircraft and used round headed rivets. He found that the take off speed was reduced by around 10 knots. The pilot notified Grumman of the discovery, who did a number of tests, and confirmed the finding...ever since they've left the rivets raised on the underside of their seaplanes.

Just too bad I'm a year late.

[mark775]

"The point of the golf balls dimples is to create better streamlining around a poor aerodynamic shape by inducing turbulence ahead of the separation point. This is highly dependent on the Reynolds number. The golf ball happens to operate at Reynolds numbers where turbulation is an advantage. There exist other Reynold #s for which the golf ball would be better off without the dimples."
This is how I´ve understood it - and the induced turbulence actually causes the total disturbed flow (comet tail) to be smaller. Nothing about lift. On a boat, the longer you can keep the flow laminar, the better, and it´s only going to be the front few feet. It is my take that you should get that front of your Balto-plate burnished well and then don´t bother with less than maybe 200 grit - it won´t matter because flow is non-laminar anyway. On a planing hull, more important yet, is to insure that the trailing edge is dead strait. Shark Skin, magic potions, teflon, etc. don´t do anything. Boaters everywhere swear by whatever product du jour simply because, of course, the bottom doesn´t have a beard when they first put the boat in the water, then, after a month, they swear they are slowing down because the super slick stuff is wearing off (it actually washed off almost immediately, and in any event, did not extend laminar flow and therefore was in non-laminar flow doing absolutely nothing), but are actually just dragging fur with them. America´s Cup racers and such are in a completely different league and have the money to experiment. When something actually works that applies to us, we will learn of it soon.
The float plane rivet thing is just to break suction when the plane is trying to take off - there is actually more drag.

[plebusmaximus]

Surfboards in the 80s were released with a "Speed" finish.

Rather than a gloss finish they were done in a satin or matt finish for similar reasons.

[Joakim]

This is how I´ve understood it - and the induced turbulence actually causes the total disturbed flow (comet tail) to be smaller.

On a boat, the longer you can keep the flow laminar, the better, and it´s only going to be the front few feet. It is my take that you should get that front of your Balto-plate burnished well and then don´t bother with less than maybe 200 grit - it won´t matter because flow is non-laminar anyway.

Yes there is a clear difference in Reynolds Number of a golf ball and a boat. A boat has about 100 times larger Reynolds number. However the main issue is, that skin friction is not very important for the golf ball, but it is very important for boats. Dimples INCREASE skin friction, but by doing so, in a very special case of a ball at suitable Reynolds number, they reduce the pressure drag (form drag) much more.

In a case of a boat pressure drag is very low or directly related to wave pattern (=residuary drag, wave making drag) or lift (planning hulls), thus there is nothing that could be reduced in a similar fashion. And the skin friction is a major part of resistance, thus you really don't want to increase that.

Actually the part of the hull where flow is still laminar does not have to be smoother than rest of the hull. A laminar flow "doesn't see" small dimples and bumps any more than a turbulent flow does. As the imperfections grow the transition from laminar to turbulent happens earlier at some point and also the skin friction of the turbulent section increases, when the surface is no longer "hydraulically smooth". This limit for the size of the dimples and bumps can actually be lower for the turbulent area than for the laminar area.

It is not a good idea to ignore the smoothness of the non-laminar area, since it is very well known how roughness adds resistance in turbulent flow (e.g. pipe flows and flow over a flat plate).

Joakim

[FramerDan]

A bit of a side question, I've never really thought about designing a fast boat so I've never considered laminar flow in water.

1. since water does not compress, might laminar flow be easier to achieve than in air? You wouldn't get the annoying rebound effect of air compressing. Could also be the opposite I guess.

2. it seems like dimples on a golf ball are dimples for the fact that the ball has no front, and might spin any direction. On a boat you might use either small ribs, or long gouges.

3. The idea of a some spoilers on cars is to tumble the air over the back, reducing the same large void behind the vehicle. You might infer that small turbulent eddies allow more time for surrounding fluid to fill the void. (many small eddies are better than one big eddy)
- not that it would speed you up enough to be worth the work, but maybe more from curiosities standpoint, do you think you could reduce the drag, not by dimpling the entire underside of the boat, but by watching where the turbulence starts as you move your boat through the water, and applying ridges there, or slightly before.

Of course the turbulent points on a boat change depending on water conditions, speed of boat, and of course pitching side to side...not to mention who knows what's going on along your keel.

I understand joakim's point of resistance in turbulent flow, but there is something to be said for calculated (if you can) placement of turbulence control.

[marshmat]

Hi FD,

Compressibility in air is not a significant factor at low Mach numbers. At the speeds that cars, boats and most light aircraft travel, it is common to model air as being incompressible. This approximation breaks down when you start dealing with commercial airliners, 500km/h bullet trains, and other bodies travelling at significant fractions of the speed of sound.

The dimples on the golf ball serve to force a transition to turbulence, thus keeping the flow attached farther back on the ball, thus reducing the pressure drag. This works only in the very narrow range of Reynolds number in which a golf ball operates; you accept a small increase in viscous drag as a tradeoff for a substantially larger decrease in pressure drag. For streamlined bodies, or for anything outside of that small Re range, the advantages are lost.

Some of the crazy jet-skiers have been known to put longitudinal gouges in their hulls using 20-grit sandpaper. The idea here is the same as with the seaplane's rivets- the resistance is probably increased somewhat, but it tends to "un-stick" the hull from the water, making it easier to jump waves.

I very much doubt that any of this turbulence-inducing stuff would be of any benefit on a normal planing or displacement hull, that stays in constant contact with the water. Still, I'd love to see some experimental data, if anyone has an old power skiff they feel like butchering....

[mark775]

Joakim, I wasn´t suggesting ignoring smoothness in non-laminar areas - just that going all the way to 600-1000 grit and burnish, like a lot of small racing sailboats do, is probably moot. So, your belief is that a very smooth surface in, let´s say, the front third of that J21 won´t keep the flow laminar longer, thereby increasing speed? Or that the ridges of 200 grit in the non-laminar aft two-thirds are not smooth enough to stay out of said flow? My impression has been that the non-laminar flow is actually quite thick - on the order of milimeters at the minimum and therefore wouldn´t know 200 grit scritches from 2000. The only reason that I would have recommended going further than roller-stipple is for confidence, headgames against opponents, the chance of knocking off really big stuff (runs, drips), and to learn the surface of the boat. Am I lost here? Kiitos - Mark

[Joakim]

Joakim, I wasn´t suggesting ignoring smoothness in non-laminar areas - just that going all the way to 600-1000 grit and burnish, like a lot of small racing sailboats do, is probably moot. So, your belief is that a very smooth surface in, let´s say, the front third of that J21 won´t keep the flow laminar longer, thereby increasing speed? Or that the ridges of 200 grit in the non-laminar aft two-thirds are not smooth enough to stay out of said flow? My impression has been that the non-laminar flow is actually quite thick - on the order of milimeters at the minimum and therefore wouldn´t know 200 grit scritches from 2000. The only reason that I would have recommended going further than roller-stipple is for confidence, headgames against opponents, the chance of knocking off really big stuff (runs, drips), and to learn the surface of the boat. Am I lost here? Kiitos - Mark

In the laminar region roughness does not increase drag, but it may cause an early transition to turbulent region. The roughness size that effects transition can be estimated with k=120*nu/V (SI units), thus for water (nu=1e-6 m^2/s) and 6 kn (3 m/s) speed k=40 um.

For the turbulent region clear equations for the effect of roughness are available. The smoothness required for "hydraulically smooth" (=no effect compared to totally smooth) depends mainly on speed. For a 6 kn speed the roughness allowed is about 25 um.

These roughnesses are defined as grain sizes. You can see the grain sizes of different sandpapers from here: http://www.cs.rochester.edu/u/roche/rec.wood.misc/grit.sizes.html

The AC-boats where finished all the way with P600 (25 um grain) and models to be towed in a towing tank are finished with P300 or P400. I think P400 (grain size 35 um, BUT will result to a much smoother surface) is certainly enough for most keelboats. I don't see any sense in using a finer sandpaper for any region, since a well finished P400 sanding is already laborious and very smooth.

If your boat is much faster (say 15 kn), you might want to go for clearly finer sandpapers.

Joakim

[Village_Idiot]

- not that it would speed you up enough to be worth the work, but maybe more from curiosities standpoint, do you think you could reduce the drag, not by dimpling the entire underside of the boat, but by watching where the turbulence starts as you move your boat through the water, and applying ridges there, or slightly before.


You need RANDOM PROTRUSIONS on the surface, to break up the eddies before they get too large to disturb laminar flow. See my earlier post. I suspect the protrusions need to be large enough to be rather unsightly.

[venomousbird]

You need RANDOM PROTRUSIONS on the surface, to break up the eddies before they get too large to disturb laminar flow. See my earlier post. I suspect the protrusions need to be large enough to be rather unsightly.

Barnacles for example. . . ? Just kidding!
Maybe the idea of dimples on a sail could be investigated?

[speedboats]

From what I'm reading you're suggesting dimples (or pimples) on a sail or displacment vessel? What of a planning craft. I've often been asked and given a similar response to what was aforementioned, but the thought of aerating the water under the hull to reduce surface friction is what we try hard to achieve, only it has to be controlled and uniform (symetrically). Has anyone experimented of have thoughts on weather the dimples would trap air at say above 60mph, or turbulate enough to reduce surface tension of the water on the hull? Of would this just increase the nett drag?

[venomousbird]

I was wondering what effect the flow of air under the boat would have myself.

[speedboats]

alot of the so-called 'tunnel' boats are trying to capitalise in the 'wings-in-ground-effect' concept. I have routed exhaust under the hull bottom. Not sure of the benifits (although the boat vibrated and reverberated oddly at non-planning speeds), 'cause if we traveled in anything but a straight line the jet pump would suck air and cavitate, hence we changed to a through-transom exhaust quick smart. Perhaps this'll work in a 'stepped' type arrangement. But 'drizzling' small air bubbles under the hull makes sense on paper, would dimples under the hull help trap these and in effect reduce the friction coefficent

[Pakal]

Good morning everyone - i remembered this discussion last night and i thought i could give you an update about an actual boat that is already on sale in Greece.The manufacturer is AirHull SA and you can find interesting photos and videos on their web site, www.airhull.com .

[cthippo]

...but the thought of aerating the water under the hull to reduce surface friction is what we try hard to achieve, only it has to be controlled and uniform (symetrically). Has anyone experimented of have thoughts on weather the dimples would trap air at say above 60mph, or turbulate enough to reduce surface tension of the water on the hull? Of would this just increase the nett drag?

I wonder if you could use small strakes near the bow to create cavitation in the water and hence bubbles. I'm thinking something like vortex generators on aircraft.

[Mr Efficiency]

I'd be interested to hear the opinions of the folk here about the concept illustrated in Fig. 3 on this site.........

http://docs.hydrofoils.org/SAS03.pdf

[Joakim]

I'd be interested to hear the opinions of the folk here about the concept illustrated in Fig. 3 on this site.........


I guess that would only work for high displacement and/or low speed hulls. Otherwise air will leak out when you trim up to shorten LWL.

Are there any papers showing the claimed reduction of drag? Did they take different trims into account?

[claydog]

Interesting thread, just to toss out a little food for thought, On the TV show Mythbusters they did a test on whether a dimpled car would get better MPG. they covered a car with about an inch of modeling clay, ran the car on a closed course to get a MPG figure. Then they cut dimples( about 1/2 " deep and 4" in dia.) in the clay all over the car puting the removed clay in the car to keep the weight constant and ran the same course at the same speed and got a significant improvement.

[rxcomposite]

This topic is an extention of Golf Ball Fluid Dynamics (http://www.boatdesign.net/forums/boat-design/golf-ball-fluid-dynamics-9566.html) TS Inquisitor 11/28/2005

[rwatson]

There is already a product on the markey to dimple your car, but the results arent that straight forward

http://ecomodder.com/forum/showthread.php/popular-mechanics-busts-fast-skinz-dimple-wrap-claims-8321.html

It would be interesting to know what speeds the mythbusters went in their tests. certainly, at consistant high speeds, dimples create more drag from what I can remember. The dimple effect has been examined for over a hundred years, so one would think that what isnt dimpled, shouldnt be ???

[kach22i]

www.airhull.com .
Cool.

http://www.airhull.com/LH2Uploads/DefaultItemPics/270_1.jpg

[hoytedow]

There is already a product on the markey to dimple your car, but the results arent that straight forward

http://ecomodder.com/forum/showthread.php/popular-mechanics-busts-fast-skinz-dimple-wrap-claims-8321.html

It would be interesting to know what speeds the mythbusters went in their tests. certainly, at consistant high speeds, dimples create more drag from what I can remember. The dimple effect has been examined for over a hundred years, so one would think that what isnt dimpled, shouldnt be ???So if your car has been in a hailstorm it should go faster?